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Excerpts From FSF Cabin Air Study[The following is excerpted from a Flight Safety Foundation study on the risk of Tuberculosis transmission aboard aircraft. It, quite necessarily, goes into great detail as to the quality of cabin air and how that may affect such transmissions. All emphasis has been supplied by the editor of http://AirlineSafety.Com. To read the entire study (Adobe Acrobat .pdf file), go to: Entire Study.—Ed. ASC] F L I G H T S A F E T Y F O U N DAT I O N Vol. 33 No. 4–5 For Everyone Concerned with the Safety of Flight July–October 1998 CABIN CREW SAFETY Guidelines Enable Health Authorities to Assess Risk Of Tuberculosis Transmission Aboard AircraftBased on eight investigations, U.S. health authorities believe that the risk is low for transmission of tuberculosis aboard transport-category aircraft. Nevertheless, the World Health Organization will publish new guidelines by late 1998 for assessing the need to notify passengers and crewmembers who may have been exposed to a person with active TB. FLIGHT SAFETY FOUNDATION • CABIN CREW SAFETY • JULY–OCTOBER 1998 Tuberculosis (TB) is an airborne disease, and discussion of the risk of TB transmission — or spread of infection — aboard aircraft leads to questions about cabin-air quality. During the last 10 years, U.S. research on cabin-air quality has been prompted by concerns other than TB, such as complaints of discomfort and unexplained health symptoms from some passengers and crewmembers. Several
investigations in the 1990s by the airline industry, the U.S. government,
academic researchers and independent interest groups have focused on airborne
microbiological concentrations and other air-quality measures aboard
transport-category aircraft. Basic measurements, such as carbon dioxide levels,
typically surpassed U.S. regulatory requirements. More investigations are under
way, however, and a new industry standard for cabin-air quality is under
development.1 The
medical director of one U.S. airline said that typical aircraft
environmental-control systems (ECSs) are designed, maintained and operated to
provide a healthy supply of air to crewmembers and passengers. Cris Bisgard, M.D., of Delta Air Lines, said, “If you compare air quality in an aircraft cabin to a standard office building, the airplane has far more air exchanges per hour than an office, and the air that comes into the aircraft cabin is sterile. Cabin air that is recirculated through a [high-efficiency particulate air (HEPA)] filter compares to recirculated air used in operating rooms and infectious-disease containment facilities.”2 Some medical researchers who investigated the risk of transmission of TB aboard transport-category aircraft during the early 1990s said that ECSs probably helped to prevent the spread of airborne bacteria that cause TB infection.3 Although ECSs are designed to meet the standards of worldwide aviation authorities, airlines and aircraft manufacturers have been challenged by various interest groups in recent years to re-examine these standards and conduct research to determine whether ECSs cause discomfort, fatigue and various health symptoms that occasionally have been reported by passengers and crewmembers. Continuing cabin-air research and the new industry standard should help settle debates about cabin-air quality by the early 21st century. Aircraft ECSs Balance Several Requirements Aircraft
ECSs provide functions that include fresh-air supply, cabin-air circulation,
cabin heating, cabin cooling, cabin pressurization, and lavatory and galley
vents. In addition to normal cabin-air quality requirements, aircraft flight
decks have special ECS-related operational and safety requirements, such as
adequate cooling of equipment and removal of smoke or vapors. These requirements
are met by systems that provide 100 percent outside air, recirculation systems
with filtration, and/or higher rates of air exchange than needed in the cabin. Components
of recent transport-aircraft ECS designs have been engineered to provide a
high-quality fresh-air environment, cabin pressure at or below 8,000 feet above
sea level, and a comfortable temperature. The source of fresh air is the
atmosphere outside the aircraft. Outside air is compressed by the turbine
engines and a portion — bleed air — is diverted via the pneumatic system to
air-cycle machines (packs), which cool the air. The temperature of bleed air typically is 482 degrees Fahrenheit (250 degrees Celsius) and it is then cooled to about 234 degrees Fahrenheit (112 degrees Celsius) at a pressure of 450 pounds per square inch (32 kilograms per square centimeter).4 Bleed air is cooled and the pressure is reduced to make it suitable for the aircraft cabin. Each pack is a collection of heat exchangers, turbines, compressors and other components that take bleed air from the turbine-engine compressors and condition it for distribution to the main deck and flight deck. This cooled air is ducted to an air-mix chamber. From the air-mix chamber, the air is directed to various cabin zones within the airplane. Early jet airliners used 100 percent outside air. This type of system continually exhausts all cabin air through outflow valves while the cabin is pressurized and replenishes the cabin with outside air. Beginning in the 1970s, designs with air recirculation were developed to make airplanes more fuel efficient. On airplanes with air recirculation, some of the air exiting the cabin is filtered and reintroduced into the air-mix chamber. The rest of the air exiting the cabin is ducted overboard. Because outside air supplied to the cabin is taken from the aircraft’s engines, any reduction in bleed-air usage increases the engine’s efficiency and reduces fuel consumption and operating costs. Typical cabin-air filters trap nearly all airborne particles. Filters also may be designed to remove from Recirculated cabin air specific aerosols (liquid droplets) that could contaminate bleed air in case of a malfunction, such as a pinhole leak of hydraulic fluid from an engine or auxiliary power unit (APU). Air filters do not remove vapors, but separate odor-removal filters help eliminate specific gaseous contaminants in some designs. During ground operations, the APU provides bleed air for ECS functions, or an external ground air-conditioning unit may supply preconditioned air. Engineers
who designed air-recirculation systems sought to balance cabin-air quality and
greater fuel efficiency. Their work responded to concerns about rising fuel
prices and fuel availability, affordable airline-ticket prices, depletion of
natural resources and environmental issues. The same concerns changed
engineering priorities for homes, appliances, motor vehicles, manufacturing
plants and office buildings. In one 1987–1994 investigation of the microbiological composition of cabin air on 45 flights, the researchers said, “Because the high compressor temperatures effectively kill any living organism in the intake air, the air supply is virtually sterile as it enters the cabin air-distribution system. Cabin air is compressed from ambient air at high altitudes. The low humidity at altitude means that the moisture content in the air supply is also quite low. Relative humidities approximate those found in the U.S. southwestern deserts— commonly 10 percent to 20 percent. Such low humidities do not favor microbiological growth.”5 The
researchers said that normal airline-cabin air-exchange rates in 1994 typically
ranged from 15 per hour to 20 per hour, which compared to 12 air exchanges per
hour in a typical office building and five air exchanges per hour in a
typical home. The
researchers said, “The microbiologic flora [organisms] within an airline cabin
under cruise conditions almost certainly cannot come from external air. Instead,
[they are] supplied by the occupants and by those residual organisms present on
cabin furnishings at the beginning of each flight.
The amount of contamination is relatively small. It is normally an
order of magnitude less than that found on city buses and streets. …
Microbiological concentrations appear to be related to [passenger] activity
within the cabin.” Air-recirculation
systems were developed primarily to improve fuel economy, but have several
benefits that airlines consider in choosing equipment, including lower operating
costs, increased range, reduced emission of exhaust gases into the
atmosphere, slightly higher cabin-air humidity and lower ozone concentrations in
some situations.6 Aircraft
with air-recirculation systems have been designed to provide comfortable airflow
to the cabin with recirculation fully turned on, continuously mixing equal
amounts of fresh air and filtered cabin air, then pumping it overboard through
outflow valves. Flight crews typically can select 100 percent outside air
temporarily to increase the rate of air exchange for comfort during some
operating conditions, to remove odors or to purge smoke or vapors. On a medium-capacity transport aircraft, one pack is designed to maintain the required fresh-air supply with adequate heating and cooling, and a second pack is provided for reliability and redundancy, but can be used for faster cooling or ventilation rates. In some large aircraft, two packs are needed to provide proper airflow, and a third pack provides redundancy and gives the flight crew the ability to provide faster-than-normal air exchanges or temperature adjustments. Some epidemiological investigations of aircraft-related TB transmission said that typical patterns of cabin airflow apparently help to dilute and filter out airborne bacteria.7 Air
from the air-mix chamber commonly enters near the cabin ceiling, circulates
around the cabin and exits near the cabin floor. Much less air moves along the
length of the inside of the cabin. Research generally shows that cabin airflow
patterns do not entirely eliminate the risk that airborne bacteria will travel
from one section to another, but cabin-air circulation, in combination with air
filtration, significantly reduces the likelihood. The
preliminary findings of a 1994 investigation by the Department of Environmental
Health at the Harvard University School of Public Health, for example, said, “Reduced
amounts of outdoor air [aboard the aircraft studied] do not necessarily
translate to poor air quality and increased risk of disease. Air cleaning and
removal of pollutants mitigate some of the effects of decreasing dilution air.
Even with recirculating ventilation systems, oxygen is not depleted, nor does
carbon dioxide increase to levels that interfere with respiration. Of
concern, however, is the adequacy of the strategies used (i.e., recirculation
and filtration) to offset the effects on air quality of reducing the amount of
outdoor air produced. … It is evident from our investigation that aircraft
ventilation systems [were] not balanced by sections of the cabin.”9 Researchers
said that the preliminary bacteria-related conclusions of the investigation,
which should not be considered comprehensive, were that more work is needed to
characterize exposure to infectious agents in aircraft cabins; airborne
bacterial concentrations were slightly higher in airport terminals than during
any of 22 flight segments, except three samples taken during deboarding;
that overall bacterial counts on airplanes with recirculating air-handling
systems tended to be higher than those with 100 percent outside air; and that
bacteria recovered were those typically shed from human skin and mucous
membranes, and levels were within the range commonly seen in public
environments such as schools and office buildings. Latest
Cabin-air Filters Eliminate TB Bacteria Cabin
air-filtration systems — whether certified to HEPA standards or previous
standards — have been designed to enhance passenger and crew health and
comfort by controlling these contaminants, he said. “The efficiency of our
HEPA aircraft filters compares very well with HEPA filters we manufacture for
use by hospital patients,” Lundquist said. “There is no way to prevent
transmission of some diseases aboard aircraft, but we can reduce the probability
of someone becoming ill if they are sitting far away from an infectious
passenger. A HEPA recirculation filter will help to prevent other passengers
from getting ill.” Lundquist
said that HEPA filters on transport-category aircraft remove particles with an
efficiency higher than 99.97 percent at 0.3 micron (one micron is one-thousandth
of a millimeter), significantly reducing the level of airborne-particulate
contamination. HEPA filters provide the microbial equivalent of outside air to
the passenger cabin, he said. The average bacterium has a diameter of about one
micron, and strains of M. tuberculosis, which cause TB infection, range from 0.2
to one micron in diameter, Lundquist said. (By comparison, the diameter of an
average human hair is about 75 microns.) “Some
people today want 20 cubic feet [0.6 cubic meter] per minute of outside air per
passenger, but there is a two percent to four percent increased cost of fuel per
year if you don’t recirculate cabin air,” Lundquist said. “The advantage
of a recirculation system aboard an aircraft is that you can filter the air so
that what comes out the filter actually is cleaner than bleed air. Secondary
benefits of recirculating through a filter are that normally low relative
humidity increases a small amount for greater comfort and reduces ozone levels.” Because
of the physical properties of airborne particles, Lundquist said, HEPA filters
also remove particles smaller than the openings between fibers of filter
material. Viruses are 10 times to 100 times smaller than bacteria, for example,
but research shows that they are trapped by HEPA filters. “In
HEPA design, there is a ‘most-penetrating particle size’ at which the filter
is least efficient,” Lundquist said. “Some viruses get very close to
molecular level in size. But when viruses are bombarded by air molecules, they
move laterally, not in a straight line. The more lateral motion, the higher the
rate of filter efficiency because if a particle touches any fiber in the filter
as it passes through, it will be captured. That means we can filter out
particles even smaller than 0.3 microns. That is why the HEPA filter is 99.9995
percent efficient for viruses, even though they are smaller than bacteria.” Scientists
first realized that cabin-air filtration could be effective because of U.S.
Department of Transportation (DOT) research about the effects of tobacco smoke
aboard U.S. aircraft, he said. “We looked at the dispersion of nicotine [from
cigarette smoke in aircraft cabins],” said Lundquist. “This data told us how
readily something airborne will disperse up and down the aisle, what we call
diffusional transport. We found that the circulation of air [from] ceiling to
floor is so much greater than along the length of the cabin … it is the
dominant airflow pattern. That was great news. There was not a lot of axial
mixing — nicotine levels varied by a ratio of 400 to one in different parts of
the cabin. This told us that improved filtration of cabin air would be a benefit.
We then did some mathematical studies and used the DOT nicotine-dispersion data,
working with Boeing on committees of the American Society of Heating,
Refrigerating and Air-Conditioning Engineers [ASHRAE]. We proved analytically
that a better filter would reduce the dispersion of contaminants throughout an
aircraft.” In
early 1998, United Airlines became the first major airline to announce plans to
install HEPA filters throughout the airline’s fleet.11
Other airlines also have been specifying HEPA filters for new aircraft in
recent years and retrofitting some aircraft, he said. Other recent-generation
filters have provided similar benefits in cabin-air quality, Lundquist said, but
HEPA technology has become the “gold standard” because of the preference for
this technology in health care. HEPA
filters typically are disposable. Among
other advances in aircraft ECS designs recently described by aircraft
manufacturers are distribution systems with more main-deck air-distribution
zones; ventilation rates that can be regulated based on passenger density in
different zones; normal and high-flow operating modes for rapid cabin-clearing
of cigarette smoke or odors, if needed; and ECSs that use 100 percent outside
air more efficiently than previous designs. Hospitals
also generally choose from two basic types of ventilation systems for dilution
and removal of contaminated air: single-pass systems and recirculating systems,
said one study of mathematical models for medical-facility environments. The
report said, “In single-pass systems the supply air is uncontaminated, fresh
outside air, and after it passes through the ventilated area, 100 percent of
that air is exhausted to the outside. In a recirculating system, a small portion
of the exhaust air is discharged to the outside and is replaced with fresh
outside air, which mixes with the portion of exhaust air that was not discharged
to the outside. A minimum of six [air changes per hour] is recommended for TB
isolation rooms and treatment rooms. Where feasible, this airflow rate should be
increased to 12 [air changes per hour] or more, and in areas where the nature of
work is exceptionally hazardous, such as autopsy rooms, airflow rates of 15–25
air changes per hour have been recommended. … HEPA filtration units or
ultraviolet germicidal irradiation can be used as a supplement to ventilation
control measures in settings where adequate airflow cannot be provided with the
general ventilation system alone.”12
These rates compare to typical
aircraft systems that provide approximately 20 air exchanges per hour. Future
Standard to Define Cabin-air Quality In
the United States, Federal Aviation Regulations (FARs) Part 25.831 says,
“Under normal operating conditions and in the event of any probable failure
conditions of any system which would adversely affect the ventilating air, the
ventilation system must be designed to provide a sufficient amount of
uncontaminated air to enable the crewmembers to perform their duties without
undue discomfort or fatigue, and to provide reasonable passenger comfort. For
normal operating conditions, the ventilation system must be designed to provide
each occupant with an airflow containing at least 0.55 pounds [0.25 kilograms]
of fresh air per minute. Crew and passenger compartments must be free from
harmful or hazardous concentrations of gases or vapors.” FARs
also specify cabin-air limits for carbon monoxide (not more than one part in
20,000 parts of air), carbon dioxide (not more than 0.5percent by volume, sea
level equivalent, during flight) and ozone concentrations (not more than 0.25
parts per million by volume, sea level equivalent, at any time above 32,000
feet, or 0.1 parts per million, sea level equivalent, time-weighted average
during any three-hour interval above 27,000 feet). In
Europe, the Joint Aviation Requirements (JARs) include the following standards
for aircraft ventilation. JARs Part 25.831 says, “Each passenger and crew
compartment must be ventilated and each crew compartment must have enough fresh
air (but not less than 10 cubic feet [0.28 cubic meter] per minute per
crewmember) to enable crewmembers to perform their duties without undue
discomfort or fatigue.” Advisory Circular-Joint (ACJ) 25.831 (a) says, “The
supply of fresh air in the event of the loss of one source, should not be less
than 0.4 pounds [0.18 kilograms] per minute per person for any period exceeding
five minutes. However, reductions below this flow rate may be accepted provided
that the compartment environment can be maintained at a level which is not
hazardous to the occupant.” JARs
Part 25.831 says, “Crew [compartment] and passenger-compartment air must be
free from harmful or hazardous concentrations of gases or vapors. In meeting
this requirement, the following apply: (1) Carbon monoxide concentrations in
excess of one part in 20,000 parts of air are considered hazardous. For test
purposes, any acceptable carbon monoxide detection method may be used. (2)
Carbon dioxide in excess of 3 percent by volume (sea-level equivalent) is
considered hazardous in the case of crewmembers. Higher concentrations of carbon
dioxide may be allowed in crew compartments if appropriate protective breathing
equipment is available. [Yves Morier, regulations director of the Joint Aviation
Authorities (JAA), said that JAA has proposed an amendment to adopt the text of
FARs 25.831 (b) (2) regarding carbon dioxide concentration during flight.13
JAA received public comments in June 1998 and expects to finalize the
amendment in early 1999, said Morier.14 The effective date of
the amendment to the FARs regarding carbon dioxide concentration was Jan. 2,
1997.] In
1994, Alan R. Hinman, M.D., M.P.H., director of the National Center for
Prevention Services at the U.S. Centers for Disease Control and Prevention (CDC),
responded to questions about aircraft air quality before an aviation
subcommittee of the U.S. House of Representatives. Hinman said that CDC’s
National Institute for Occupational Safety and Health (NIOSH) had applied
extensive experience from investigating indoor environmental quality under the
health hazard evaluation (HHE) program to study cabin-air quality in
transport-category aircraft. “In
1991, in response to a request by the Association of Flight Attendants [AFA],
NIOSH conducted an HHE to investigate potential causes of headache, dizziness,
blurred vision, mental confusion and numbness reported by employees [of one U.S.
airline],” Hinman said. “NIOSH assessed cabin air quality and reviewed
employee medical records and company incident logs to determine whether toxic
gases or lack of oxygen caused these symptoms. Measurements of levels of carbon
monoxide, ozone, carbon dioxide, nitrogen dioxide, oxygen, temperature,
humidity, total particulate and volatile organic compounds did not reveal an
environmental cause for the symptoms reported. Review of employee medical
histories also did not indicate a work-related etiology [cause or origin] for
these illness incidences. NIOSH recommended that the airline continue to
monitor cabin air for carbon monoxide levels and that further investigation
should examine the roles of other environmental, ergonomic and psychosocial
occupational stressors.”15 A
1994 study commissioned by the Air Transport Association of America (ATA)
collected cabin-air-quality data during flights aboard two types of
transport-category aircraft designed to use 100 percent outside air and two
types of transport-category aircraft designed to use a combination of outside
air and cabin air Recirculated through filters. The
study evaluated contaminants (respirable particulates, biological organisms
[bacterial and fungal] and volatile organic compounds) and environmental
parameters (such as carbon dioxide levels, relative humidity, temperature and
noise). The study said, among other findings, that the aircraft environments
reviewed were relatively free of dust and other particles that are likely to
cause health effects; that levels of airborne microorganisms were well below
NIOSH-recommended levels; and that no bacterial or fungal respiratory pathogens
were isolated by a medical laboratory that studied air samples.16 Since
the NIOSH health-hazard evaluation and the ATA study, however, AFA has
continued to monitor reports of health symptoms from its members and to
discuss with airlines, aircraft manufacturers and engineering groups a new
standard for cabin-air quality. “AFA
is still very interested in the issue of air quality,” said Candace Kolander,
AFA’s coordinator for air safety and health. “Complaints from flight
attendants vary over time, but air quality remains high on the priority list of
issues for AFA.”17 General
public interest in aircraft-cabin air quality has not abated since the
mid-1990s, said Tony Giometti, ASHRAE’s public relations manager.
Representatives of flight-attendant unions from the United States and Canada,
manufacturers, engineers and other groups participated in sessions on this topic
during the society’s June 1998 meeting in Toronto, Ontario, Canada.18 The
focus of attention, Giometti said, has been work on ASHRAE’s proposed Standard
161P, Air Quality Within Commercial Aircraft. A 20-member standards committee
comprises representatives of airlines, aircraft manufacturers, airline pilots,
flight attendants, environmental-control-system engineers, scientists, the
traveling public and other knowledgeable individuals and interest groups, he
said. ASHRAE
expects that work on the proposed standard, begun in June 1995, will require
another two years.19 ASHRAE
Standard 161P will apply to commercial passenger air-transport aircraft
certified under FARs Part 25. The standard will define the requirements for air
quality in air-carrier aircraft that carry 19 or more passengers, and will
specify methods for measuring and testing air quality to verify compliance. The
society believes, however, that it may prove difficult to satisfy every
person who has expressed concern. “Considering
safe operation of the aircraft, the diversity of sources and contaminants in
aircraft-cabin air, and the range of susceptibility in the population,
compliance with this standard will not necessarily ensure acceptable
aircraft-cabin air quality for everyone,” ASHRAE said.20 Part
of the problem has been a common tendency to ignore significant differences
between “moving” and “built” environments, ASHRAE said. The committee
developing the proposed standard for cabin-air quality believes that the amount
of air provided per cubic foot per minute and the number of air changes per hour
are not directly comparable between aircraft and buildings, for example.
Giometti said that additional ASHRAE research on cabin-air quality is under way.© — FSF Editorial Staff We Encourage Reprints Articles in this publication may, in the interest of aviation safety, be reprinted, in whole or in part, in all media, but may not be offered for sale or used commercially without the express written permission of Flight Safety Foundation’s director of publications. All reprints must credit Flight Safety Foundation, Cabin Crew Safety, the specific article(s) and the author(s). In keeping with FSF’s independent and nonpartisan mission to disseminate objective safety information, Foundation publications solicit credible contributions that foster thought-provoking discussion of aviation safety issues. If you have an article proposal, a completed manuscript or a technical paper that may be appropriate for Cabin Crew Safety, please contact the director of publications. Reasonable care will be taken in handling a manuscript, but Flight Safety Foundation assumes no responsibility for submitted material. The publications staff reserves the right to edit all published submissions. The Foundation buys all rights to manuscripts and payment is made to authors upon publication. Contact the Publications Department for more information. CABIN CREW SAFETY Copyright © 1998 FLIGHT SAFETY FOUNDATION INC. ISSN 1057-5553 Suggestions and opinions expressed in FSF publications belong to the author(s) and are not necessarily endorsed by Flight Safety Foundation. Content is not intended to take the place of information in company policy handbooks and equipment manuals, or to supersede government regulations. Staff: Roger Rozelle, director of publications; Mark Lacagnina, senior editor; Wayne Rosenkrans, senior editor; John D. Green, copyeditor; Karen K. Ehrlich, production coordinator; Ann L. Mullikin, assistant production coordinator; and David A. Gzelecki, librarian, Jerry Lederer Aviation Safety Library. Subscriptions: US$60 (U.S.-Canada-Mexico), US$65 Air Mail (all other countries), six issues yearly. • Include old and new addresses when requesting address change. • Flight Safety Foundation, 601 Madison Street, Suite 300, Alexandria, VA 22314 U.S. • Telephone: (703) 739-6700 • Fax: (703) 739-6708 Visit our World Wide Web site at http://www.flightsafety.org
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