IAQ in Airliner Cabins
A question I have received frequently over the past 15 years is, "How good is the air in airplane cabins?" Normally the person asking the question observes that it is a very small space with often crowded conditions and it seems that the level of contamination has to be high. When smoking was discontinued on domestic routes, I figured the concerns expressed might fade away. Actually they have gotten more frequent. The nature of concerns have moved from comfort to health related.
Facts about Aircraft Cabin Air
Commercial aircraft have a complex Environmental Control System that provides ventilation, heating, cooling, cabin pressurization and humidity and contaminatin control. The design of that system is quite unlike that of HVAC systems used in buildings and other transportation modes. The design utilizes "Air Cycle Air-conditioning Equipment" that is powered through the EBAS (engine bleed air system). The American Society of Heating Refrigeration and Air Conditioning Engineers and the US FAA have established performance specifications for the operation of such systems and have studied their performance extensively during the past decade. Some findings include:
- 10 CFM of fresh air is provided by the system for each occupant of the aircraft. (Office building standards provide as little as 5 CFM per occupant)
- An additional 10 CFM is supplied each occupant as re-circulated and reprocessed air. Thus, half is fresh or outside air and half is reprocessed.
- This results in about 12.5 air changes per hour (ACH) compared to a modern building that will have a maximum of 3-4 ACH. Even though an aircraft cabin is much more densely populated than most buildings, the greater ACH should result in significantly cleaner air.
- Higher rates of air change would carry a risk of bringing in excessive ozone due to ozone plumes at the normal flight altitude. Higher rates would also be costly due to the energy required to produce bleed air from the engines.
- Fresh air comes from engine bleed, reaches a minimum of 400 degrees F and therefore is sterile.
- Re-circulated air (which has a high microbial content) is passed through a HEPA filter that removes 99.7% of particles larger than 0.3 microns in size. Since microorganisms (including viruses) are larger than 0.3 microns, it is assumed that reprocessed supply air is free of any organisms and dust free.
- ASHRAE studies suggest that levels of bacteria and fungi in aircraft in flight are lower than levels in typical homes.
- Cabin humidity levels are 20-30% (uncomfortably low) and lead to minor respiratory complaints.
- Cabin levels of carbon dioxide are at the high end of normal at 1050 ppm but well below the ACGIH action level of 5,000 ppm.
- Supply air is furnished at a velocity of 500 feet per minute. This tends to mix the air by row and limits the mixture of air between rows. This is believed to limit the possibility that particulates and other contaminates will move between rows.
Aircraft Air Ducts
Aircraft have a complex system of air ducts that carry stale air back to the central air unit for reprocessing and a mixture of fresh (outside) and reprocessed air to cabin occupants, crew and other zones of the aircraft that are pressurized and conditioned.
Often, environmental conditions in the aircraft cause the conditioned air to reach dew point which will lead to water condensing on surfaces of these air ducts. This is validated by the occasional releases of water vapor that can be seen coming from the vents in the form of a cloud like haze or droplets of water from air supply vents. It is not documented in public literature how moist these ducts become in normal operation or how long they remain wet. In the event duct surfaces remain wet for periods of several hours or more there is an excellent chance that microbial growth including bacterial and fungal spores from cabin activities and occupants could amplify on these surfaces and reach high concentrations.
It has been suggested that a growth inhibiting treatment applied by fogging into the ducts so as to treat all interior surfaces would inhibit such growth and prevent the buildup of excessive concentrations of organisms. Tests of surface concentrations of organisms of interest should be conducted to determine the appropriate application frequency to obtain adequate control. We would estimate that infrequent treatment (monthly or less frequently) would be satisfactory. This concept has not been investigated.
Interior Cabin Surfaces
Interior surfaces in commercial aircraft are exposed to a high biological load. Cabin population is significantly denser than commercial, residential or public buildings. It is reasonable to assume that these surfaces accumulate a high concentration of microorganisms between cleanings. Where there is moisture from human emissions, spills or other releases, the load of bioburden may well amplify significantly. There is a high possibility of transfer of this microbial load to passengers and others.
Some airlines fog or spray a disinfectant onto these surfaces to inhibit the growth and spread of such biological matter. This would seem to reduce the load of organisms that passengers would be exposed to. Tests of surface concentrations of organisms of interest should be conducted to determine the appropriate application frequency to obtain adequate control. This again is an area that has not been well studied.
Other Surfaces
Surfaces that are exposed to human contact or on which germs may have been deposited should be wiped down with a disinfectant/cleaner at every opportunity. The more frequent the disinfection, the greater the protection for the next passenger. For example, this technique has been used on school buses by spraying and wiping interior surfaces at the end of each work shift. Studies completed by the Research Triangle Institute demonstrate a significant reduction of respiratory complaints for children in a day care center and their immediate families through consistent cleaning and disinfecting of the center and its furnishings.
Summary
Although there are several areas where cleaning and disinfecting products and technology might well improve cabin environmental quality as outlined above, there is no published literature that documents any risk that would motivate an airline to spend significant funds to achieve an enhanced level of cabin air quality. What has been published to date states that risks associated with exposure to microorganisms in commercial aircraft are less than when in a residence or other building.
Dated: June 1, 2003
Mr. Baker's field of expertise is the control of contamination in air-conditioning and ventilating systems by mold, mildew and bacteria. He writes and speaks frequently about the efficacy, legal risks, and regulatory issues involved in various control strategies. He serves on ASHRAE Technical Committee TC 2.3, TC 2.4, TC 9.8, and Sampling of Airborne Particulate Concentration in Commercial and Residential Buildings GPC 17P. He also serves as a member of ASTM D22.06 (Indoor Air Quality) and is on the Board of Directors of the Indoor Air Quality Association and the Florida Public Health Foundation. Because HVAC applications encompass new uses from an U.S. EPA regulatory standpoint, Mr. Baker works closely with the EPA and industry groups, including serving as the chair of the IAQ committee of the Consumer Specialty Products Association, to help formulate policy in this area.
Bob Baker is Chairman and CEO of BBJ Environmental Solutions, Inc., a company specializing in providing clean air through environmentally responsible products, such as BBJ MicroBiocide , BBJ Micro Coil Clean , "FreshDuct Odor Eliminator ", and BBJ Mold and Mildew Remover™ as well as the revolutionary new Power Coil Clean™. For additional information, Mr. Baker can be reached at (800) 889-2251 or through the company web site at http://www.bbjenviro.com.
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