Encasement: A New Option for Asbestos Hazard Control in Canada

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Protecting Personnel from Pandemic
Influenza American Industrial Hygiene Conference
and Exposition, Toronto, 2009 John H Murphy
BSc MHSc MBA ROH CIH GradIOSH President,
Resource Environmental Associates
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Module 1 Introduction to the Course
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1. Introduction to the Course
1.1 Instructor and Class Introductions 1.2 Backgro
und to Course Development 1.3 Scope of the
Government of Ontario Study 1.4 Types of Work
Environments Examined 1.5 Subject Matter of this
Course 1.6 Curriculum Overview 1.7 Learning
Objectives 1.8 Course Materials 1.9 Housekeeping 1
.10 Any Questions?
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1.1 Instructor and Class Introductions

• John H. Murphy
• President, Resource Environmental Associates
  Limited
• Bachelor of Science, Physiological Psychology and
  Radiation Biology, University of Toronto, 1984
• Master of Health Science, Occupational
  Environmental Health, University of Toronto, 1985
• Post-graduate Studies, Occupational Epidemiology,
  University of Western Ontario, 1987-88
• Master of Business Administration, University of
  Toronto, 1993
• Certified Industrial Hygienist (ABIH)
• Registered Occupational Hygienist (CRBOH)
• Graduate Member, UK Institution of Occupational
  Safety and Health

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1.2 Background to Course Development

• In January 2008, the Ontario Ministry of
  Government and Consumer Services contracted
  Resource Environmental Associates Limited to
  conduct a pandemic influenza risk assessment.
• The principal goals of the study were
• to rate relative risks in Government of Ontario
  work settings, and
• provide recommendations concerning
  non-pharmaceutical / environmental infection
  prevention and control measures, in case of a
  pandemic influenza outbreak.

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1.2 Background to Course Development

• Non-pharmaceutical or environmental infection
  prevention and control measures
• control measures and procedures, focused on the
  work environment, building systems, and work
  processes, intended to protect personnel from
  potential workplace and work-related exposure to
  influenza virus, and thereby reduce infection
  risks.
• These types of controls are intended to minimize
  the transmission of viral material shed from a
  person to the air, to a surface, or to the skin
  or clothing of one person to another in
  contrast to medically-oriented preventive
  measures such as immunization or prophylactic
  drug treatments.

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1.2 Background to Course Development

• Recommendations were to
• be proportionate to risks
• reflect the state of scientific knowledge related
  to influenza infection prevention and control
• Cover preparatory actions to take now, and
  actions to take if an outbreak occurs
• The methods developed in this study, and the
  recommendations arising, are the basis for this
  course.

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1.3 Scope of the Government of Ontario Study

• Sixteen workplace locations across Ontario were
  examined for the purpose of collecting
  information on work settings and work activities
  performed by Government of Ontario personnel.
• The workplace locations were pre-determined by
  the client, with a view to covering a range of
  different working environments representative of
  government operations.
• Information gathered at these work settings was
  used for risk assessment and control planning
  purposes.

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1.4 Types of Work Environments Studied
Youth Psychological Assessment Institute (Clinical, Residential, School)
Disability Support Program Office
Customer Service Centres (Walk-in Resource Centre, Counter Service, Interview Rooms)
Provincial Court House
Highway Weigh Scale Facility
Provincial Park
Residential School for Handicapped Youth
Provincial Police Detachment
Probation Office
Ontario Science Centre (Hands-on Science and Technology Attraction / Museum)
Forest Fire Fighting Aviation Services
Provincial Offences Inspection and Enforcement Activities
Youth Correctional Centre
Adult Detention Centre (Holding Jail)
Adult Correctional Centre (Long Term Jail)
Corporate Administrative Office Settings (Class A and Class B Buildings)
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1.5 Subject Matter Covered in this Course

• Basic information about influenza pathology and
  health effects.
• Review of the state of science regarding routes
  by which employees may be exposed to influenza
  virus in the course of their work, and the
  relative importance of these exposure routes from
  a prevention perspective.
• The efficacy and practicality of various actions
  to minimize exposure.
• Methods for workplace influenza virus exposure
  risk assessment.
• Recommendations with respect to ways and means of
  preventing and controlling transmission of
  influenza in work settings.

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1.6 Curriculum Overview
Module 1.0 Introduction to the Course Module
2.0 Influenza 101 Module 3.0 Modes of Exposure
and Transmission Module 4.0 Occupational Risk
Assessment Module 5.0 Influenza Prevention and
Control Measures Module 6.0 Preparing for and
Responding to an Influenza Pandemic
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1.7 Learning Objectives

• There are specific learning objectives for each
  of the 5 course modules that follow this
  Introduction.
• The learning objectives will be presented at the
  start of each module.

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1.8 Course Materials

• Detailed course curriculum
• Course slides
• Instructor resume
• List of reference materials (end of course
  slides)

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1.9 Housekeeping

• Morning break, lunch, afternoon break
• Fire safety
• Cellphones / PDAs
• Asking questions

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1.10 Any Questions?
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Module 2 Influenza 101
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2. Influenza 101
Learning Objectives 2.1 What is
Influenza? 2.2 Characteristics of the
Virus 2.3 Disease Mechanism 2.4 Infectious
Dose 2.5 Comparison with the Common Cold 2.6 Is
Influenza a Serious Health Risk? 2.7 What is an
Influenza Pandemic? 2.8 Why the Concern over a
Possible Influenza Pandemic? 2.9 Could a
Influenza Pandemic be Severe? 2.10 Whats the
Connection between Avian Influenza and a Human
Influenza Pandemic? Questions
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Learning Objectives

• 1. Understand the symptoms of influenza.
• Understand the sequence of events leading to the
  disease.
• Know how influenza differs from the common cold.
• Understand the potential severity of a pandemic
  influenza, and the basis for these estimates of
  severity.
• Understand the meaning of a pandemic.
• Understand the role of avian influenza in driving
  current concern over a potential influenza
  pandemic.

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2.1 What is Influenza?

• Influenza (also known as the flu) is a contagious
  respiratory illness caused by influenza viruses.
  
• Common flu symptoms are
• Fever  (usually high)
• Headache
• Extreme tiredness
• Dry cough
• Sore throat
• Runny or stuffy nose
• Muscle aches
• Stomach symptoms, such as nausea, vomiting, and
  diarrhea, also can occur but are more common in
  children than adults.
• Infection can last from a few days to about 2
  weeks.

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2.2 Characteristics of the Virus

• Influenza viruses range in size from about 20
  nanometers (2/100th of a micron, an example being
  rhinovirus) to 120 nanometers (12/100th of a
  micron, an example being influenza A virus)

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2.3 Disease Mechanism

• The disease mechanism involves the following
  steps
• Delivery of a critical dose of the virus to the
  target tissues in the lungs
• Entry of the virus into the cells of the target
  tissues
• Reproduction of the virus inside the target
  tissue cells, using the biomolecules of the cell
  as building blocks
• Death of the host cells due to depletion of
  biomolecules and cell rupture caused by the
  massive quantity of virus particles created
  inside the cell
• Dead cells cellular contents white blood
  cells blood plasma fluid in lungs
• Significant fluid in lungs results in impaired
  gas exchange and laboured breathing

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2.4 What Constitutes an Infectious Dose

• A limited amount of data is available on what
  constitutes an infectious dose in humans. One
  study estimated the 50 human infectious dose
  (HID50) for influenza A administered by the
  aerosol route to be 0.6 to 3 50 tissue culture
  infectious dose (TCID50) units (Alford 1966).
• The TCID50 is a quantitative unit for virus,
  representing an unknown number of virus particles
  observed to infect 50 of replicate cell
  cultures, each receiving the same volume of
  virus.
• Another study estimated the HID50 for a different
  strain of influenza administered intranasally to
  be 127 to 320 TCID50 units (Murphy 1973).
• Based on the assumption that a TCID50 unit
  corresponds to more than one virus particle, it
  has been estimated that 44,000 particles on
  average are emitted in coughs, although only
  0.0001 of these potentially infectious particles
  remain airborne (Nicas and Sun 2006).

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2.5 Comparison with the Common Cold
SYMPTOMS COLDS INFLUENZA
Fever Rare Typically present
Aches Absent or mild Typical, often severe
Chills Rare Fairly common
Tiredness Mild Moderate to severe
Speed of Onset Gradually (over days) Rapid, 3-6 hours
Coughing Hacking, productive cough Dry, unproductive cough
Sneezing Common Uncommon
Stuffy nose Common Uncommon
Sore throat Common Uncommon
Chest Discomfort Mild to moderate Often severe
Headache Uncommon Common
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2.6 Is Influenza a Serious Health Risk?

• Influenza can cause mild to severe illness, and
  can sometimes cause death.
• 10 to 20 of the population in western nations
  contracts seasonal influenza in a typical year,
  and this may result in about 3 deaths in 10,000
  cases.
• Most infected people recover from seasonal
  influenza within without requiring medical
  treatment.
• In the very young, the elderly, and those with
  other serious medical conditions, influenza
  infection can exacerbate underlying health
  problems, cause pneumonia, and result in death.

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2.7 What is an Influenza Pandemic?

• A pandemic is an epidemic that affects a
  large geographic region, or even the entire
  world.
• An epidemic is an outbreak of disease that
• causes much higher rates of illness than the
  typical background rates and / or
• has more severe health consequences than the
  usual form of the disease.
• The term influenza pandemic refers to an
  outbreak of influenza
• for which there is low human immunity
• that causes more severe illness than is typically
  associated with influenza (due to limited human
  immunity) and
• affects large geographic regions, or the entire
  world.

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2.8 Why the Concern over a Possible Influenza
Pandemic?

• There is a recorded history of periodic influenza
  pandemics.
• In the past 120 years, there have been four
  influenza pandemics
• The Asiatic Flu", 18891890
• The "Spanish Flu", 19181919
• The "Asian Flu", 195758
• The "Hong Kong Flu", 196869
• The absence of an influenza pandemic for 40 years
  may mean that we are overdue for one.
• The appearance of human cases of avian influenza
  has raised concerns that the virus causing avian
  influenza could eventually spark a human
  influenza pandemic.

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The Significance According to Google
SEARCH TERMS SEARCH TERMS GOOGLE HITS
Illness Workplace Illness Workplace 4,010,000

1 Illness Workplace Transmission 641,000
2 Common Cold Workplace 210,000
3 Influenza Workplace 585,000
4 Pandemic Workplace 509,000
5 Flu Workplace 1,620,000
6 Pandemic Workplace Preparedness 84,500
7 Venereal Disease Workplace Illness 9,170
8 Conjunctivitis Bacterial Workplace 5,470
9 Conjunctivitis Viral Workplace 4,880

Sum (19) Sum (19) 3,669,020
(Sum (19) / Illness Workplace) x 100 (Sum (19) / Illness Workplace) x 100 92
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Indicators of a Growing Concern
USA
Canada
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Influenza Pandemics, 1500 to Present

• The first recorded pandemic that was likely to
  have been influenza started in 1510 in Africa,
  and spread across Europe.
• The Asiatic Flu", 18891890. First reported in
  May of 1889 in Russia. By October, it had reached
  the Caucasus. It rapidly spread west and hit
  North America in December 1889, South America in
  February 1890, India in February 1890, and
  Australia in March 1890. It had a very high
  attack and mortality rate. Believed to be caused
  by the H2N8 type of flu virus.
• The "Spanish Flu", 19181919. First identified
  early March 1918 in US troops training in Kansas,
  by October 1918 it had spread to become a
  world-wide pandemic on all continents. Unusually
  deadly and virulent, it ended nearly as quickly
  as it began, vanishing completely within 18
  months. In six months, 25 million were dead.
  Believed to be caused by the H1N1 virus.

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Influenza Pandemics, 1500 to Present

• The "Asian Flu", 195758. First identified in
  China in late February 1957. Spread to the
  United States by June 1957. Caused about 70,000
  deaths in the United States. Caused by the H2N2
  virus.
• The "Hong Kong Flu", 196869. First detected in
  Hong Kong in early 1968 and spread to the United
  States later that year. Caused about 34,000
  deaths in the United States. Caused by the H3N2
  virus, which still circulates today.

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More on Influenza Virus Types

• There are three types of influenza viruses A, B
  and C. Influenza A and B viruses cause seasonal
  influenza epidemics. Influenza type C infections
  cause a mild respiratory illness and are not
  thought to cause epidemics.
• Influenza A viruses are divided into subtypes
  based on two proteins on the surface of the
  virus hemaglutinin (H) and neuraminidase (N).

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Other Events that Have Contributed to the Concern
Over a Pandemic
2005 Seven Oaks Legionnaires, Toronto, 21 deaths
1997 First Human Cases, Avian Flu, Hong Kong,
18 deaths
2001 Anthrax Powder Mail Attacks, USA, 5 deaths
1976 First Human Cases, Ebola Virus, Zaire, 150
deaths
2010
2005
2000
1995
1990
1985
1980
1975
1995 Twelve Monkeys, Bruce Willis, Terry
Gilliam
2003 SARS, China, Hong Kong, Toronto, 251 deaths
1960s-1970s Victory Over Infectious Disease
1981-83 First Recognized AIDS Cases, USA
1999 First Human Cases of West Nile Virus in
North America
2004 ? Increased Government and Business
Activity on Avian Influenza Preparedness
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2.9 Could an Influenza Pandemic be Severe?

• There is no way of knowing how severe a future
  influenza pandemic could be.
• For pandemic planning purposes, it is often
  assumed that the attack rate for the type of
  influenza causing a pandemic would be comparable
  to, or higher than the attack rate for seasonal
  influenza that being 10 to 20.
• The Ontario Health Plan for an Influenza Pandemic
  projects a worst-case scenario with a 35 attack
  rate for Ontario, meaning that about 4.5 million
  people would get sick. The Plan assumes a
  mortality rate of about 0.1, which is about 3x
  the mortality rate for seasonal influenza. This
  represents about 20,000 deaths.

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2.10 Whats the Connection Between Avian
Influenza and a Human Influenza Pandemic?

• The four influenza pandemics between 1889 and
  1968 have been caused by types if influenza A
  viruses.
• The avian influenza virus is a type of influenza
  A virus, called H5N1.
• At present, it appears that avian influenza is
  not easily transmitted from birds to humans, and
  not transmitted at all between humans.
• Between 2003 and March 2009, there were only
  about 390 confirmed cases of avian influenza, and
  all with in Asia and North Africa.
• However, about 65 of these persons died.

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WHO Avian Flu (H5N1) Statistics
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2.10 Whats the Connection Between Avian
Influenza and a Human Influenza Pandemic?
(continuation)

• Influenza viruses can readily mutate, which can
  affect ease of animal-to-human, or human-to-human
  transmission.
• If the H5N1 avian influenza virus mutated in a
  way that caused easier transmission, it could
  result epidemics or a pandemic, and given the
  high mortality rate, there could be many deaths.
• However, a human influenza pandemic could also be
  caused by other types of influenza A viruses.

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The Antigenic Shift
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WHO Swine Flu (H1N1) Statistics

• As of May 25, 2009, the World Health Organization
  reports 12,515 confirmed human cases and a
  mortality rate of 0.7 (91 deaths).
• Observations made by WHO
• Very contagious, attack rate seems to be higher
  than seasonal influenza, at 22-33
• Outside of Mexico, causes mild illness
• Younger population effected
• Illness tends to be more severe in those with
  underlying medical conditions

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2. Review - Influenza 101
2.1 What is Influenza? 2.2 Characteristics of the
Virus 2.3 Disease Mechanism 2.4 Infectious
Dose 2.5 Comparison with the Common Cold 2.6 Is
Influenza a Serious Health Risk? 2.7 What is an
Influenza Pandemic? 2.8 Why the Concern over a
Possible Influenza Pandemic? 2.9 Could a
Influenza Pandemic be Severe? 2.10 Whats the
Connection between Avian Influenza and a Human
Influenza Pandemic?
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Questions?
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Module 3 Modes of Exposure and Transmission
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3. Modes of Exposure and Transmission
Learning Objectives 3.1 When is a Person
Infectious? 3.2 Routes of Virus Exposure and
Dosing 3.3 Relative Significance of Exposure
Routes 3.4 Transmission by Inhalation 3.5 Contact
Transmission Questions
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Learning Objectives

• 1. Know when a person infected with influenza is
  contagious.
• Understand the theoretical and likely routes of
  virus entry for disease initiation.
• Understand the reasons why inhalation exposure is
  likely to be a far more significant mode of
  disease transmission than self-inoculation /
  contact transmission.
• Understand the physical basis for a 2 meter
  protective zone against inhalation exposure to
  influenza.
• Understand why aerosol exposure is likely to be
  less significant than droplet exposure, and
  exceptions to this general premise.

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3.1 When is a Person Infectious?

• Most healthy adults may be able to infect others
  beginning 1 day before symptoms develop and up to
  5 days after becoming sick.
• Children may pass the virus for longer than seven
  days after becoming sick. Symptoms start one to
  four days after the virus enters the body.
• You may be able to pass on the flu to someone
  else before you know you are sick, as well as
  while you are sick.
• Some persons can be infected with the flu virus
  but have no symptoms. During this time, those
  persons can still spread the virus to others.

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3.2 Routes of Virus Exposure and Dosing
Virus Dose by Inhalation (P)
Risk of Infection
Virus Dose to Target Tissue (U)
Virus Dose by Inoculation to Mucosa (R)
Virus Dose by Ingestion (R)
U ? aP bR cR'
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3.2 Routes of Virus Exposure and Dosing
Inhalation of Air -Background Viral Concentration
Virus Dose by Inhalation (P)
Inhalation of Droplets in Discharge Path
Virus Dose to Target Tissue (U)
Virus Dose by Inoculation to Mucosa (R)
Hand to Eye / Nose / Mouth
Virus Dose by Ingestion (R)
U ? aP bR cR'
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3.3 Relative Significance of Exposure Routes
Virus Dose by Inhalation (P)
Risk of Infection
Virus Dose to Target Tissue (U)
Virus Dose by Inoculation to Mucosa (R)
Empirical evidence on influenza, scientific
information on virus-host interactions, and
exposure dynamics considerations, point to
inhalation being the most significant mechanism
for transmission of infection
Virus Dose by Ingestion (R)
U ? aP bR cR' Where a gtgtgt b gtgtgt c
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3.3 Relative Significance of Exposure Routes

• For purposes of this course, we assume that an
  influenza strain causing a pandemic would likely
  be transmitted in the same way as seasonal
  influenza.
• The preponderance of evidence suggests that
  seasonal influenza is spread from
  person-to-person primarily when a non-infected
  person inhales viral droplets and droplet nuclei
  expelled by an infected person through sneezing,
  coughing or breathing (Tellier 2007).
• Therefore, to understand the rationale for, and
  how to implement measures to minimize risk of
  infection by inhalation, it is necessary to
  understand how viruses discharged from an
  infected person by coughing or sneezing can be
  inhaled by other persons.

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3.4 Transmission by Inhalation
3.4.1 Basics of Respiratory Droplets and Droplet
Nuclei 3.4.2 Virus Quantities in
Droplets 3.4.3 Importance of Proximity for
Droplet Transmission 3.4.4 Aerodynamics of
Droplets and Droplet Nuclei 3.4.5 Inhalation
Exposure in the 1-2 Meter Discharge
Zone 3.4.6 Effect of Distance from Source on
Virus Concentration 3.4.7 Environmental Fate of
Expelled Viruses 3.4.8 Risk of Aerosol
Transmission 3.4.9 Aerosol Transmission
Scenarios
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3.4.1 Basics of Respiratory Droplets and Droplet
Nuclei
Virus Dose by Direct Inhalation of Droplets
(Droplet Transmission)
Virus Dose to Target Tissue (U)
Virus Dose by Indirect Inhalation of Droplet
Nuclei (Aerosol Transmission
Empirical evidence on influenza, scientific
information on virus-host interactions, and
exposure dynamics considerations, point to both
droplet exposure and inhalation of sub-droplet
size aerosols as mechanisms for transmission of
infection
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3.4.1 Basics of Respiratory Droplets and Droplet
Nuclei

• Droplets
• airborne liquid spheres (mists) greater than 5
  microns in diameter, that are discharged from the
  nose or mouth by sneezing, coughing, talking, or
  simply exhaling air.
• Droplets discharged by coughing or sneezing are
  comprised of mucous and saliva, both of which are
  comprised of water, carbohydrates, proteins and
  lipids.
• If the individual has a respiratory microbial
  infection, the droplets will also contain the
  microbe.

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3.4.1 Basics of Respiratory Droplets and Droplet
Nuclei

• In a person with a respiratory infection, many of
  the expelled droplets will contain the infecting
  microorganism

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3.4.1 Basics of Respiratory Droplets and Droplet
Nuclei

• From the instant it is expelled, the diameter of
  a droplet becomes progressively smaller due to
  evaporation of the droplet's water fraction as
  the droplet moves through the air.
• Once the water fraction has completely
  evaporated, the resulting particle is the droplet
  nucleus (plural nuclei).

Original Droplets
Evaporating Droplets
Droplet Nuclei
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3.4.1 Basics of Respiratory Droplets and Droplet
Nuclei

• "Droplet nuclei
• airborne particles that remain when the water
  fraction has evaporated away from a mucous
  droplet.
• If the droplet did not initially contain
  microorganisms, then the droplet nuclei will
  consist solely of agglomerated molecules of
  carbohydrates, proteins, lipids, and in the case
  of larger droplets, possibly respiratory tract
  and immune system cells and fragments.

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3.4.2 Virus Quantities in Droplets

• Based solely on the relative volumes of space
  they occupy, a 5 micron droplet (volume 65
  cubic microns) could, in theory, hold between
  approximately 70 thousand and 15 million
  influenza virus particles if completely filled
  with viruses.
• volume of a sphere (1?)(p)r3
• Empirical studies suggest that the actual
  influenza virus concentration in a droplet is
  significantly less than this theoretical capacity
  limit.
• The total quantity of viruses emitted by a sneeze
  or cough (containing thousands droplets) is more
  likely on the order of 50,000 (Nicas and Sun,
  2006).

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3.4.3 Importance of Proximity for Droplet
Transmission

• The closer you are to the source of the cough or
  sneeze, the more virus you can inhale, and the
  greater the risk of becoming infected.

Figure illustrating the idea that risk of virus
exposure is high when close to someone
discharging droplets by sneezing or coughing
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3.4.3 Importance of Proximity for Droplet
Transmission

• The concentration of virus in the air declines
  rapidly with distance.
• One reason for the concentration decline with
  distance is that some of the larger visible
  droplets fall-out onto surfaces, which reduces
  the total amount of virus in the immediate air
  zone.
• Another reason is that the cloud of remaining
  droplets disperse into a larger volume of air,
  which reduces the airborne virus concentrations
  (upcoming slides).
• For this reason, keeping a 2 meter distance from
  persons who may have influenza greatly reduces
  risk of exposure to droplets and airborne
  viruses.

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3.4.4 Aerodynamics of Droplets and Droplet Nuclei

• From everyday experience, we know that a sneeze
  or cough can produce mist droplets that are large
  enough to be seen with the naked eye.
• Such droplets are 40 microns in aerodynamic
  diameter and larger (about 400x larger by
  diameter than influenza A virus).
• Sneeze and cough droplets in this visible size
  range can only travel 1 to 2 meters before
  slowing and falling out onto a horizontal surface
  under the influence of gravity.

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3.4.4 Aerodynamics of Droplets and Droplet Nuclei

• Droplets under 5 microns in aerodynamic diameter,
  and droplet nuclei produced by sneezing, coughing
  or talking, typically travel a distance of 1 to 2
  meters, come to a stop in mid-air due to fluid
  air resistance.
• These droplets are so small that the downward
  force of gravity is countered by the fluid
  resistance of the air, and as a result these
  droplets tend to remain suspended in air, and
  move with ambient air currents.

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3.4.4 Aerodynamics of Droplets and Droplet Nuclei

• If droplets under 5 microns are expelled close to
  a surface (e.g. wall, desktop, tabletop, or a
  persons clothing or skin), some will hit and
  stick to the surface, and will no longer be
  airborne.
• Droplets under 5 microns and droplet nuclei that
  dont impact onto surfaces remain airborne, and
  are only removed from the air by filtration,
  inhalation, or electrostatic attraction to
  surfaces.

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3.4.5 Inhalation Exposure in the 1 to 2 Meter
Discharge Zone

• The concentration of droplets and droplet nuclei
  in air will be highest in the pathway 1 to 2
  meters from the point of discharge.
• Beyond this zone the droplets and droplet nuclei
  lose momentum, and disperse with air currents.
• As a result, the virus concentration in the air
  outside the 2 meter zone is expected to be
  significantly lower than inside the 2 meter zone.
  
• For this reason, it is believed that the risk of
  exposure is greatest in the zero to 2 meter zone
  around the infected individual, and this is the
  basis for many authorities recommending use of
  personal protective equipment for droplet
  protection in the 1 meter area around an infected
  individual.

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3.4.6 Effect of Distance from Source on Virus
Concentration

• One can get a sense of how virus concentrations
  diminish with distance by a simple geometric
  calculation.
• Imagine a sneeze or cough being a burst of air
  that is discharged in the shape of a cone having
  a 45 degree angle

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3.4.6 Effect of Distance from Source on Virus
Concentration

• The air volume of a cone is calculated by the
  following formula
• V 1/3 (p r2 h)
• Where
• V volume
• h height of the cone (or in the case of our
  example, distance from point of discharge)
• d diameter of the base of the cone
• r d/2
• p the pi constant, 3.14

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3.4.6 Effect of Distance from Source on Virus
Concentration

• If, for sake of this example, we imagine that
  there are 100,000 viruses discharged in a sneeze
  or cough, and those are more-or-less uniformly
  spaced apart as they are spread in the conical
  shape, the initial concentration in the zone 0.3
  meters from the point of discharge (i.e. the
  nasal openings or mouth) would be about 263
  viruses per cubic centimeter of air (assuming
  uniform distribution in the volume of space).

h 0.3 meters a 45o d 0.22 meters r 0.11
meters V 0.004 m3 100,000 viruses / V 263
viruses/cm3
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3.4.6 Effect of Distance from Source on Virus
Concentration

• However, as the distance from the point of
  discharge increases, the volume occupied by the
  cone increases, and the 100,000 viruses are
  dispersed through over a larger volume, with the
  result being that by 1 meter the concentration
  has dropped to about 1.6 per cubic centimetre
  (about 0.6 of the starting concentration),

h 1 meters a 45o d 0.77 meters r 0.385
meters V 0.62 m3 100,000 viruses / V 1.6
viruses/cm3
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3.4.6 Effect of Distance from Source on Virus
Concentration

• By 3 meters (volume of cone 4.2 cubic metres),
  the concentration is about 0.24 virus per cubic
  centimeter, which is only 0.09 of the starting
  concentration.

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3.4.6 Effect of Distance from Source on Virus
Concentration
Figures illustrating the idea that airborne virus
concentration decreases rapidly with distance
from the mouth and nose
70
3.4.7 Environmental Fate of Expelled Viruses

• If the original droplet contained microorganisms,
  then the dried agglomeration forming the droplet
  nuclei will also include the microorganisms.
• Many influenza viruses survive and remain
  infectious for some time after droplet
  desiccation, and droplet nuclei can harbour
  infectious viruses for minutes, hours or even
  days after formation.

71
3.4.8 Risk of Aerosol Transmission

• While airborne virus concentrations are certainly
  highest in the pathway of a sneeze or cough,
  there is some evidence that certain respiratory
  viral infections can be contracted from exposure
  to airborne droplet nuclei that have traveled
  through the air a considerable distance from the
  presumed point of droplet discharge (Moser et al.
  1979).
• In addition, it is well established that persons
  occupying the same homes or buildings as persons
  infected with tuberculosis (which, while not a
  virus, is similarly discharged from the airway in
  droplets, which in theory should behave similarly
  to any other droplet) can become infected without
  having close contact with the infected persons
  (Riley 1974).
• There is not sufficient information to know if
  this is a significant risk for influenza, but
  clinical experience suggests it is not.

72
3.4.8 Risk of Aerosol Transmission

• If it is possible for infection to occur from
  exposure to viruses at a great distance from the
  source of discharge, there is likely to be some
  degree of dose-response proportionality, and
  therefore, risks of infection are expected to be
  higher when all of the following conditions are
  present
• There is a high occupant density of infected
  persons in the space.
• The infected persons are in the infectious stage.
• There is inadequate ventilation, which allows a
  progressive rise in the airborne concentrations
  of virus.
• the type of virus survives for a sufficient
  amount of time while airborne to allow a build-up
  of the airborne concentration of infectious
  virus.
• Non-infected persons are present in the
  environment long enough to inhale an infectious
  dose (Council of Canadian Academies CCA,
  2007).

73
3.4.8 Risk of Aerosol Transmission
74
3.4.9 Aerosol Transmission Scenarios

• Transmission by droplet nuclei at large distances
  from the source is referred to as aerosol
  transmission
• In an indoor environment, it is possible for
  concentrations of airborne influenza virus to
  reach infectious concentrations in two scenarios
• Scenario 1 Several Cases in Small Air Spaces
• Scenario 2 Many Cases in Poorly Ventilated
  Larger Air Spaces

75
3.4.9.1 Scenario 1 Several Cases in Small Air
Spaces

• Scenario
• One or more active influenza cases (exhaling
  virus by coughing or sneezing).
• Small air space (e.g. inside a vehicle, cabin of
  an airplane, living quarters inside a ship).
• Under these conditions it is possible for virus
  concentrations to progressively rise, and to
  reach infectious concentrations.
• What constitutes an infectious concentration is
  unknown. It is simply known that infections can
  occur in this scenario.

76
Illustration of Increasing Virus Air
Concentrations Due to Coughing
77
3.4.9.2 Many Cases in Larger Air Spaces

• Scenario
• Many active influenza cases (exhaling virus by
  coughing or sneezing)
• Larger air space that does not have a high rate
  of stale air removal and fresh air introduction
• Under these conditions it is possible for virus
  concentrations to progressively rise, and reach
  infectious concentrations.

78
3.5 Contact Transmission
3.5.1 Defining the Concept 3.5.2 Ways to
Contaminate Skin 3.5.3 Ways to Contaminate
Objects (Fomites) 3.5.4 Ways to Contaminate
Surfaces 3.5.5 Virus Attachment to Substrates and
Adhered Particles 3.5.6 Necessary Conditions for
Infection by Self-Inoculation 3.5.7 Virus
Survival Outside the Body 3.5.8 Virus
Accumulation on High-Contact Surfaces 3.5.9 Risk
Factors for Infection by Contact Transmission
79
3.5.1 Defining the Concept

• Contact transmission refers to any scenario
  where influenza virus is introduced into the
  mouth or nose by
• a contaminated hand or finger being inserted into
  the mouth or nose
• a contaminated object being inserted into the
  mouth or
• mouth-to-mouth contact between infected and
  non-infected persons

80
3.5.2 Ways to Contaminate Skin

• The skin of the hands or fingers can become
  contaminated with influenza virus in the
  following ways
• being sneezed, coughed, or spat upon by an
  infected person (droplet contact)
• making contact with contaminated body fluids such
  as saliva, nasal secretions, feces, and blood
• touching an object that has been contaminated by
  an infected person (by sneezing, coughing, or
  contact with contaminated hands)
• hand-to-hand contact with a contaminated hand.

81
3.5.3 Ways to Contaminate Objects (Fomites)

• An object can become contaminated with influenza
  virus in the following ways
• being sneezed, coughed or spat upon by an
  infected person
• being touched by a person with a contaminated
  hand and
• contact with contaminated body fluids, such as
  saliva, nasal secretions, feces, blood.

82
3.5.4 Ways to Contaminate Surfaces

• Viruses can be deposited onto surfaces in many
  ways, including
• impact and fallout of droplets produced by human
  (or animal) sneezing, coughing, talking, spitting
  or laboured breathing
• adherence of airborne droplet nuclei to surfaces
  by electrostatic attraction
• transfer from virus-contaminated skin (e.g.
  hands)
• transfer from another contaminated inanimate
  object (e.g. a tissue, stapler, article of
  clothing, utensil, hand-held radio, etc.)
• deposition by insect or animal vectors (some
  viruses but not influenza are transmitted by a
  multi-step process involving several hosts and
  vectors)

83
3.5.5 Virus Attachment to Substrates or Adhered
Particles

• When viruses are deposited onto surfaces they can
  adhere directly onto the surface material itself
  (e.g. adhered onto the varnish on a piece of
  wooden furniture), or can be attached to a
  particle of dust or dirt that is on that surface
  (which in turn may allow re-entrainment to the
  airstream)

84
3.5.6 Necessary Conditions for Infection by
Self-Inoculation

• Viruses are expelled from humans and animals to
  the environment on a constant basis, and for this
  reason viruses can be ubiquitous on surfaces.
• However, the fact of a surface having viral
  contamination does not necessarily mean that
  contact with the surface will cause an infection.
  
• Several conditions must be satisfied for
  infection to occur via self-inoculation with
  viruses on surfaces (next slide).

85
3.5.6 Necessary Conditions for Infection by
Self-Inoculation

1. After deposition onto the surface, the virus must
   be capable of surviving and remaining infectious
   long enough to allow uptake onto a persons skin
   by contact with that surface.
2. There must be a sufficient quantity of the virus
   present on the surface (whether skin, an object,
   or surface), and taken-up onto a persons skin,
   to constitute an infectious dose
3. The virus must survive and remain infectious on
   the persons skin long enough for
   self-inoculation to occur
4. There must be self-inoculation of an infectious
   dose onto mucous membranes and
5. The inoculated viruses must migrate from the
   mucous membranes to the target tissues of the
   respiratory tract and give rise to infection.

86
3.5.6 Necessary Conditions for Infection by
Self-Inoculation

• There is no published information available on
  the significance of these mechanisms in
  transmission of influenza.
• Theoretical considerations lead us to conclude
  that these mechanisms are unlikely to be
  significant in relation to direct inhalation of
  discharged droplets and droplet nuclei.

87
3.5.7 Influenza Virus Survival Outside the Body

• Research into the survival of viruses on surfaces
  has shown that most viruses exhibit different
  rates of survival on different surfaces.
• There is some research that suggests that
  influenza viruses can survive on non-porous
  surfaces for 24-48 hours, 8-12 hours on cloth,
  paper and tissues and for about 5 minutes on
  hands (Ministry of Health and Long-Term Care,
  2007).
• Most of this research is laboratory-based, and
  survival findings appear to be influenced by the
  test conditions, surfaces, recovery methods, and
  environmental conditions.
• One finding that is consistent across the
  research literature is that once a virus has left
  its host environment (e.g. the respiratory
  tract), the percentage of the original population
  of dispersed viral particles that survives and
  remains infectious decreases with time.

88
3.5.7 Influenza Virus Survival Outside the Body
89
3.5.7 Influenza Virus Survival Outside the Body

• Since there are time limits to virus survival on
  surfaces, one can conclude that contact with
  any particular surface or object need not be
  viewed as a potential risk if there is certainty
  that no other person has come into contact with
  it for a couple of days.
• Contact includes tactile contact, oral
  contact, clothing to surface contact, and any
  actions that could deposit droplets onto the
  surface, such as sneezing, coughing, spitting or
  talking.
• This has practical value in making determination
  of the need to disinfect shared equipment,
  communications devices, tools, desks, etc.
  between users.

90
Example Using Time as a Disinfectant

• If
• a walkie-talkie is used by person A on Friday
  and placed into a charging stand at the end of
  the work day at 430 p.m. on Friday, and
• nobody is anywhere near the walkie-talkie over
  the weekend,
• then any virus deposited onto the walkie-talkie
  on Friday by
• person A should be non-viable by 800 a.m.
  Monday
• morning, and there would be no need for person
  B to
• disinfect the walkie-talkie before using it on
  the job on
• Monday.

91
3.5.8 Virus Accumulation on High-Contact Surfaces

• Since many types of viruses can survive on
  surfaces for long periods of time in indoor
  environments, the concentration of surface
  contamination, and the total surface area
  contaminated, can both increase over time if
  infected persons are present and in contact with
  such surfaces.
• In certain high occupancy, high activity
  environments such as schools and daycares, the
  concentration of persons and the nature of
  activities can theoretically result in a
  progressive rise in virus concentrations on
  surfaces, and a large portion of the occupants
  experiencing surface contact viral exposure in a
  short time period.
• There is no evidence that this is a signficant
  transmission mechanism for influenza.

92
3.5.8 Virus Accumulation on High-Contact Surfaces
93
3.5.9 Risk Factors for Infection by Contact
Transmission

• Based on the foregoing, the risk of surface
  exposure to viral material in the workplace is a
  function of,
• occupant density in the work space, and
• the degree to which there is shared contact with
  surfaces and objects.
• In work spaces where there is primarily single
  user contact with surfaces and objects (e.g.
  private offices, private vehicles), there should
  be less risk of exposure to second party
  viruses.
• As noted previously, contact transmission is
  unlikely to be a significant mode of transmission
  for influenza.

94
3. Review - Modes of Exposure and Transmission
3.1 When is a Person Infectious? 3.2 Routes of
Virus Exposure and Dosing 3.3 Relative
Significance of Exposure Routes 3.4 Transmission
by Inhalation 3.5 Contact Transmission
95
2.5.10 The Provenance of Hand Hygiene
Recommendations
Arrows point to the sources cited as authority
for hand hygiene recommendations
Weak evidence that HHgt11x / day reduced SARS
transmission amongst HC workers
Lau JT, Tsui H, Lau M, Yang X. SARS
transmission, risk factors, and prevention in
Hong Kong. Emerg Infect Dis 200410587-92
WHO Non-pharmaceutical interventions for
pandemic influenza, national and community
measures (2006)
Morens DM, Rash VM. Lessons from a nursing home
outbreak of influenza A. Infect Control Hosp
Epidemiol. 199516275-280.
Spatial pattern of outbreak suggested contact
transmission, but could equally be droplet or
aerosol
Canadian Pandemic Influenza Plan, Annex F (June
2006)
10 studies evaluating effect of hand hygiene on
upper respiratory infections in children and
students some studies show no effect, others
show possible weak effect
Ontario Health Plan for an Influenza Pandemic,
July 2007, Chapter 6
The School Hand Hygiene Studies
CDC Interim Pre-pandemic Planning Guidance
Community Strategy for Pandemic Influenza
Mitigation in the United States, Feb 2007
96
2.5.11 Lines of Research Bearing on Hand Hygiene
Effectiveness
Arrow thickness signifies the relative amount of
research
Upper respiratory infections in children
Research on correlation between hand hygiene
practices and rates of infection
Absences / Illness Rates (all causes)
Gastro-intestinal infections
Wound infections
Research on virus survival on surfaces
Upper respiratory infections in children
Case reports suggesting contact transmission of
disease
Hand hygiene recommendations for influenza
Gastro-intestinal infections
Eye infection
(But not very good evidence for influenza)
Upper respiratory infections in children
Hand hygiene intervention studies
Gastro-intestinal infections
Wound infections
97
Questions?
98
Module 4 Occupational Risk Assessment
99
4. Occupational Risk Assessment
Learning Objectives 4.1 Relationship of
Exposure Risk Assessment to Routes of
Transmission 4.2 Developing a Relative Risk
Mathematical Model 4.3 Use of the Model for Field
Data Collection 4.4 Results of the Model in the
Government of Ontario Study 4.5 Using the Model
to Assess Impacts of Controls Upgrades 4.6 Making
Exposure Controls Recommendations Using
Professional Judgment Questions
100
Learning Objectives

1. Understand the relationship between routes of
   transmission and methods for exposure risk
   assessment.
2. Understand the logical basis for the REA
   influenza virus exposure model, and the
   limitations of this model.
3. Understand the rationale for a professional
   judgment-based approach to influenza exposure
   control in a work setting.

101
4.1 Relationship of Exposure Risk Assessment to
Routes of Transmission
Virus Dose by Inhalation (P)
Risk of Infection
Virus Dose to Target Tissue (U)
Virus Dose by Inoculation to Mucosa (R)
Empirical evidence on influenza, scientific
information on virus-host interactions, and
exposure dynamics considerations, point to
inhalation being the most significant mechanism
for transmission of infection
Virus Dose by Ingestion (R)
U ? aP bR cR' Where a gtgtgt b gtgtgt c
102
4.1 Relationship of Exposure Risk Assessment to
Routes of Transmission
Simplification 1
Virus Dose by Inhalation (P)
Risk of Infection
Virus Dose to Target Tissue (U)
Virus Dose by Inoculation to Mucosa (R)
Empirical evidence on influenza, scientific
information on virus-host interactions, and
exposure dynamics considerations, point to
inhalation being the most significant mechanism
for transmission of infection
U ? aP bR Where a gtgtgt b
103
4.1 Relationship of Exposure Risk Assessment to
Routes of Transmission
Simplification 2
Risk of Infection
Virus Dose to Target Tissue (U)
Virus Dose by Inhalation (P)
Empirical evidence on influenza, scientific
information on virus-host interactions, and
exposure dynamics considerations, point to
inhalation being the most significant mechanism
for transmission of infection
U ? aP
104
4.2 Developing a Relative Risk Mathematical Model

• REA developed a mathematical model to (1)
  organize the relationships of several variables
  to the risk of getting influenza in a work
  setting, (2) identify key variables for field
  data collection, and (3) assess the effects of
  different transmission control measures on risk
  outcomes.
• Our influenza virus exposure model essentially
  treats a virus as a simple particle, and assumes
  that risk of contracting influenza is
  proportionate to the virus dose, which is
  proportionate to virus exposure.
• Ultimately, the purpose of the risk model was to
  calculate a risk value for influenza virus
  exposure and infection for the various work
  settings examined in order to estimate potential
  risk differences for those settings.

105
REA Model Used for Ranking Influenza Exposure
Risks
REA WORKPLACE VIRUS EXPOSURE RANKING MODEL REA WORKPLACE VIRUS EXPOSURE RANKING MODEL REA WORKPLACE VIRUS EXPOSURE RANKING MODEL
VARIABLE SYMBOL FORMULA
VIRUS EXPOSURE BY ALL ROUTES U U ? ( aP bR ), where a gtgt b
Quantity of Virus Inhaled P P ? ( P1 P2 )
Quantity of Virus Inhaled via Droplet Exposure (Droplet Transmission) P1 P1 ? ( M x H x G )
Quantity of Virus Inhaled from Ambient Air (Aerosol Transmission) P2 P2 ? ( K x Y )
Quantity of Virus Inoculated to Mucosa (Contact Transmission) R R ? (S1 x N1) (S2 x N2)

106
REA Model Used for Ranking Influenza Exposure
Risks
REA WORKPLACE VIRUS EXPOSURE RANKING MODEL DROPLET TRANSMISSION REA WORKPLACE VIRUS EXPOSURE RANKING MODEL DROPLET TRANSMISSION REA WORKPLACE VIRUS EXPOSURE RANKING MODEL DROPLET TRANSMISSION
VARIABLE SYMBOL FORMULA
Quantity of Virus Inhaled via Droplet Exposure (Droplet Transmission) P1 P1 ? M x H x G
Frequency of Subjects Exposure to Persons Who Are Sneezing or Coughing M M 1, Low 5, Medium 10, High
Extent of Sneezing or Coughing by Sick Persons H H 1, Low 5, Medium 10, High
Average Proximity to Points of Discharge G G 1, Low Density Area 5, Medium Density Area 10, High Density Area

107
REA Model Used for Ranking Influenza Exposure
Risks
REA WORKPLACE VIRUS EXPOSURE RANKING MODEL AEROSOL TRANSMISSION REA WORKPLACE VIRUS EXPOSURE RANKING MODEL AEROSOL TRANSMISSION REA WORKPLACE VIRUS EXPOSURE RANKING MODEL AEROSOL TRANSMISSION
VARIABLE SYMBOL FORMULA
Quantity of Virus Inhaled from Ambient Air (Aerosol Transmission) P2 P2 ? (K x Y)
Duration of Exposure K K 1, Brief 8, Full Shift
Viral Loading of the Ambient Air Y Y ? (D1 x B) (T x X)
Average No. of Infected Persons Present D1 D1 ? D2 x E
Average No. of Persons in the Area D2 D2 Estimated or Measured
Average Risk that Person is Infected E E 1, Regular Population 5, At Risk Population
Fraction of Time Expelling Viruses B B Estimate or Use a Constant
Extent of Surface Dust Re-entrainment T T 1, Low 5, Medium 10, High
Viral Loading of Surface Dusts X X ? (D1 x B) I2
Average Quantity of Virus on Objects I2 I2 ? M x H
Frequency of Subjects Exposure to Persons Who Are Sneezing or Coughing M M 1, Low 5, Medium 10, High
Extent of Sneezing or Coughing by Sick Persons H H 1, Low 5, Medium 10, High

108
REA Model Used for Ranking Influenza Exposure
Risks
REA WORKPLACE VIRUS EXPOSURE RANKING MODEL CONTACT TRANSMISSION REA WORKPLACE VIRUS EXPOSURE RANKING MODEL CONTACT TRANSMISSION REA WORKPLACE VIRUS EXPOSURE RANKING MODEL CONTACT TRANSMISSION
VARIABLE SYMBOL FORMULA
Quantity of Virus Inoculated to Mucosa R R ? ( S1 x N1 ) ( S2 x N2 )
Quantity of Virus on Hands S1 S1 ? ( I1 x C1 ) ( I2 x C2 )
Frequency of Hand to Mucosa Contact N1 N1 1, Low 5, Medium 10, High
Quantity of Virus on Objects Touched to Mucosa S2 S2 5, rare sharing of objects 10, frequent sharing of objects
Frequency of Object to Mucosa Contact N2 N2 1, Low 5, Medium 10, High

109
REA Model Used for Ranking Influenza Exposure
Risks
REA WORKPLACE VIRUS EXPOSURE RANKING MODEL CONTACT TRANSMISSION REA WORKPLACE VIRUS EXPOSURE RANKING MODEL CONTACT TRANSMISSION REA WORKPLACE VIRUS EXPOSURE RANKING MODEL CONTACT TRANSMISSION
VARIABLE SYMBOL FORMULA
Quantity of Virus on Hands S1 S1 ? ( I1 x C1 ) ( I2 x C2 )
Average Quantity of Virus on Hands of Others I1 I1 ? (C1 x (I2 x C2) (M x H)
Average Frequency of Hand to Hand Contact C1 C1 1, Low 5, Medium 10, High
Average Quantity of Virus on Objects I2 I2 ? M x H
Average Frequency of Hand to Object Contact C2 C2 1, Low 5, Medium 10, High
Frequency of Subjects Exposure to Persons Who Are Sneezing or Coughing M M 1, Low 5, Medium 10, High
Extent of Sneezing or Coughing by Sick Persons H H 1, Low 5, Medium 10, High

110
Variables Used in REA Model to Assess Controls
Effectiveness
REA WORKPLACE VIRUS EXPOSURE CONTROL MODEL REA WORKPLACE VIRUS EXPOSURE CONTROL MODEL REA WORKPLACE VIRUS EXPOSURE CONTROL MODEL
TRANSMISSION PREVENTION VARIABLES SYMBOL FORMULA
Ventilation Factors Reducing Virus in Air W1 W1 ? ( j x l x k )
Dilution Rate j Ordinal, professional judgment
Air Sterilization Efficiency k 0 None, 1 Perfect Efficiency
Filtration Efficiency l 0 None, 1 Perfect Efficiency
Factors Limiting Virus-to-Mucosa Contact W2 W2 ? c x d
Use of Hand and Eye Protection c Ordinal, assumed available
Proximity to Hand Hygiene d Ordinal, field observations
Frequency of Environmental Disinfection W3 Modeled values
Respirator Usage W4 Modeled values
Social Distancing W5 Modeled values
Dedicated Use Factor W6 Modeled values
Restriction of Activity W7 Modeled values
111
4.3 Use of the Model for Field Data Collection

• Field data collected on a range of variables
  deemed relevant to potential for exposure to
  influenza virus via inhalation or inoculation
• types of occupancies within the work setting
• types of activities carried out in each occupancy
• types of objects and surfaces in each occupancy
• ventilation systems
• employee and public occupant densities (i.e.
  range and typical numbers of persons present in
  the space)
• hours of occupancy
• whether the work setting visited was comparable
  to other work settings of the same category, or
  whether there were notable differences
• existing infection prevention and control
  procedures

112
4.4 Occupancy as the Unit for Risk Estimation

• In the course of our field visits, it became
  evident that for most personnel, the potential
  for exposure would largely be related to the
  occupancies and settings they worked in. This
  was deemed to be the case for the following
  reasons
• potential for exposure to influenza virus is
  proportionate to
• the potential for coming into contact with
  persons infected with influenza and / or
• the likelihood of being in a work space with high
  airborne virus concentrations and
• the potential for (1)(a) and/or (1)(b) is largely
  determined by job duties and
• there is a high correlation between job duties
  and the type of occupancy and setting in which
  the work is performed.
• As a result, we chose to use occupancies within
  work settings as the unit for measuring relative
  risk of exposure to influenza virus.

113
4.5 Use of Field Data to Calculate Relative Risks
in Different Settings

• Upon completion of field visits, we prepared an
  inventory of occupancies within each of the work
  settings visited, in order to identify the
  complete range of same across the entire set of
  locations visited. 
• Using our risk model, we then calculated a risk
  value for influenza virus exposure and infection
  for each of the occupancies, using the following
  equation U ? aP bR .
• These relative risk ratings were prepared in
  response to the clients desire for estimates of
  potential risk differences for the various work
  settings examined.
• In addition, it was anticipated that risk ratings
  may enable the prescription of progressively more
  stringent infection prevention and control
  measures as a function of the magnitude of
  potential influenza exposure risk.

114
4.5 Use of Field Data to Calculate Relative Risks
in Different Settings

• For the purpose of these calculations, all
  control variables were assumed to have a value of
  1 (in other words, risk modeling assumed the
  absence of infection prevention measures).
• In order to calculate relative risks using our
  model, it was necessary to assign ordinal values
  (ratings) to many of the variables shown in
  Table 1, to reflect the judged magnitude of the
  value of each variable for each occupancy.
• These are simply numeric values intended to
  indicate qualitative magnitudes for the judged
  value of the variable in a particular occupancy.
  For example, 1 Low, 5 Medium, 10 High. The
  assignment of specific numeric values for low,
  medium and high is arbitrary (for example, 1, 2
  and 3, or 1, 10 and 100 could equally be used).
• The choice of any particular set of numeric
  values affects the overall product values
  calculated by the model (and hence the apparent
  degree of differences between ranked scores), but
  has no effect on the relative rankings of those
  values when applied across a range of
  occupancies.

115
4.6 Results of the Model
116
Calculated Relative Risks for Contracting
Influenza in Different Typical Government Work
Settings
WORK SETTING RISK SCORE
Patient Care Room 5968
Patient Exam Room 5968
First Aid/Sick Room 5968
Public Information Centre 2360
Restaurant 2260
Classroom 2240
Public Waiting Room 2045
Swimming Pool 2045
Public Cafeteria 2035
Commercial Sales Desk 2015
Library 2015
Residential Common Area 1365
Residential Living Room 1365
Residential Entertainment Rm 1365
Gymnasium 1365
Gatehouse/Roadway Booth 655
Residential Dining Room 653
WORK SETTING RISK SCORE
Service Counter 561
Reception Desk 561
Shipping Receiving Areas 525
Offices Open Concept 407
Servery in an Office Area 407
Warehouse 381
Office Corridor /Hallway 381
Mechanical Room 381
Outdoor Work 381
Tunnels Between Build