Exposure generated by cough released droplets in the indoor environment a comparison among four ventilation systems

1
Chapter 1: Introduction


1.1 Background and Motivation


People in the developed countries spend more than 80 %of their time indoors (Robinson and
Nelson, 1995). Many of the indoor pollutants are suspended particles in air. The human
respiratory tract handles 10 l/min of air and 3000 cycles of inhalation – exhalation per day.
Suspended droplets can penetrate into human respiratory system carried by inhaled air potentially
causing acute or chronic health effects. Viruses have been identified as the most common cause
of infectious diseases acquired within indoor environments, in particular those causing respiratory
and gastrointestinal infection.

Particles present in the indoor environments can originate from outdoors penetrating through
building envelope drawn in by ventilation systems or can be emitted from the indoor source.
Indoor source can be cooking, building materials, consumer products or occupants. Respiratory
secretions from an infected person can be aerosolized through expiratory activities (breathing,
talking, coughing, sneezing and vomiting) and dispersed through indoor environment. Each of
these expiratory activities produces different size distribution of droplets, amount of infectious
agents and initial velocities. Coughing and sneezing produces much higher number of droplets
than breathing and talking although the latter two are much more frequent. These expiratory
pathogen laden respiratory droplets are believed to be responsible for the epidemic spread of
several respiratory tract infections. Epidemiologic studies implicate the droplet nucleus
mechanism in the transmission of tuberculosis, measles, influenza, smallpox, chickenpox and
SARS (Li et al., 2007). Vomiting can spread 10
7
virus particles per ml of vomit (Baker et al.,
2001). Spread of viral infections through atomized vomit is a significant route of infection in

2
diseases which causes frequent vomiting such as Norwalk Like Virus (NLV), also vomiting by
SARS infected person on the corridor of Metropol Hotel in Hong Kong is believed to have caused
a series of infections. Infected individuals can release 10
12
virus particles per gram of feces
(Baker et al., 2001) or up to 10
5
CFU/ml of bacteria.

Motion of infectious droplets in a ventilated room depends on ventilation air pattern, droplet size,
density, number, pollution source location, etc. Among these parameters the ventilation air
pattern is the most important parameter influencing airborne infectious disease transmission in the
indoor environment (Morawska 2006). Droplets in the air are subjected to Brownian forces,
gravity, turbulent diffusion, inertial forces, RH, thermal gradients, electrical forces,
electromagnetic radiation. Depending on the droplet size above mentioned forces have different
magnitude of influences on droplets dispersion.

Several technologies have been used to reduce the amount of infectious droplets in the indoor air.
Ultraviolet radiation (UV) in the wavelength of 254nm is germicidal and has been used for air
disinfection within indoor environment (Riley, 1972). Efficiency of UV lighting depends on:
intensity of UV radiation, the species of organisms, and RH. Wavelength of UV radiation is
irritating to skin and eyes and it cannot be permitted to impinge on people in doses above the
limit recommended by National Institute of Occupational Safety and Health (NIOSH, 1972). Due
to this restriction UV lighting is placed between occupants head and ceiling. This influences the
efficiency of UV lights and technology have not been applied outside medical facilities where
risk of outbreak of airborne infectious diseases is high (Bloch et al., 1985; Gustafson et al., 1982,
Catanzaro, 1982). Room air filtration is another technology applied for air purification and
disinfection. The effectiveness of in-room filtration depends on: single-pass filter efficiency, air

flow rate through the filter and on other features of the indoor environment particularly the
relative positions of the source and receptor and the indoor air flow patterns generated by air

3
conditioning and ventilation system (Miller-Leiden et. al., 1996). Although this process have
shown high efficiency to droplets with diameter less than 3μm when filter is placed close to the
occupant, while for droplets with diameter of less than 1μm the efficiency is low when HEPA
filter is not used. Using HEPA filters increase energy consumption necessary for fan operation to
overcome high pressure drop of filter. This technology also has not found any application outside
medical facilities. Two main reasons preventing usage of UV lighting and room air filtration in
office building are the low efficiency and high amount of energy they require for operation. In the
indoor environment where risk of infectious disease spread is not high usage of these
technologies is not economically justified.

Ventilation air patterns can be used to prevent airborne infectious disease spread. Several
ventilations systems are currently used in office environments. These systems generate very
different air patterns in the indoor environment. Mixing Ventilation (MV) is the most commonly
used ventilation system, which supplies the air with high momentum to induce mixing in the
room. Displacement Ventilation (DV) is ventilation system which supplies air with low velocity
from the supply diffuser mounted on the floor. Air supplied in this way moves on the floor until it
reaches heat sources which entrain this air into their boundary layers and due to thermal
buoyancy displaces upwards. Under-floor Ventilation system (UF) supplies the air from diffusers
mounted on the floor. Air is supplied with high momentum to ensure sufficient upward
momentum while air mixes inside indoor environment. Personalized Ventilation (PV) is designed
to deliver conditioned (cool and clean) outdoor air to the breathing zone of the occupant. The
amount of inhaled personalized air has been shown to depend on the design of the Air Terminal
Device (ATD), its positioning in regard to the occupant, the supply flow rate of the personalized
airflow, as well as the difference between the room air and the Personalized Ventilation airflow
temperature, size of target area, etc. (Faulkner et al., 1999; Melikov et al., 2002). The optimal
performance for the most of the ATD has not exceeded 50–60% of clean air in each inhalation

4
although it was pointed out by Bolashikov et al., (2003) that PV ATD can be designed to
achieve100 % breathing zone air delivery effectiveness. When Personalized Ventilation is
compared to total volume mechanical ventilation potential for energy savings has been
demonstrated (Melikov, 2004; Sekhar et al., 2005). This technology is suitable for commercial
office buildings and has potential to reduce risk of airborne infectious disease transmission in the
indoor environment while reducing overall energy consumption of air conditioning and
mechanical ventilation system

Multidisciplinary literature review by Li et al. (2007) concluded that: ―there is no evidence/data
to support specification and quantification of the minimum ventilation requirements in schools,
offices, and other non-hospital environments in relation to the spread of airborne infectious
diseases. The knowledge gap is obvious.‖ Quantification of ventilation rates will influence
ability of a particular ventilation system to control (reduce) airborne infectious disease
transmission by reducing exposure of the healthy occupant(s) to the infectious agents released by
expiratory activities of the infected occupant(s). Ability of the ventilation system to control
(reduce) exposure to airborne infectious agents is protective ability of a ventilation system. Since
several ventilation systems are currently used protective ability of each one should be evaluated
and compared to other systems. In the present literature there is no methodology proposed for
evaluation and comparison of the protective ability of different ventilation systems. Several
experimental studies (Qian et al., 2006; Qian et al. 2008; Nielsen et al 2009; Nielsen et al., 2010)
or Computational Fluid Dynamic studies (CFD) (Li et al., 2005; Xie et al., 2009, Zhu et al., 2007;
etc.) compared protective ability of two or more systems for a specific scenario, but no
comprehensive comparison of protective ability of commonly used ventilation systems has been
performed so far. This identifies the necessity for development of the methodology for evaluation
of protective ability of ventilation system. This methodology should be then applied to compare

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different ventilation systems for the same supply flow rate as well as variation of protective

ability of any of the system with variation of the supply flow rate. Occupant density needs to be
included into the analysis because disease propagation (number of new cases) will depend on the
number of occupants in the indoor environment under consideration. It is important to establish
relationship between number of occupants in the indoor environment, ACH generated by the
particular ventilation system and prevention of the airborne transmitted disease propagation.

Previous studies examined ventilation efficiency of different system and provided knowledge
regarding mechanisms (flow field generated) of air delivery to the breathing zone deployed by
different systems. These studies were performed under the steady state using tracer gas (SF6) to
quantify different ventilation indices. Cough release is episodic event which generates unsteady
state in the environment. When cough is released saliva droplets (with or without infectious
agents) move through the environment due to high initial momentum (cough velocity can be up to
22 m/s, while average cough velocity is 10 m/s) before sufficient momentum decay occurs and
ventilation streamlines reestablish and start to carry them. There were no previous comprehensive
experimental studies about protection mechanisms (flow field generated) engaged by different
ventilation systems when cough release occurs in the indoor environment. Since cough droplets
have high initial momentum, distance between infected occupant (infector) and exposed healthy
occupant are important parameters that impact on the flow field generated by cough that will
cause exposure. No previous studies have been conducted to establish influence of the infector-
susceptible distance on the exposure generated by infectors cough release and flow fields
generated to provide protection of the susceptible occupant using the multiphase flow
approach to simulate cough droplets. Study performed by Melikov et al., (2009)
investigated influence of distance between infected coughing person and exposed person
using tracer gas to simulate cough release.

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In order to conduct a study of the cough released droplets exposure changes due to changes of the
ventilation system experiments need to be conducted in the indoor environment capable of
changing ventilation modes. Cough release need to be simulated as a multiphase flow because
decay of the cough velocity is much more intensive when only gas phase (commonly simulated

using SF6) is used. Liquid droplets are able to preserve higher momentum and reach larger
distances from the source for the same boundary conditions compared to gas phase. Simulated
saliva or real human saliva needs to be used to properly simulate nonvolatile saliva
characteristics. Cough release droplets can evaporate only to droplet nuclei size which than cause
exposure, but when, for example, water is used to simulate cough droplets evaporation is
complete and there is no nonvolatile residue. When gas (SF6) is used to simulate cough
evaporation is neglected. To assess the risk of airborne infectious disease transmission, exposure
of the occupants to cough released droplets need to be measured. This might be achieved by using
a Breathing Thermal Manikin (BTM) to simulate convective boundary layer generated around
heated body and inhalation – exhalation flow in the breathing zone. Size distribution of cough
droplet and droplet nuclei need to be measured in the breathing zone of the BTM to estimate the
risk of infection because different droplet sizes have different deposition characteristics in
different parts of the respiratory track. Some of the previous experimental studies (Chao and Wan
2006; Wan and Chao 2007; Qian et al.; 2006; Cermak and Melikov 2007; Qian et al., 2008; Chao
et al. 2008; Nielsen 2009) were conducted with some of the above mentioned equipment, but no
study so far used all these equipment to study changes of the exposure to cough released droplets
with the application of different ventilation systems.

Epidemiological investigation of airborne infectious disease transmissions in the indoor
environment (e.g. Riley et al., 1978; Catanzaro 1982; Nardell et al., 1991; Nicas 2000) is usually
faced with several uncertainties. Occupants (infector and all susceptible occupants) move in the
indoor environment and change distances, heights (sitting or standing) and position (facing each

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other; back to back, or any other possibility in between) in respect to each other. Changes of these
parameters distance, height and position will cause changes of the susceptible occupants‘
exposure, but these changes are generally unknown. When a particular air delivery system is used
(e.g. MV) type of supply diffusers (perforated, jet and multi-nozzle, conical diffusers, swirl and
rectangular 4-way), size of return grilles, positions and number of supply diffusers and return
grilles relative to each other will influence air patterns and turbulence levels generated in the

indoor environment. When cough is released these air patterns interact with multiphase cough
flow and influence dispersion of potentially infectious cough droplets. Different interactions
among multiphase cough flow and generated air patterns will cause different levels of exposure of
susceptible occupants. Although generated air patterns under steady state environment can be
known, due to changes of the distance, height and position of infector and susceptible occupants‘
exposures of the occupants are generally unknown. Another important parameter influencing
exposure is distribution of cough releases (frequency) throughout exposure time, which is
generally also unknown. In order to simplify investigation and overcome these uncertainties
assumption of perfectly mixed environment and steady release of potentially infectious expiratory
droplets are commonly used. These assumptions can lead to large risk underestimation (Nicas,
1996). Wells-Riley approach (Riley et al., 1978) or modifications of the original equation
incorporating other effects (Nazaroff et al., 1998; Rudnick and Milton 2003; Fisk et al, 2005;
Noakes 2006; Noakes and Sleigh 2009) was used in several studies to calculate risk of airborne
infectious disease transmission. Although some of the studies (Noakes and Sleigh 2009) divided
indoor environment into zones, each of the zones was treated as perfectly mixed while source was
treated as steady constant release which is simplification of the episodic cough release occuing in
the real environment. Wells-Riley equation needs to be augmented to cater for heterogeneity and
unsteadiness in the indoor environment in order to be applied for evaluation of risk of airborne
infection for different air delivery systems and various supplied flow rates.


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1.2 Research Objectives

The aim of this research is to evaluate ability of different ventilation systems to provide
protection to the susceptible occupant form the airborne infectious disease transmission due to
cough released infectious agents. The objectives are as follows:

1. Develop a methodology to evaluate the efficiency of reduction in concentration of cough
released droplets in the breathing zone of the occupant and apply this methodology to

evaluate protective ability of four ventilation systems at various air supply flow rates.

2. Evaluate the validity of perfectly mixed assumption in the breathing zone commonly
used in estimating the probability of getting infected for different ventilation systems at
different air flow rates.

3. Augment the Wells-Riley equation for unsteady heterogenic environment and
demonstrating its applicability to different ventilation systems and infectious loads.

4. Examine how the protective performance of different ventilation systems vary with the
height of cough release (sitting and standing position of the infector) and distance
between infected and susceptible occupant.

5. Examine flow field characteristics of the interactive two-phase flows between cough
released droplets and room air flow generated with different ventilation systems.

1.3 Scope of Work


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This study is in the field of ventilation but results from this study have application on medical
problem of control of airborne infection disease transmission. The scope of work and the
structure of discussion in each chapter are described briefly as follows:

 Literature review. This study is not independent of, but based on, previous research on
motion of expiratory droplets in indoor environment. Chapter 2 provides useful
information on experimental design used to study the dispersion of expiratory droplets
and methods adopted to evaluate risk of airborne infectious disease transmission.

 Research methodology. The work comprised a series of three related studies: (i)

development and application of methodology for evaluation of overall protective ability
of different ventilation systems and reduction of assumption necessary to evaluate
probability of getting infected using Wells-Riley approach; (ii) evaluation of distance
between infector and susceptible on the protection from cough released infectious
droplets achieved with different ventilation systems; (iii) investigation of flow field
characteristics of the resulting flow field between cough released droplets and room air
flow generated influencing reduction droplet concentration in the breathing zone. The
research methodology is described for the series of experiments, which includes
experimental design (facility and instrument) and data analysis (data and statistical
analysis). The discussion of research methodology is presented in Chapter 3.

 Chapter 4 describes the overall influence of ventilation system on control of
airborne infectious disease transmission. A evaluation methodology is proposed for the
calculation of overall averaged probability of getting infected for a susceptible occupant.
The method is adopted to two different risk assessment models for the indoor
environment supplied with a particular ventilation system. These results were than

10
compared to calculations conducted with the perfectly-mixed assumption, and Basic
Reproductive number variations are shown for different levels of occupancy.

 Chapter 5 describes the influence of distance between infector and susceptible on
the protection from cough released infectious droplets achieved with ventilation.
The influence of protective performance of different ventilation systems on height (sitting
and standing position of the infector) and distance between infected and susceptible
occupant was studied.

 Chapter 6 (and Appendix 1 and 2) documents the potential exposure and flow field
characteristics generated by direct cough at different infector - susceptible distances
in the indoor environment supplied with various ventilation systems. The interaction

between cough and the flow fields generated by different ventilation systems and their
consequent impact on exposure from the direct cough at several infector – susceptible
distances were studied.

 Conclusion and Recommendation. The objectives are reviewed and a summary of
significant findings is presented. These include: (i) the contributions of the new
methodology for evaluation of ability of ventilation system to control airborne infection
disease spread; (ii) the validity of the proposed augmentation of Wells-Riley equation;
(iii) evaluation of protective performance of different ventilation systems (overall and
infector-susceptible distance dependant); and (iv) flow field characteristics responsible
for protection of susceptible occupant. Lastly, some suggestions for further research and
the development of ventilation system for airborne infection transmission control are
given in Chapter 7.


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Chapter 2: Literature Review

2.1 Airborne Infectious disease transmission
2.1.1 Definitions
The modes of infectious disease transmission vary by type of organism, and some infectious
agents may be transmitted by more than one route: some are transmitted primarily by direct or
indirect contact and others by the droplet routes or airborne routes (CDC, 2003).

Definition adopted by CDC (2003): ―Droplet transmission is, technically, a form of contact
transmission, and some infectious agents transmitted by the droplet route also may be transmitted
by the direct and indirect contact routes. However, in contrast to contact transmission, respiratory
droplets carrying infectious pathogens transmit infection when they travel directly from the
respiratory tract of the infectious individual to susceptible mucosal surfaces of the recipient,
generally over short distances. The maximum distance for droplet transmission is currently

unresolved, but historically, the area of defined risk has been a distance of <3 feet around the
patient and is based on epidemiologic and simulated studies of selected infections (Feigin, 1982;
Dick, 1987).‖

Definition adopted by CDC (2003): ―Airborne transmission occurs by dissemination of either
airborne droplet nuclei or small particles in the respirable size range containing infectious agents
that remain infective over time and distance. Microorganisms carried in this manner may be
dispersed over long distances by air currents and may be inhaled by susceptible individuals who
have not had face-to-face contact with (or been in the same room with) the infectious individual
(Coronado et al., 1993; Bloch et al., 1985; LeClair et al., 1980; Riley, 1959).‖


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Li et al. (2007) in a review co-authored by large group of engineers, microbiologists and
epidemiologists on the role of ventilation on airborne transmission of infectious agents adopted
this definition: ―airborne transmission refers to the passage of microorganisms from a source to a
person through aerosols, resulting in infection of the person with or without consequent disease.
Aerosols are a suspension of solid or liquid particles in a gas, with particle size from 0.001 to
over 100μm (Hinds, 1982). Infectious aerosols contain pathogens. A droplet nucleus is the
airborne residue of a potentially infectious (micro-organism bearing) aerosol from which most of
the liquid has evaporated (Wells, 1934).‖

Droplet size is another variable which has been discussed. Droplets traditionally have been
defined as being >5μm in size. Particles arising from desiccation of suspended droplets, know as
droplet nuclei, have been associated with airborne transmission and defined as <5μm in size
(Duguid, 1946). ―Large droplets‖ were first defined as droplets larger than 100μm in diameter by
Wells (1934). Wells found that under normal air conditions, droplets smaller than 100μm in
diameter (―small droplets‖) would totally dry out before falling to the ground 2 m away. Xie et al.
(2007) conducted a study revisiting droplet size originally referred by Wells and proposed the
following size definitions: ―large-droplet‖ diameter >60μm, ―small droplet‖ diameter <60μm and

‗droplet nuclei‘ diameter <10μm.

2.1.2 Infectious agents transmissible by aerosols
Droplet nuclei can remain suspended in air for long periods and can travel considerable distances,
so close contact is not always necessary for transmission to occur. Examples of infections
transmitted in this manner include pulmonary TB, measles and chicken pox (Beggs, 2003).
Respiratory viruses such as influenza and respiratory syncytial virus are mainly spread by droplet
nuclei and droplet transmission (Ayliffe et al., 1999). Although respiratory viruses are transmitted
through the air, other non-respiratory viral infections, such as chicken pox and measles (Riley et

13
al., 1978) are also spread by the airborne route. The airborne route also contributes to the spread
of viral gastro-enteritis (Gellert and Glass, 1994).

There are only three respiratory diseases, i.e. measles, chicken pox (varicella) and tuberculosis
that are widely recognized as being primarily spread by airborne transmission, but many others,
such as those due to the influenza virus and the respiratory syncytial virus, are also probably
spread via airborne transmission (Yassi and Bryce, 2004). Tang et al. (2006) in review on factors
affecting aerosol transmission of infection points out that pathogens, such as parvovirus B19,
enteroviruses and the organisms of atypical pneumonias, Chlamydophila pneumoniae, Coxiella
burnetti and Legionella pneumophila, have the potential to be transmitted via aerosols as their life
cycle involves replication at some point in the respiratory tract.

One set of infection control guidelines for healthcare settings suggested that only tuberculosis
(Mycobacterium Tuberculosis,TB), measles (rubeola virus) and chickenpox (varicella zoster
virus, VZV) should be considered as ‗true‘ airborne infectious diseases (CDC, 2003). It is likely
that other infectious agents may also behave as ‗airborne‘, given a favorable environment, e.g.
whooping cough (Bordetella pertussis), influenza virus, adenovirus, rhinovirus, Mycoplasma
pneumoniae, SARS coronavirus (SARS-CoV), group A streptococcus and Neisseria meningitides
(Tang et al., 2006).

In multidisciplinary review of the 40 studies by Li et al. (2007), the conclusion was that within
the contemporary limitations of the conclusive studies chosen, there is strong and sufficient
evidence to demonstrate the association between ventilation and the control of airflow directions
in buildings and the transmission and spread of infectious diseases such as measles, TB,
chickenpox, anthrax, influenza, smallpox, and SARS.


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Guideline for Isolation Precautions: Preventing Transmission of Infectious Agents in Healthcare
Settings 2007: ―Infectious agents to which this applies include Mycobacterium tuberculosis
(Riley et al., 1959; Beck-Sague et al., 1992; CDC, 1994; Haley et. al., 1989), rubeola virus
(measles) (Bloch et al., 1985) and varicella-zoster virus (chickenpox) (LeClair et al., 1980). In
addition, published data suggests the possibility that variola virus (smallpox) may be transmitted
over long distances through the air under unusual circumstances and airborne infection isolation
rooms (AIIRs) are recommended for this agent as well; however, droplet and contact routes are
the more frequent routes of transmission for smallpox (Wehrle et al., 1970; Gelfand and Posch,
1971; Fenner et al.,1988).‖

2.1.3 Bioaerosol infectivity and virulence
The infectious disease process in an animal host is a function of microorganism concentration
(infective dose) and virulence (disease promoting factors) that enable an agent to overcome the
normal physical and immunologic defenses of the host (Cole and Cook, 1998). The infectious
dose of a pathogen is the number of organisms required to cause infection in susceptible host.

Data from research performed on biological warfare agents suggest that both bacteria and viruses
can produce disease with as few as 1-100 organisms (e.g. brucellosis 10-100, Q fever 1-10,
tularaemia 10-50, smallpox 10-100, viral haemorrhagic fevers 1-10 organisms) (Franz et al.,
1997). M. tuberculosis may need only a single organism to cause disease, and as many as 3000
organisms can be produced by a cough or talking for 5 min, with sneezing producing many more

(Fitzgerald and Hass, 2005). For many common agents, the infectious dose almost certainly
varies between individual pathogens and their hosts, e.g. immuno-compromised hosts may not
only be more susceptible to infection with a lower infectious dose, but may also be a more
infectious source, as the pathogen is poorly controlled by the defective immune system. This may
allow higher pathogen loads to be disseminated into the surrounding environment in some cases,

15
possibly leading to super-spreading events, such as described in some SARS outbreaks (Wong et
al., 2004; Yu et al., 2005; Li et al., 2005; Yu et al., 2004; Li et al., 2005).

Measles is a highly contagious viral disease that is spread by the airborne route. The infective
dose is small, and as few as 4 doses per minute from an infected person can initiate an epidemic
(Riley, 1980). Additionally, rubella (German measles) and varicella (chickenpox) viruses can be
readily spread by aerosols in indoor air. During major epidemics, influenza hospitalizations for
persons at high risk may increase two to five times (CDC, 1992) placing health care workers at
increased risk for infection. Small infective doses are thought to be responsible because of the
rapidity with which the disease spreads throughout a population. The natural airborne
transmission of respiratory infection with the coxsackie A virus type 21 was investigated and
transmission of infection was demonstrated (Couch, 1981).

Inherent in the infection process initiated by the inhalation of infectious droplet nuclei is the area
of deposition within the respiratory tract. Such deposition is influenced by hygroscopicity, which
causes an increase in the size of inhaled aerosols through moisture take up as they move within
the airways. Knight (1993) estimates that a 1.5 μm hygroscopic particle a common size in coughs
and sneezes increases to 2.0μm in diameter when passing through the nose and to 4.0μm in the
saturated air of the nasopharynx and the lung. He further theorizes that the effect of
hygroscopicity and the resultant particle size change increase retention in the tertiary bronchioles
and alveolar ducts, an effect that may be significant for viral aerosols, which are highly infectious
for that part of the lung.

2.1.4 Bioaerosol viability
When pathogenic microorganisms leave their host and are aerosolized, they are potentially
injured during the generation process. Additionally, once airborne they are outside of their natural

16
habitat and, depending on a variety of environmental factors, are increasingly subject to loss of
viability with time. Viability can be defined as the capability of a microorganism to reproduce.
Even if a microorganism remains alive, if it cannot reproduce it can be considered nonviable
because it has lost the ability to reestablish a population within a defined microenvironment.
Factors influencing the survival of bioaerosols include their suspending medium, temperature,
relative humidity, oxygen sensitivity, and exposure to UV or electromagnetic radiation.

M. tuberculosis is a hardy organism with a thick cell wall, and can survive for long periods in the
environment (Fitzgerald and Hass, 2005). Data on human corona-virus (hCV) 229E from Ijaz et
al. (1985) showed that, when airborne, this virus had a survival half-life of about 3 h at an RH of
80%, 67 h at an RH of 50% and 27 h at an RH of 30%, at 20
o
C, suggesting that high RH above
80% is most detrimental to survival of this coronavirus. More recently, it has been shown that
SARS-CoV can remain infectious in respiratory specimens for more than seven days at room
temperature (Lai et al., 2005). Similarities with other viruses of nosocomial importance, i.e. other
RNA, lipid enveloped, respiratory viruses such as influenza, suggest that such organisms can
survive for long enough in aerosols to cause disease, especially when associated with biological
fluids such as mucus, faeces and blood.

The studies by Meselson et al. (1994) and Yu et al. (2004) are striking, as they showed how the
virus-laden aerosols could spread a few kilometers in a city or between buildings that are 60 m
apart, due to wind flows.

2.1.5 Bioaerosol size

Infectious bioaerosol particles may exist as (1) single bacterial cells or spores, fungal spores, or
viruses; (2) aggregates of several cells, spores, or viruses; or (3) biologic material carried by
other, nonbiologic particles (Navalainen et al., 1993). Microorganisms span wide size ranges. In

17
general, infectious microorganisms range from 0.3 to 10μm for bacterial cells and spores, 2.0 to
5.0μm for fungal spores, and 0.02 to 0.30μm for viruses. Specific pathogen sizes include 0.3 to
0.6 x 1 to 4μm for M tuberculosis; 0.3 to 0.90 x 2.0 to 20μm for L pneumophila, (Brenner et al.,
1984) and 0.09 to 0.12μm for influenza virus (Murhpy and Kingsburg, 1991).

2.2 Dispersion of the contaminants released inside the indoor environment supplied with
total volume systems

2.2.1 Dispersion of exhaled contaminants in indoor environment supplied with total volume
systems
In a study of dispersal of exhaled air in displacement ventilated rooms (Bjørn and Nielsen, 2002)
numerical simulation show that the simulated personal exposure is very sensitive to variations in
the heat output of both the exposed person and the exhaling person, and to the cross-sectional
exhalation area and pulmonary ventilation of the exhaling person. The exhalation does not
necessarily follow the boundary layer flow close to the body, but is able to ‗‗break free‘‘ and
penetrate the breathing zone of other persons if persons are at distance up to 0.4 m.
Computational Fluid Dynamics (CFD) simulation by Gao and Niu (2007) findings performed for
transient flow and experiment for steady state is in qualitative agreement these. Air exhaled
horizontally through the mouth results in much larger exposure than air exhaled through the nose.
Experiments show that air exhaled through the mouth can be ‗‗locked‘‘ in a thermally stratified
layer, where concentrations can be several times higher than the return concentration. If the
vertical temperature gradient is larger than approximately 0.4-0.5
0
C/m, this layer can settle in
breathing zone height, but the boundary layer flow around the heated body can still offers some

protection in this situation. It should be noted that this phenomenon was only observed in cases
where exhalation was conducted through the mouth. With exhalation through the nose, the
exhaled air was observed to follow the boundary layer flow and the thermal plume to the upper

18
part of the room, resulting in the typical two-zone concentration distribution. Results gained
through CFD simulation by Gao and Niu (2005) are in agreement with these experimental results.
These results indicate that stratification of the exhalation flow is not an acute problem in most
normal situations.

The potentially infectious droplets exhaled from the patients with pulmonary tract infection can
cause cross infection especially in hospitals. Performance of wall mounted and ceiling mounted
downward mixing ventilation and displacement ventilation system in hospital ward was examined
by Qian et al. (2006) using a personal exposure index (defined as the ratio between pollutant
concentration in the breathing zone for the reference ventilation system (usually mixing
ventilation) and ventilation system under investigation under steady state conditions). Qian et al.
(2006) established that Mixing Ventilation and downward ventilation systems have personal
exposure index of 1 while Displacement Ventilation (DV) depending on the relative positions
between the source and target show variations of personal exposure index. When DV is used, it is
possible to obtain a very high ventilation index for some source-target position, but it cannot be
achieved in all situations (Nielsen 2009). Qian et al. (2008) conducted the study of exhaled
pollutant dispersion in hospital ward with downward ventilation and found that due to the
interaction of the upward thermal plume and the downward supply airstream, which caused
strong mixing, the supply air was incapable of pushing down the source patient‘s exhaled
contaminants, and produced a downward unidirectional flow. It was pointed out that results in the
study by Qian et al. (2008) are only qualitative and they only represent the trends since droplets
are simulated by tracer gas, physics of large droplets was fully neglected as well as deposition,
evaporation and coagulation of small and large droplets.

2.2.2 Dispersion of particles released inside the indoor environment supplied with total

volume systems

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Gao and Niu (2007) conducted a CFD study of particle dispersion inside the room with Mixing
Ventilation (MV), DV and Underfloor system (UF). They concluded that particle concentration is
almost uniform except at the points close to the ceiling where there is clean supply airflow. The
larger the particles are, the lower the mean indoor concentrations. For particles smaller than 5μm,
human exposure is nearly equal to the outlet concentration. However for larger particles, human
exposure is much lesser than the exhaust concentration. For DV human exposure is greatly lower
than that with MV for 0.1, 1.0, 2.5, and 5μm particles, but larger for 10 and 20μm particles. With
UF the vertical stratification for 0.1, 1.0, and 2.5μm particle also appears and human exposure is
lower than that with MV, but slightly higher than that with DV. But owing to the weak carry-up
effect, particles larger than 5μm are unable to be entrained to the upper level (Gao and Niu,
2007). With particle sources located within an internal heat source, desirable vertical
concentration stratification appears in DV and UF for particles up to 10μm. The advantageous
principle of DV and UF that there is a less polluted occupied zone for non-passive gaseous
pollutants is also applicable to particles whose diameters are less than a certain value which
depends on the strength of the buoyancy force (Gao and Niu, 2007).

2.3 Personalized ventilation
2.3.1 General overview
Personalized ventilation (PV) is designed to provide clean and cool air close to each occupant
(Melikov, 2004). Various ATD designs have been developed and studied previously. Two small
nozzles placed at the back corners of a desk generating two symmetrical jets or two linear
diffusers placed at the front desk edge generating two jets, one toward the occupant‘s body and
the second vertically, directed slightly away from the occupant (Sodec & Craig, 1990; Arens et
al., 1991; Bauman et al., 1993; Faulkner et al., 1993, 1999, 2002; Tsuzuki et al., 1999; Cho et al.,
2001; Melikov et al., 2002, 2003; Cermak & Melikov, 2003, 2004). ATD with a rectangular or
circular opening mounted on a movable arm-duct which allows for changes of the distance

20
between the ATD and the person as well as the direction of the personalized flow (Melikov et al.,
2002; Bolashikov et al., 2003), a flat ATD mounted on the top of a PC monitor allowing for
change of personalized flow direction in a vertical plane (Melikov et al., 2002), a small nozzle
integrated with the flexible support of a commercially available headphone supplying air very
close to the mouth and the nose (Bolashikov et al., 2003), or combinations of some of these ATDs
(Kaczmarczyk et al., 2004) have been studied. Several other designs, such as a round nozzle
attached to the chest blowing air against the face (Zuo et al., 2002), a displacement ATD placed
below the desk (Loomans, 1999; Izuhara et al., 2002), a ventilation tower system (Hiwatashi et
al., 2000), a partition integrated fan-coil unit (McCarthy et al., 1993; Jeong & Kim, 1999; Chiang
et al., 2002; Levy, 2002), Desktop Personalized Ventilation Air terminal Devices (Pantelic et al.,
2009; Tham and Pantelic 2010) and textile terminals (Nielsen et al., 2007) have all been tested.

Energy saving potential of PV has been studied by Sekhar et al. (2003). The results indicate that
the use of a secondary PV system in conjunction with a primary air-conditioning system not only
enhances thermal comfort and IAQ acceptability but can reduce energy consumption by 15-30%.

2.3.2 Dispersion of contaminants in the indoor environment supplied with personalized
ventilation
Non-uniformity in velocity and temperature field and differences in pollution generated in spaces
will depend on the airflow interactions as well as the location of pollution sources. This results in
considerable variation in occupants‘ exposures (Melikov, 2004). Physical measurements have
shown that a significant decrease of contaminant concentration in inhaled air with PV in
comparison with Total Volume Systems (TVS) has been achieved but only at high supply flow
rates of outdoor air (Faulkner et al., 1993, 1999, 2002; Melikov et al., 2002, 2003; Zuo et al.,
2002; Cermak & Melikov, 2003, 2004; Cermak et al., 2004; Bolashikov et al., 2003). The airflow
interaction, i.e., whether the personalized flow is transverse to, assisting or opposing the transient

21
flow of exhalation, the buoyancy driven boundary flow and the ventilation flow are of major

importance for minimizing mixing in spaces. With careful consideration of these factors affecting
airflow patterns, pathogen laden droplets can be removed faster from the indoor air either
exhausting them faster or increasing the deposition rate. Available knowledge suggests that in
rooms with mixing ventilation the use of PV will always protect the occupants from airborne
transmission of infectious agents and will be superior to mixing ventilation alone (Melikov et al.,
2003). In rooms with displacement ventilation, however, PV promotes mixing of the exhaled air
with room air (Melikov et al., 2003; Cermak et al., 2004). A similar effect may occur in rooms
with underfloor air distribution (UF) (Cermak & Melikov, 2003, 2004; Cermak et al., 2004).
Cermak and Melikov (2007) reported that PV in conjunction with underfloor ventilation is more
effective in protecting occupants from airborne pathogens released by exhalation than when an
underfloor ventilation strategy is applied alone. Owing to the non-uniform environment created
by PV, undesirable transport of exhaled pathogens can occur when an infected individual uses PV
while the other occupants in the space do not use PV for protection. However, these studies
examined particular air terminal device (ATD) designs which create a particular flow field.
Nielsen et al. (2007) reported that the protective ability of Personalized Ventilation (textile
terminals) used in conjunction with vertical ventilation from ceiling-mounted terminals achieved
an increased efficiency of personal protection by factors of 5 up to 35.

2.3.3 Dispersion of particles in room with personalized ventilation
In the study conducted by Faulkner et al. (1993) room particle transport and the efficiency of
removing tobacco smoke particles in the room with ceiling supply mixing ventilation and
Personal Environmental Module was investigated. The study concluded that only the high supply
rate of outdoor air (40 l/s) directed to the breathing zone provided enhanced ventilation. When 10
l/s were used particle removal efficiencies were essentially equivalent to those of a conventional
ventilation system with thoroughly mixed indoor air.

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Distribution of particles with size of 0.5 – 10μm in a room ventilated by personalized ventilation
system and ceiling supply system was investigated by the means of CFD by Zhao and Guan

(2007). Particle source was assumed to be one person sitting indoor, with a generating rate of
5000 particles/s. Zhao and Guan (2007) concluded that for the particles smaller than 2μm, a
strong-enough personalized ventilation which can disperse the thermal plume is an effective
ventilation mode to remove particles. The personalized ventilation with less air supply volume
has no obvious advantage compared to ceiling supply. For the particles bigger than 7.5μm,
Personalized Ventilation may not be the best ventilation mode. It may have bigger particle
concentration in breathing zone than that of ceiling supply ventilation.


2.4 Expiratory droplet dispersion in indoor environment
2.4.1 Size distribution of expiratory droplets
Expiratory human activities such as: breathing, coughing, sneezing, or laughing result in droplet
generation by wind shear forces. The significance of each of these activities in the spread of
infection depends on a number of factors, including: (1) the number of droplets it produces, (2)
their size, (3) content of infectious agents, and (4) the frequency of its performance. The content
of infectious agent expelled by an infected person depends, among other factors, on the location
within the respiratory tract from where the droplets originate (Morawska, 2006).

Six studies that have reported size distribution of droplets emitted during coughing and sneezing
were reviewed. They are Duguid (1946), Louden and Roberts (1967), Papineni and Rosenthal
(1997), Chao et al. (2009), Morawska et al. (2009) and Xie et al. (2009). These studies used
different measurement apparatus to measure droplets of different size bins and may be clustered
into two groups: the first group that targeted the full range of released droplet size bins and the

23
second group that was focused on smaller droplet size bins, released by expiratory activity.
Among the first group, studies by Duguid (1946) and Chao et al. (2009) reported very similar size
distribution profile and mean droplet size while the results by Xie at al. (2009) show a
distribution shift into the larger droplet size range while maintaining similar shape of the
distribution profile. A full range of droplet size distribution is more realistic and this is adopted

for this study.

2.4.2 Dispersion of expiratory droplets generated by coughing
In the scenario studied using CFD by Gao and Niu (2006) where two occupants were facing each
other at a distance of 1.2 while one occupant sneezed 1 s was enough for it to reach the breathing
region of the exposed person. Gao and Niu (2007) concluded that horizontal airflow (sneezing) is
able to penetrate the protection of the boundary layer flow enclosing the exposed person and
neutralize the positive effect of DV created by drawing fresh air from the lower level.

Four CFD simulations were performed by Zhu et al. (2006), examining dispersion of droplets
particles with diameters of 30, 50, 100, 200, 300 and 500μm in the room with mixing ventilation
(air conditioning device was placed on the wall close to the ceiling). In the case when two persons
were sitting 1.54 m apart, simulation results for the droplets of 30μm suggest that after been
coughed out the motion of these droplets were affected by the room airflow. 30μm droplets tend
to remain airborne for prolonged periods and final point of deposition onto the surfaces inside the
room cannot be predicted. Some droplets of 50μm in diameter impacted on the face and neck of
the person opposite, while the remainder passed by and then dropped to the floor. For 100μm
droplets simulation results suggested that more than 80% impacted on the lower neck area of the
person opposite, while the remainder dropped to the floor soon after passing their body. Saliva
droplets of 200μm in diameter started to fall before reaching the person opposite, and nearly
100% of these droplets impacted on the chest of the person opposite. However, the saliva droplets

24
of 300μm or more in diameter did not descend significantly, and nearly 100% of them impacted
on the neck of the opposite person. Most saliva droplets of 500μm in diameter impacted on the
face of the opposite person with little or no observable gravitational effect.

Sun and Ji (2007) performed CFD simulation of the manikin coughing. Four different cases were
examined with mixing and displacement ventilation systems. The CFD results show that droplets
larger than 100μm generated by the sitting manikin body fall to the ground or onto the bed if the

coughing burst are in the horizontal direction. From a horizontal cough, expelled droplets of
80μm fall initially but dry up to nuclei while still some distance from the ground. Therefore,
droplets of 80μm can stay airborne and become the contaminant source from the lower part of the
ventilated room. Results from Sun and Ji (2007) also suggest that, droplets larger than 300μm fall
to the bed if the cough is directed upwards from a lying manikin. From an upward cough from a
manikin lying down, expelled droplets <100μm dry up to nuclei during their passage upwards and
are then dispersed around the room by the air flow. For droplets smaller than 50μm, because of
the fast evaporation time, their transport and dispersion show similar characteristics to those of
small passive particles. Sun and Ji, (2007) showed that the ventilation set-up has a great influence
on removing the cough-expelled droplets of different sizes. Mixing ventilation shows an almost
equal ability to remove 1μm droplets and the nuclei of droplets with original sizes of 100 and
80μm, respectively. However, displacement ventilation appears, in contrast, to remove 1μm
droplets and the nuclei from large droplets of 100μm and 80μm, differently. Small passive
droplets show two-zonal distribution in a room with displacement ventilation, and the ventilation
air removes them with very high efficiency. The nuclei of the large droplets, however, are subject
to gravitational forces and show a tendency to settle down in the displacement ventilation the air
flow of which has difficulty in carrying the nuclei upwards and removing them.


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Zhu et al. (2006) also studied motion of cough droplets from a lying person while ventilation inlet
was positioned on the wall near the ceiling above the head of the lying person. CFD Simulation
suggests that 30μm droplets were dispersed through out the room and deposit randomly to the
surfaces. The dispersion of droplets narrowed down with the increase in the droplet size from
50μm onward, and 300μm droplet dispersion was limited to the pillow area. 500μm droplets
impacted directly to the ceiling.

Sun et al. (2007) performed CFD study of droplet dispersion (50, 80 and 100μm) from a source at
the middle of test room with no initial velocity in the long room with air supply was positioned at
the wall close to the ceiling and exhaust was placed on the opposite wall close the floor. Results

show that for droplets less than 50μm, the dispersion feature is dominant due to their very short
evaporation time and small settling velocity, therefore evaporating droplets of these sizes
distribute in a similar manner as the neutral aerosol particles. For droplets as large as 100μm, the
settling feature is dominant due to the longer evaporation time and considerable large dropping
velocity, their distribution consequently behave like large depositing particles in a room scale.
For droplets between these two sizes, the distribution tends to be at the lower part of the room
than that of small neutral aerosol particles. Within this size range, a lower initial position of the
droplets in the room results in a higher deposition rate to the floor.

Zhao et al. (2005) examined transport of 1μm droplets generated by respiratory system from a
standing person in the room with mixing ventilation strategy using unsteady drift flux model.
Results from the study indicate that coughing may produce particles or droplets, which will
transport over a long distance. Coughing with outlet velocity of 20 m/s will cause the particles or
droplets to transport over a distance longer than 3m, and larger outlet velocity will have even
longer distance in shorter time. At the distance of 3 m from the person‘s mouth concentration of
1μm droplets were 25%.