EXPOSURES ON AIRCRAFT

In the aircraft cabin, both passengers and cabin crew may be exposed to numerous air contaminants, including CO from engine exhaust, O₃ that enters with outside air, organic compounds generated by emissions from materials in the cabin and the human body, and infectious agents, allergens, irritants, and other contaminants of biological origin. Air quality incidents¹ have been reported during which passengers and cabin crew were exposed to other contaminants. Such incidents can result from inadvertent releases of engine oils, hydraulic fluids, deicing fluids, and their decomposition products into the air that enters the cabin. In general, the frequency of such incidents is not known because many of the data are considered proprietary by the airlines and were not made available to the committee and because there are no comprehensive, systematic methods for the collection of exposure and health effects information. AFA has reported a frequency of 7.6 incidents per 10,000 flights of a single airline on the basis of review of many sources of information, including reports filed by flight attendants, insurance companies, the Occupational Safety and Health Administration (OSHA), and medical professionals (Witkowski 1997).

In addition to passenger and cabin crew exposures to airborne contaminants, physiological stressors are inherent in flight, may contribute to complaints about cabin air quality, and may exacerbate underlying health problems. For instance, aircraft cabins are pressurized to an equivalent altitude of 5,000– 8,000 ft, relative humidity is typically below 20%, seating is often cramped, and people may experience jet lag.

Many guidelines and standards established for air quality in aircraft cabins are applicable to routine exposures in other indoor and outdoor environments. Table 1–1 lists contaminants that may be encountered under routine conditions in aircraft and the exposures recommended or legally established by various organizations. The organizations include FAA, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), the Environmental Protection Agency (EPA), OSHA, and the American Conference of Governmental Industrial Hygienists (ACGIH). FAA is the only organization with regulatory authority to establish standards for the aircraft cabin environment. ASHRAE has committees that provide guidelines on exposure in indoor environments, including those of aircraft (ASHRAE 2001). EPA promulgates national ambient air-quality standards (NAAQSs) for outdoor air. OSHA establishes permissible occupational exposure limits (PELs), and ACGIH recommends threshold limit values (TLVs) to protect worker health. Because the limits established by OSHA and ACGIH are intended for application in workplaces populated by healthy adults of working age, they are not intended to apply in situations where infants, children, the elderly, or those with medical conditions might be exposed; these subpopulations, included among aircraft passengers, are addressed by EPA's NAAQSs.

TABLE 1–1

Limits on Contaminants That May Be Found in Aircraft Cabin Air.

REGULATORY ASPECTS OF CABIN AIR QUALITY

FAA has regulatory authority over the operation of civil aircraft, including aviation safety, stemming from the congressional passage of the Federal Aviation Act in 1958 (Public Law 85–726). In 1970, Congress passed the Occupational Safety and Health (OSH) Act which was intended to ensure safe and healthful working conditions (Public Law 91–516). Under the OSH Act (Section 4[b][1]), federal agencies were granted the right to exercise jurisdiction over their own workers. In 1975, FAA asserted its jurisdiction over the safety and health of cockpit and cabin crew (40 FR 29114, DOT 1975). Specifically, FAA stated in 40 FR 29114:

Every factor affecting the safety and healthy working conditions of aircraft crew members involves matters inseparably related to the FAA's occupational safety and health responsibilities under the [Federal Aviation] act. With respect to civil aircraft in operation, the overall FAA regulatory program, outlined in part above, fully occupies and exhausts the field of aircraft crew member safety and health.

It is important to note that FAA regulatory authority over occupational safety and health applies when an aircraft is “in operation.” In operation is defined as the time starting when the aircraft is first boarded by a crew member, preparatory to a flight, to when the last crew member leaves the aircraft after completion of the flight, including stops on the ground during which at least one crew member remains on the aircraft even if the engines are shut down (40 FR 29114, July 10, 1975).

In addition to its regulatory authority over cockpit and cabin crew, FAA is authorized to protect the health and safety of passengers, as expressed in 49 USC 40101D and 49 USC 44701A, which provide FAA with broad authority to maintain the safety and security of air commerce.

As a result of that regulatory authority over safety and health, FAA has promulgated specifications for air quality in commercial aircraft in Federal Aviation Regulations (FARs): 14 CFR 21, 14 CFR 25, 14 CFR 121, and 14 CFR 125). Those regulations address O₃, CO, carbon dioxide (CO₂), ventilation, and cabin pressure. Regulations in 14 CFR 25 are airworthiness standards for commercial aircraft; they are intended as design specifictions for aircraft that are subject to certification under 14 CFR 21.² In contrast, 14 CFR 121 is intended as an operational standard and applies to domestic, flag (foreign), and supplemental air carriers. (Appendix C contains the FARs that are relevant to cabin air quality.) Regulations similar to the U.S. regulations established by FAA are applied to European aircraft by the European Joint Airworthiness Authority (JAA) and are termed Joint Aviation Regulations.

The air-quality design specifications in 14 CFR 25 are for ventilation, O₃, CO, and CO₂. The ventilation standard (Section 25.831) requires that the ventilation system be designed to provide enough uncontaminated air to enable crew members to perform their duties without undue discomfort or fatigue and to provide reasonable passenger comfort. Specifically, for normal operating conditions, the ventilation system must be designed to provide each occupant with an airflow containing at least 0.55 lb of “fresh” air per minute, equivalent to 10 ft³/per min (cfm) at 8,000-ft cabin altitude. The ventilation standard was revised in June 1996 to include cabin occupants. Before June 1996, Section 25.831 had specified only that for the crew compartment (the cockpit) a minimum of 10 cfm of fresh air per crew member (pilots and flight engineers) was required (61 FR 28683).³ This ventilation standard was revised in June 1996 to include cabin occupants. FAA determined that the change in the standard was needed because cabin crew members, who are active during flights, must be able to perform their duties in the cabin without discomfort and fatigue. In addition, FAA concluded that fresh airflow in the aircraft is necessary to provide adequate smoke clearance in the event of smoke accumulation due to a system failure.

The ventilation standard also specifies that the air of the cockpit and cabin must be free of harmful or hazardous concentrations of gases or vapors (14 CFR 25, Section 831). According to the standard, CO concentrations in excess of 1 part in 20,000 parts of air (50 ppm) are considered hazardous, and CO₂ concentrations during flight may not exceed 0.5% by volume (sea-level equivalent⁴ ) (5,000 ppm) in compartments normally occupied by passengers or crew members. In 1996, the CO₂ regulation was reduced from 3% by volume (sea-level equivalent) (30,000 ppm) on the basis of the 1986 NRC committee's recommendation that 30,000 ppm was much higher than was recommended for other indoor environments (61 FR 63952).

The FAR (Section 25.832) states that aircraft cabin O₃ concentrations during flight must be shown not to exceed 0.25 ppm by volume (sea-level equivalent) at any time above flight level 320 (that is, 32,000 ft or 10.7 km) or to exceed 0.1 ppm by volume (sea-level equivalent) for a time-weighted average (TWA) during any 3-h interval above flight level 270 (27,000 ft or 9 km). The regulation is expressed in more detail as an operational standard in 14 CFR 121. They are based on complaints of crew members and passengers about discomfort due to high O₃ concentrations at high altitudes (DOT 1980)⁵ .

In addition to design standards for ventilation, CO, CO₂, and O₃, FAA requires a cabin-pressure altitude of not more than 8,000 ft at the maximal operating altitude of the aircraft under normal conditions (Section 25.841). The standard was published in the FARs in 1964 (29 FR 18291, December 24, 1964), but no rationale was ever provided.

14 CFR 121, unlike 14 CFR 25, is an operational standard that not only describes the appropriate O₃ concentrations in the cabin at particular altitudes, but also specifies ventilation requirements (Section 121.219). Specifically, Section 121.219 states that each passenger or crew compartment must be “suitably” ventilated. In addition, Section 121.219 states that CO concentrations may not be more than 1 part in 20,000 parts of air (50 ppm), and fuel fumes may not be present.

HISTORY OF PREVIOUS CABIN AIR-QUALITY STUDIES

The NRC committee that produced The Airliner Cabin Environment (NRC 1986) addressed some of the same issues as the current committee. That committee was tasked with determining whether characteristics of cabin air could be responsible for health problems of passengers and cabin crew. The cabin air characteristics looked at included the quantity of outside air, the quality of onboard air, the extent of pressurization, the characteristics of humidification, and the presence of contaminants, such as bacteria, fungi, environmental tobacco smoke, CO, CO₂, and O₃. In response to its tasks, the 1986 committee concluded that “empirical evidence is lacking in quality and quantity for a scientific evaluation of the quality of airliner cabin air or of the probability of health effects of short or long exposure to it” (NRC 1986). The committee proposed numerous conclusions and recommendations to FAA that addressed several air-quality issues, including environmental tobacco smoke (ETS), CO₂, O₃, ventilation, and the need for exposure and health monitoring. Those most relevant to the current committee's task are discussed below.

• Ventilation. The 1986 report concluded that if the current equipment were used under full passenger loads, ventilation would be at the minimum for acceptable indoor air quality when smoking was not permitted and other contaminant sources were not present. The 1986 report recommended that because ventilation rate was one of the controlling factors for cabin air quality and because air-quality data were insufficient, FAA should implement a data-collection program that measures airflow and contamination in aircraft cabins.
• Carbon dioxide. The 1986 report recommended a review of the CO₂ standard of 30,000 ppm, which was much higher than standards for other indoor environments, including workplaces.
• Humidity. The 1986 report found no conclusive evidence of extensive or serious adverse health effects of low relative humidity. Therefore, it did not recommend supplemental humidification of cabin air.
• Ozone. The 1986 report concluded that O₃ in aircraft could reach concentrations above those in the FARs. The committee recommended that FAA conduct a review to ensure that cabin O₃ concentrations comply with the regulations.
• Environmental tobacco smoke. The 1986 report recommended a ban on smoking in all commercial domestic flights.
• Bioaerosols. Because of the lack of data on bioaerosols in aircraft cabins, the committee could not conclude whether exposures posed a health hazard. Because of concern regarding the potential transmission of infectious agents, particularly while an aircraft is on the ground and the ventilation system is not operated at full capacity, the 1986 report recommended a regulation that requires removal of passengers from an aircraft within 30 minutes after a ventilation failure or shutdown on the ground and maintenance of full ventilation whenever onboard or ground air conditioning is available. In addition, the 1986 report recommended that maximal airflow be used with full passenger loads to decrease the potential for microbial exposure and that recirculated air be filtered.
• Volatile organic compounds. The 1986 committee found no studies on the concentrations of volatile organic compounds (VOCs) or substances that might be emitted from disinfectants or cleaning materials.
• Pressurization. The 1986 report concluded that current pressurization criteria and regulations are adequate to protect the traveling public. However, it also concluded that the medical profession should use a more efficient system to warn those with medical conditions who might be at greater risk because of reduced pressure.
• Data collection. The 1986 report concluded that there was a lack of data for a scientific evaluation of aircraft cabin air quality and associated health effects. It recommended that FAA establish programs for the systematic measurement (by unbiased groups) of CO, respirable particles (RSPs), biological agents, O₃, ventilation rates, and cabin pressure. It also recommended that FAA establish a program to monitor health effects of cabin crew.

After the 1986 report, FAA adopted several of that committee's recommendations. In 1988, Congress passed Public Law 100–202 banning smoking on commercial flights with durations less than 2 h. In 1989, legislation banned smoking on nearly all domestic flights with durations of less than 6 h (Public Law 101–164). In 1996, FAA reduced the CO₂ standard from 30,000 to 5,000 ppm, on the basis of recommendations of the 1986 committee (61 FR 63952). In response to the committee's recommendation that FAA establish a program for the measurement of exposure variables, the Department of Transportation sponsored a study by Nagda et al. (1989) to evaluate health risks posed by exposures to ETS—including nicotine, RSPs, and CO—and other contaminants (O₃, bacteria, fungi, and CO₂) on randomly selected smoking and nonsmoking flights. FAA interpreted the 1986 committee's use of the term program to mean a one-time study (DOT 1987). The current committee finds it regrettable that FAA interpreted the term that way, since the 1986 committee's clear intent was to establish continuing monitoring and surveillance.

Since the 1986 report and FAA's response to its conclusions and recommendations (DOT 1987), a number of studies have made air-quality measurements on aircraft during commercial flights. Table 1–2 presents a summary of some of the studies, showing the principal contaminants measured and reported concentrations (CSS 1994; ASHRAE/CSS 1999; Dechow 1996; Dechow et al. 1997; Haghighat et al. 1999; Lee et al. 1999; Nagda et al. 1989; Nagda et al. 1992; Nagda et al. 2001; Spengler et al. 1997; Waters et al. 2001). Collectively, the studies presented in Table 1–2 measured a large number of contaminants and characteristics, including O₃, CO₂, CO, nitrogen oxides (NO_x), particulate matter (PM), VOCs, formaldehyde, bacteria, fungi, temperature, humidity, and cabin pressure altitude. Nagda et al. (1989) was the only study to measure air exchange rates, the rate at which cabin air is replaced with outdoor air. The type of aircraft, the number of flights on which measurements were made, and how flights were selected for monitoring differed among studies. In addition, measurement techniques varied considerably. Most of the studies showed that relative humidity was below 20 to 30% minimum levels specified by ASHRAE for comfort (ASHRAE Standard 55–92; CSS 1994; Haghighat et al. 1999). Most of the studies showed that CO₂ concentrations were higher than ASHRAE standard 62, which would result in concentrations of about 1,100 ppm or less (ASHRAE standard 62; Haghighat et al. 1999; Nagda et al. 1989). (It is important to note that ASHRAE Standards 55–92 and 62 are not explicit for aircraft, but rather are recommended levels for indoor buildings.) Table 1–2 indicates that average relative humidity ranged from 10.0%–42.6%, and average CO₂ concentrations from 386 to 1,756 ppm. Spengler et al. (1997), the only study that made concomitant measurements in other forms of transportation—including trains, interstate buses, and subways—determined that concentrations of contaminants that were measured in aircraft cabin environments (CO₂, CO, particles, VOCs, O₃, and nitrogen dioxide, bacteria, and fungi) were similar to those observed in other forms of public transportation.

TABLE 1–2

Contaminant Concentrations Reported in Published Studies.

In addition to the studies presented in Table 1–2, two independent inquiries into cabin air quality were recently conducted, by the British House of Lords (House of Lords 2000) and by the Australian Senate Rural and Regional Affairs and Transport References Committee (Parliament of the Commonwealth of Australia 2000). Those inquiries were motivated by complaints and growing public concerns regarding cabin air quality and associated health effects. The House of Lords inquiry focused on air travel in general; the Australian Senate committee examined principally the British Aerospace 146 (BAe 146) aircraft and concerns about exposures to engine oils. The House of Lords inquiry concluded that there was no significant impact of air travel on health for the majority of travelers, but it also noted the lack of knowledge on issues regarding the healthfulness of cabin air, and it recommended that the government commission further research and that passengers be notified of health concerns regarding flying. The Australian Senate committee offered much stronger conclusions, including that cabin and cockpit crew flying BAe 146 aircraft suffered occupational health effects of exposures to constituents of engine oils that took a long time to recognize. The committee's recommendations were therefore more extensive and included the redesign of the BAe 146 air circulation system, the introduction of regulations specifying air-quality monitoring and compulsory reporting guidelines for all passenger aircraft, and the development of a research program to study the effect of aircraft cabin air on cabin crew and passengers.

Although more data on cabin air quality have been collected since the 1986 NRC report, they have not been collected in a systematic manner that would conclusively address many of the questions that were raised in the 1986 report. For that reason and because of concerns about other potential exposures in aircraft that were not addressed in the 1986 report (including leaks of engine oil, hydraulic fluid, and deicing fluid, and exposure to pesticides), another NRC study was commissioned.

ORGANIZATION OF THE REPORT

The remainder of this report is organized into seven chapters. Chapter 2 presents information on the purpose and operation of the aircraft environmental control system (ECS). Chapter 3 addresses sources of chemical contaminants in aircraft; it focuses on the different types of exposures (including exposure sources inside and outside the aircraft) and how the ECS can act as a contaminant source. Chapter 4 reviews exposure and health data on biological agents; biological agents associated with hypersensitivity diseases are discussed first, followed by a discussion of agents that cause infectious diseases. Chapter 5 presents health effects of exposure to chemical contaminants; its emphasis is on the toxicology of chemical contaminants that are of greatest concern. Chapter 6 reviews the current database of health-surveillance and epidemiology studies. Chapter 7 addresses air-quality measurement techniques and applications that are available for gathering the necessary data. Chapter 8 explores the approaches needed to address the outstanding questions regarding aircraft cabin air quality; specifically, it lays out a surveillance and research program that integrates health-surveillance and air-quality monitoring techniques.

REFERENCES

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Footnotes

1

The committee defines air-quality incidents as events that result in the intake of potential contaminants, including engine oils and hydraulic fluids, through the environmental control system into the cabin.

2

Certification is the process by which the FAA ensures that the design of aircraft complies with statutes and that these regulations and standards are met by manufacturers and air carriers in the course of designing, producing, operating, and maintaining aircraft.

3

It is important to note that this amended standard does not apply to existing aircraft types—whether produced in the past or the future. Rather, it applies to aircraft types (designed and certified after June 5, 1996) and derivatives for which an application for certification was filed on or after this date of the regulation.

4

Sea-level equivalent refers to conditions of 25°C (77°F) and 760 mm Hg (15 psi) (FAR 25, Section 832).

5

This regulation specifies that FAA will conduct spot checks to ensure the effectiveness of O₃ control devices (14 CFR 25 and 121. Airplane cabin O₃ contamination. Fed. Regist.45(14):3880–3885. January 27, 1980).