CHAPTER 2
INFECTION CONTROL
2.1 INTRODUCTION
As noted in Chapter 1, a
unique aspect of hospital HVAC systems is their role in mitigating the spread
of airborne contaminants that could cause infections. Hospital-acquired
infections (HAIs, also referred to as nosocomial infections) have a significant
impact on patient care. Mortality rates from HAIs are significant and affect
the overall cost of health care delivery. In the United States, HAIs occur in
an estimated 4% to 5% of admitted patients; at an estimated annual cost
approaching $7 billion. It is generally agreed that 80 to 90% of HAIs are
transmitted by direct contact, with 10% to 20% resulting from airborne
transmission (representing 0.4% to 1% of admitted patients) (Memarzadeh 2011a).
Transmission of airborne
hazards is influenced by factors beyond the control of the engineer that
include movement of patients, undiagnosed patients, visitors, concentration of
patients, and patient susceptibility. The nature of infectious pathogens, the
modes of transmission, the causation of infections, and the relationships to
HVAC system design are complicated and not fully understood. HVAC systems can
affect the distribution patterns of airborne particles by diluting or
concentrating them, moving them into or out of the breathing zones of
susceptible persons, or by accelerating or decelerating the rate of growth of
airborne microbes. Improperly operated and maintained HVAC systems can even
become a reservoir for microorganisms.
Surgical site infections (SSIs)
are caused by a variety of sources, including deposition of particles either
directly on the patient or on the staff and equipment, which are then
transferred to the surgical site.
This chapter is intended to give a
brief overview of the ways HVAC systems can help mitigate airborne infections.
A more thorough discussion of this subject can be found in The Environment
of Care and Health Care-Associated Infections: An Engineering Perspective (Memarzadeh
2011a).
2.2 ROLE OF
HVAC SYSTEMS IN INFECTION CONTROL
Although the majority of nosocomial infections are caused by factors
such as poor hygiene practice, lack of hand cleaning, or surface-to-surface
contamination, there is some risk of infection spread via HVAC systems;
consequently, codes and guidelines covering design and operation of health care
HVAC systems have been developed. Some of the ways that potentially infectious
microorganisms can be spread in a health care environment include
- sneezes and coughs,
- inhalation,
- contact,
- deposition in surgical site or open wound,
- water mist, and insect bite.
HVAC systems can impact HAIs by affecting
- dilution (by ventilation),
- air quality (by filtration),
- exposure time (by air change and pressure differential),
- temperature,
- humidity,
- organism viability (by ultraviolet [UV] treatment), and
- airflow patterns.
The science of controlling infections caused by airborne microorganisms
is a complex mixture of engineering, particle physics, microbiology, and
medicine. The rates at which particles settle are a function of their size,
shape, density, and of course, air movement. Turbulence within a room increases
the residence time of larger particles in the air, hence the desire for laminar
airflows in operating rooms. Particles or aerosols below about 1μm in size are
virtually unaffected by gravity and stay in suspension because of Brownian
motion. Aerosols generated by coughs and sneezes generally affect other persons
only within 3 to 6 ft [0.9 to 1.8 m]. It is virtually impossible for HVAC
systems to exert control at this close exposure; therefore, transfer must be
controlled using personnel protection and/ or isolation.
The viability of microorganisms
embedded in water droplets (aerosols) is affected by temperature, humidity, and
air velocity. Memarzadeh (2011a) summarizes and provides an extensive
bibliography on this subject.
Although the number of airborne-spread
infections is a small percentage of the total number of HAI infections, the
number is significant enough to warrant care in the design of HVAC systems for
health care facilities. Therefore, many aspects of current HVAC design codes
and guidelines are motivated by a desire to reduce infections caused by
inhalation and deposition.
2.3 HOW THE HUMAN BODY IS AFFECTED BY AIRBORNE CONTAMINANTS
The normal human body has excellent protection
systems to prevent respiratory infections. There are several layers of
”filtration” starting with the mouth and windpipe (which have moist surfaces—a
mucous layer—to attract and collect particles before they enter the trachea)
and beyond that further collection within the bronchi (Figure 2-1). Infections
such as pneumonia affect the lungs, where they can thrive and enter the
bloodstream if they manage to penetrate the body’s defense systems. Pneumonia
is a deep-seated infection once it reaches the lungs and presents a serious
risk to anyone who encounters it, particularly patients who may be immune
suppressed and less able to resist its presence.
Airborne particles can affect the human body
and the health care environment in several ways. These particles can be toxic
chemicals, allergens, or infectious agents. Airborne contaminants may enter a
building from the outdoors or from processes that occur inside the building.
Harmful substances from the outdoors may enter the space through openings in
the building envelope or through outdoor air intakes. Outdoor air in an urban
environment can contain fumes from motor vehicles or emissions from
building-related equipment such as diesel generators. Facilities in rural areas
may be near pesticides and agricultural dust. Equally hazardous are emissions
from materials and processes inside a building. Exhalations of patients create
yet more sources of potentially infectious agents.
The building itself may produce
toxic or annoying fumes and allergens. Usually, contaminants from medical
processes are carefully
contained in laboratories or specialized procedure or storage rooms, but
it is important that the HVAC system maintain isolation of these substances
from the building occupants. Other potential sources of air quality problems
are off-gassing from materials inside the building and from activities such as
cleaning. Sensitive, vulnerable patients (as well as visitors and staff) may
suffer from exposure to such substances, and it is incumbent upon the HVAC
system to minimize their transmission and delivery (although controlling them
at the source is the preferable strategy, when feasible). People vary widely in
their sensitivities to various allergens, and allergic reactions can exacerbate
other medical conditions. Common airborne allergens include mold, fungal
spores, and tree and grass pollen.
A virulent infection in a hospital setting will find the body an easier
target than normal. People recovering from an operation are vulnerable, as
their immune system is preoccupied with repairing the body after surgery. A
hospital is also a place with more infectious agents than normal, because
building occupants are likely to have a variety of illnesses, some of which
relate to infectious diseases. These are a threat to all other patients, staff,
and visitors.
Infectious diseases (also known as transmissible diseases or
communicable diseases) comprise clinically evident illness resulting from the
infection, presence, and growth of pathogenic biological agents in an
individual host organism. Infectious pathogens include some viruses, bacteria,
fungi, protozoa, multicellular parasites, and aberrant proteins known as
prions.
The term infectivity describes the ability of an organism to enter,
survive, and multiply in the host, while the infectiousness of a disease
indicates the comparative ease with which the disease is transmitted to other
hosts. Transmission can occur in various ways including physical contact,
contaminated food, body fluids, objects, airborne inhalation, or through a
vector organism.
Infection occurs when all of the following elements are present: an
infectious agent, a source of the agent, a susceptible host to receive the
agent, and most critically, a way for the agent to be transmitted from the
source to the host. The interaction among these elements is known as the “chain
of infection,” or “disease transmission cycle,” terminology that emphasizes the
necessary linkages among all elements. Generally, the severity of the impact of
exposure to microorganisms is described by the following relationship:
Thus, the severity of the impact
on persons exposed is determined by the length of the exposure, virulence of
the microbes to which they are exposed, location of the exposure, and quantity
of microorganisms;
whereas, the impact is
mitigated or exacerbated depending upon the vitality of the exposed person.
Generally, the methods by which HVAC systems are intended to reduce HAIs are
all designed to address components of this simple equation.
The infectious agent is the microorganism that
can cause infection or disease. The ability to penetrate the respiratory system
relates to size of particle. Figure 2-2 indicates that, while particle sizes
around 20 μm are generally stopped at the nose, those of 0.6 to 2 μm can reach
the lower respiratory tract (alveoli).
A reservoir is the place where the agent
survives, grows, and/or multiplies. People, animals, plants, soil, air, water
and other solutions, instruments and items used in clinical procedures can
serve as reservoirs for potentially infectious microorganisms. There are two
sources of infectious agents in a hospital or health care setting:
- Endogenous source: the causative agent of the infection is present in the patient at the time of admission to the hospital but there are no signs of infection. The infection develops during the stay in the hospital as a result of the patient’s altered resistance or through introduction of microbes into normally sterile areas, such as insertion of an intravenous catheter into a vein, or from a surgical procedure.
- Exogenous source: infection occurs from introduction of microbes into or on the patient from an outside source. For example, the patient may acquire infectious agents from the hands of staff or from contaminated equipment and subsequently develop an infection.
The infectious agent can be transmitted via tiny droplet nuclei (< 5 μm)
containing microorganisms that remain suspended in the air and that can be
carried by air currents greater distances than large droplets (e.g., measles, M.
tuberculosis). The susceptible host then inhales these droplets. The
droplet nuclei may remain suspended in the air for long periods of time.
The route by which the infectious agent leaves the reservoir is called
the exit. An infectious agent can exit the reservoir through the mucous
membranes (e.g., eyes, nose, and mouth), and the respiratory tract (e.g.,
lungs), or through droplets that come from these sites.
The place of entry is the route by which the infectious agent moves into
the susceptible host. An airborne infectious agent can enter the susceptible
host through mucous membranes (e.g., eyes, nose, mouth) or the respiratory
tract (e.g., trachea, bronchi, lungs).
This is the route of many disease agents that cause respiratory
illnesses, such as the common cold, influenza, tuberculosis, measles, mumps,
rubella, pertussis, Haemophilus influenzae type b (Hib), and
pneumococcal disease (pneumonia). The respiratory tract is the most important
portal and the most difficult to control.
Airborne transmission via aerosols is important in some respiratory
diseases. Aerosols are particularly dangerous because their size (1 to 5 μm)
allows them to be drawn deep into the lungs and retained. Turbulent air may
also spread large particles from contaminated soil or from objects such as
clothing and floors.
In general, the following factors affect whether or not an infection
occurs in a particular situation:
- Aerosol and droplet transmission dynamics
- Nature of dust levels
- Health and condition of individual’s nasopharyngeal mucosal linings
- Population density
- Ventilation rate
- Air distribution pattern
- Humidity
- Temperature
- Susceptibility
- Length of exposure
- Number of infected people producing contaminated aerosols
- Infectious-particle settling rate
- Lipid or nonlipid viral envelope or microorganism cell wall
- Surrounding organic material
- UV radiation or antiviral chemical exposure
- Vitamin A and D levels
- Microorganism resistance to antibiotic or antiviral therapy
- Type and degree of invasive procedures
- Spatial considerations
- Contact with carrier
- Persistence of pathogens within hosts
- Immunoepidemiology
- Genetic factors
2.4 RISK MANAGEMENT APPROACH TO INFECTION CONTROL
Risk assessment and management provide a
systematic approach to discovery and mitigation of risks facing an organization
or facility. The goal is to help objectively state, document, and rank risks
and prepare a management plan for implementation. Risk management techniques
are used to identify appropriate countermeasures, options, or alternatives for
a known or anticipated situation.
The general principles that should be
considered for any risk assessment are
- identifying the risk,
- estimating the level of exposure,
- estimating the probability of risk occurrence,
- determining the value of the loss,
- ranking risks, and
- identifying vulnerabilities
The approach to infection control
and environmental control outlined in FGI (2010) considers the susceptibility
of patients versus the degree of environmental contamination. Such an infection
control risk assessment (ICRA) requires communication between clinical and
facility staff and includes both design and remediation issues to protect
patients and staff. ICRA is described in more detail in Chapter 11. Risk
assessment design strategies for infection prevention and control include
consideration of the patient population served, range and complexity of
services provided, and settings in which care is provided. Other variables
include status (e.g., infectious, susceptible, or both); the area under
consideration (e.g., isolation or protective); the type of filtration,
ventilation, and pressurization; and the operations and maintenance procedures
and management that are in place. Risk assessment design strategies for
environmental controls include the use of positive pressure environments (PPEs)
for the health care professional, the type of isolation necessary (e.g.,
protective or contaminant), and the ventilation standards applicable to the
type of facility being assessed (Kosar 2002).
2.5 SURGICAL SITE
INFECTIONS
as a result of the trend toward less invasive procedures. In complicated
cardiac, vascular, orthopedic, and prosthetic and transplant surgeries
requiring lengthy procedures, there is a higher risk of SSI. SSIs are
classified as incisional or organ/space. Incisional infections are further
divided into superficial (skin and subcutaneous tissue) and deep (deep soft
tissue, bone, muscle, and fascia) (Horan et al. 1992; Edwards et al. 2008).
Organ/space SSIs are associated with high morbidity and mortality.
The source of the SSI pathogen(s) is usually the patient’s skin, mucous
membranes, or bowel and rarely from another infected site in the body
(endogenous sources). Organisms associated with SSIs vary with the type of
procedure and the anatomic location of the operation. Exogenous sources of SSI
pathogens may include members of the surgical team (e.g., hands, nose, or other
body parts); contaminated surfaces in the operating room; the air; and
contaminated instruments, surgical gloves, or other items used in the surgery
as shown in Figure 2-3. Exogenous organisms are primarily aerobic staphylococci
or streptococci species.
Operating rooms (ORs) are one of
the most critical areas for infection control; this is where patients are
opened to the surrounding environment while in an immune-suppressed condition.
The patient is vulnerable to attack from any infectious agents that get into
the room and to the surgical site. Although clean conditions can be created by
2.6 PROTECTING
POPULATIONS
atients with severely compromised immune
systems, such as bone marrow transplant and HIV patients. The HVAC systems for
PPE rooms are designed to preclude airborne pathogens through the use of
high-quality filtration, high air exchange rates, anterooms, and
pressurization.
Airborne infection isolation (AII) rooms are
designed for patients diagnosed as having communicable contagious diseases.
These rooms are designed with anterooms, are under negative pressure, and have
a dedicated direct exhaust system. Optionally, the exhausts from these rooms
have HEPA filters to control the dispersion of airborne pathogens to the
surrounding environment. The emergency waiting room is an area of particular
risk because both immune-compromised and undiagnosed contagious patient
populations often coexist there. This type of space has high air exchange rates
and all air is exhausted directly outside. PPE, AII, and ED (emergency
department) waiting rooms are discussed in Chapter 8.
Procedurally, staff and patients
wear facemasks to mitigate potential airborne contamination—particularly
short-range droplet transmission from sneezing, talking, coughing, or just
breathing. Special procedures such as surgeries entail more sophisticated PPEs
to protect both staff and patient, including fully-ventilated suits for
procedures on high-risk contagious patients. The size of a particle affects its
ability to penetrate the respiratory system.
2.7 AIR CHANGE
RATE/ DILUTION
One method of reducing the time and/or number
of microbes to which a person is exposed is by increasing the dilution rate of
clean air into a space. This reduces the exposure time of microorganisms
generated within the room by objects, staff, or the patient. Table 2-1
indicates the length of time it takes for a room to be flushed with filtered
air, assuming perfect mixing.
As indicated, microbes can be
expected to be resident in a room for 14 minutes at 20 ach (air changes per
hour [ACH]) and 28 minutes at 10 ach. The general concept of dilution and
replacement with clean air is a fundamental driver of the ACH rates provided in
the ANSI/ ASHRAE/ASHE Standard 170 ventilation table; as shown in Table 2-2.
The entire table is shown in Chapter 3 of this manual.
These air change rates are also a
result of research on odor control and comfort (Klaus 2011). Studies in London
in the early 1900s documented lung volume and calculated outdoor air
ventilation rates to reduce odor. Other studies documented the volume and
velocity of airflows from sneezes and coughs and concluded that about 35 cfm
[16.5 L/s] per person of outdoor air was needed. However, Memarzadeh and Xu
(2012) questions current ACH standards.
2.8 NATURAL
VENTILATION
In the World Health Organization (WHO) interim guidelines (WHO 2007),
natural ventilation is considered an effective environmental measure to reduce
the risk of spread of infections in health care settings.
Recent research carried out in the
UK (Short and Al-Maiyah 2009) showed that as much as 70% by area of a hospital
can be successfully naturally ventilated, 40% using simple natural ventilation,
and 30% using advanced natural ventilation (controlled via air shafts and
ground
coupling), with as little as 7% of the hospital
area needing minimum efficiency reporting value (MERV) 14 or better filtered
air conditioning.
Where natural ventilation is to be
used in health care applications, reference to WHO and other sources is
recommended to ensure infection control is maintained. At this time, there are
few hospitals in North America using natural ventilation.
2.9 FILTRATION
The efficacy of air filters is determined
primarily by particle size, but can be affected by the relative electrical
charges of particles and filters. Bacteria typically are quite small, requiring
filters that remove particles below 1 μm in size. ANSI/ASHRAE Standard
52.2-2007 (ASHRAE 2007) specifies a test procedure for evaluating the
performance of air-cleaning devices as a function of particle size, resulting
in a minimum efficiency reporting value (MERV) for a given device. As shown in
Table 2-3, for example, MERV 14 filters remove 75% to 85% of particles in the
range of 0.3 to 1.0 μm. Table 2-4, from ANSI/ASHRAE/ASHE Standard 170-2008,
gives requirements for prefiltration and final filtration in different areas.
As indicated in Table 2-4, true
HEPA (MERV 17; 99.99%) filters are required only for protective environment
(PE) rooms; such filters are also required for pharmacies per U.S.
Pharmacopoeia General Chapter 797 (USP 2012). HEPA filters are often specified
for bone marrow and organ transplant patient rooms and orthopedic surgery. The
MERV rating system is described in detail in ASHRAE Standard 52.2-2007 (ASHRAE
2007). It is based on a test over a range of particle sizes. Standard
designations of overall percent filter efficiency are no longer commonly used.
2.10 HUMIDITY
Humidity affects the rate at which the body can release moisture into
the air—either by evaporation from the skin (sweating) or through breathing,
where it can reduce the antiseptic layer of mucous that lines the respiratory
system. The key criterion is relative humidity (RH), rather than absolute
humidity, because RH affects the rate of evaporation. Although the human body
is generally tolerant to a wide variation in RH (typically between 35% and 75%)
there can be serious concerns at the extremes of dryness (low RH) and dampness
(high RH). Either of these can occur at high or low temperatures. See Chapter 3
for a discussion of psychrometrics and Chapter 12 regarding human comfort.
Memarzadeh (2011b) states “there is no conclusive evidence that any single
factor, whether it be a specific temperature, RH or geographic location can be
universally applied to the wide variety of infectious viruses to reduce
airborne or contact transmission, but there is pervasive evidence in the
literature that the survival of viruses and other infectious agents depends
partly on levels of RH.”
Allergies and respiratory
illnesses are associated with high humidity and mold growth, particularly for
asthma and rhinitis. Asthma is a difficulty in breathing, often associated with
coughing. Rhinitis is an inflammation of the nose which causes similar effects.
Both asthma and rhinitis are now closely associated with an allergic response
to dust mites and their waste products.
Dust mites prosper in warm, damp conditions
where there is a supply of food in nutrients such as dead skin particles. The
insulating properties of soft finishes (e.g., carpets) can create a temperature
gradient with a corresponding increase in RH. Places where there are prolonged
high air humidities are likely to experience problems such as airborne fungi
and house dust mites, particularly if room relative humidity exceeds 70% for
long periods. Fungi generally grow on damp organic material but they do not
necessarily require either high air humidity or high air temperature for growth
if the substrate conditions are suitable. The growth rate is dependent upon the
nutrient, temperature, and humidity. Each mold has its special growth
characteristics. Once mold has started to grow, it is difficult to stop by
lowering the humidity, because one of the metabolic products of growth is
water, which then enables the mold to continue to grow in drier conditions.
For the purposes of air-conditioning system
design, a maximum room RH of 60% generally provides acceptable comfort
conditions for human occupancy and minimizes the risk of mold growth and dust
mites. Condensation should be avoided on surfaces within buildings that could
support microbial growth or be stained or otherwise damaged by moisture. This
may be achieved by ensuring that all surfaces are above the dew-point
temperature of the adjacent air; or that the dew point is above the surface
temperatures.
There is some evidence of a
correlation between low room humidity and symptoms associated with dryness and
irritation of the mucosa. It has been suggested that low room moisture content
increases evaporation from the mucosa and can produce microfissures in the
upper respiratory tract that may act as sites for infection. The reduction in
mucous flow inhibits the dilution and rejection of dust, microorganisms, and
irritant chemicals. This is a particular problem for wearers of contact lenses.
2.11
ULTRAVIOLET RADIATION
Ultraviolet (UV) radiation can be effective in
reducing the virulence of microorganisms and, therefore, in attempting to
reduce infection rates. The efficacy of ultraviolet systems is determined by
the following equation, which is a variant of the previous equation dealing
with the effect of microorganisms on a person. The effectiveness of the
radiation is determined by the UV resistance of the microorganism, the UV
radiation’s intensity (dose), and the length of time the microorganism is
exposed to it.
Although UV devices have been used
effectively in static situations, such as where irradiating coils, filters, and
pans, their efficacy is still in question when applied to ductwork. In a
fast-moving airstream, the length of exposure time is very short; therefore,
the intensity must be very high to kill a significant number of microorganisms.
The more
UV-resistant the microorganism, the longer and/or stronger the
irradiation must be.
More information on UV air and
surface treatment is available in Chapter 60 of the 2011 ASHRAE
Handbook—HVAC Applications and Chapter 17 of the 2012 ASHRAE
Handbook—HVAC Systems and Equipment.
2.12 AIR
MOVEMENT AND PRESSURIZATION
Another common method of mitigating the spread of infections is through
pressure relationships. As noted in Table 2-2, many rooms require positive or
negative pressure relative to adjacent spaces. As shown in Figure 2-4, the
intent of pressurization is to move potentially infectious particles from the
cleanest areas to less clean areas. A clear understanding of these areas is
essential. A hospital is not totally clean throughout, but has various
areas—from sterile-clean to semiclean to dirty.
Therefore, for example, operating rooms must be at a positive pressure
relative to adjacent corridors to prevent potentially harmful microorganisms
from entering the operating room. In contrast, airborne infection isolation
(AII) rooms must be maintained negative because patients in these rooms may be
highly infectious with diseases such as tuberculosis or SARS. Design
considerations for specialty rooms are covered in more detail in Chapter 8.
Many operations are carried out by
surgeons wearing simple masks across their lower face. In fact, it is important
that surgeons are comfortable during operations to ensure their concentration
during long or complex procedures. Clean supply air from overhead helps to
achieve this although it also tends to induce air from surfaces of anyone close
to the table and could deposit squames on or in the patient. Memarzadeh and
Manning (2003) modeled airflow in operating rooms using computational fluid
dynamics (CFD). They postulated that there is a thermal plume at the patient’s
wound site that can have the beneficial effect of deflecting the deposition of
particles away from the wound. Using the CFD model, Memarzadeh concluded that
the face velocity of the diffuser above the operating table should not exceed
30 fpm [0.15 m/s] to avoid disrupting the patient’s thermal plume. As described
in Chapter 8, limiting diffuser face velocity is one of the basic means of
designing operating room air distribution to reduce
deposition. However, field
research on this theory is limited. Kurz et al. (1996) provided indirect
evidence of the existence of a plume. It is also noted that work within the
surgical site with instruments and other devices disturbs the heat plume so
that the principal method of infection control remains the high air change rate
of well-filtered air delivered by a laminar flow system. Ongoing ASHRAE
research project RP-1397 is investigating hospital operating room air
distribution to verify CFD predictions of conditions that sustain the thermal
plume.
2.13 EFFECT OF
INFECTION CONTROL ON HVAC DESIGN
Figure 2-5 shows how the infection control
measures described in this chapter affect the design of HVAC systems by
influencing the following HVAC design parameters:
- Outdoor air quantity, including natural ventilation
- Type and location of filters
- Humidification
- UV radiation
- Chilled-water temperature
- Supply air conditions
- Supply air change rates in individual rooms
- Air distribution and velocity
- Locations of return air grilles
- Balance of supply and return/exhaust air for pressurization (including anterooms)
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