APPENDIX C: Frequently Asked Questions

Question: What types of health impacts can airport emissions cause?

Answer: In general, both gaseous and particulate matter (PM) emissions from airports can cause harm to the human respiratory and cardiovascular systems. Effects can range from minor exacerbations of existing conditions to increased risk of hospitalization and premature death. Exposure to certain pollutants also can cause skin irritations and other physical effects, especially in sensitive individuals.

Question: What are the main air pollutants of concern?

Answer: As with other transportation sources, airport sources can emit all of the criteria pollutants (CO, NO_x, VOC, SO_x, Pb, PM₁₀, and PM_2.5) including the precursors that form ozone (O₃) and other secondary pollutants including various PM species. Also, HAPs such as formaldehyde, acrolein, acetaldehyde, etc., can be emitted from various sources. However, the main pollutant from a local health risk potential is PM_2.5, with important effects of ozone at longer range and increasing concern about UFPs in the near field. Formaldehyde tends to have the most cancer risk among HAPs species. Important non-cancer effects of HAPs may exist but are challenging to quantify.

Question: What are the differences between ambient standards, exposure thresholds, and cancer risk indicators?

Answer: Ambient standards, such as the National Ambient Air Quality Standards (NAAQS) are outdoor air pollutant concentration levels maintained by the EPA to monitor air pollution levels in different regions of the country. These are generally intended to be adequately protective of sensitive subpopulations. Exposure thresholds for air pollutants generally refer to concentration levels where below those levels, human beings are not considered to be at health risk. More specifically, exposure threshold levels and limits have been used by various organizations such as OSHA, NIOSH, and American Conference of Governmental Industrial Hygienists (ACGIH) to define and recommend levels/limits for workplaces to protect workers from harmful exposure. Cancer risk factors for air pollution are factors that may directly or indirectly cause or support the formation of cancer due to pollutant exposure. These factors may include a person’s age, sex, and family cancer history, and can serve to help determine the probability (the risk) of developing cancer.

Question: Besides pollutant type, what other factors affect public health?

Answer: Besides pollutant type, emission rates, toxicity, individual and population exposure, and vulnerability attributes are important factors that can affect public health. Indeed, all of these factors are important components in properly assessing the health risk of each pollutant. Individual exposure encompasses the pathway from the source to human activity locations (e.g., homes, workplaces, etc.) as well as how long a person is exposed to the pollutants. Population exposure integrates across individual exposures to provide measures relevant to the entire population. In addition, a person’s background and condition also can play a significant role in affecting his/her health. Factors such as age, gender, pre-existing disease status, and co-exposures to other risk factors can all affect susceptibility to air pollutants.

Question: What are the significant sources of pollutants at an airport?

Answer: The significant sources of pollutants at an airport generally arise from the combustion of fuels. The highest contributing emissions sources tend to be aircraft, GSE, and GAVs. Aircraft engines are significant sources of emissions in all phases of their operation, such as approach, landing, idling/taxiing, takeoff, and climb out, but the significance for each pollutant depends on the mode (or phase) of aircraft operation.

Question: What emissions mitigation measures have airports implemented?

Answer: Airports have implemented mitigation measures to address the key pollutants and the most significant sources. To mitigate aircraft emissions, airports have implemented measures to reduce taxiing and runway holding times and electrified gates to provide preconditioned air and power. The use of electric GSE and alternative fuels (e.g., CNG) has also helped reduce emissions in gate areas. For mitigating road traffic emissions, airports are dependent on influencing individuals and other organizations to change their behavior or practices. Many airports have promoted public transport use, especially where this involves low-emitting vehicles, and have invested in consolidating certain activities, such as rental car facilities, into one location.

Question: What are the differences between criteria pollutants and HAPs?

Answer: In the 1970 Clean Air Act Amendments, the EPA established criteria, or ambient air concentrations, that define the maximum acceptable level for each of the six criteria pollutants that affect public health and the environment. The concentrations are referred to as the National Ambient Air Quality Standards (NAAQS), and the criteria pollutants include carbon monoxide (CO), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), ozone (O₃), lead (Pb), and particulate matter (PM) in two forms, inhalable (coarse) particulate matter (PM₁₀), and fine particulate matter (PM_2.5), which are very small particles with a diameter 2.5 micrometers or less.

The EPA identified 187 hazardous air pollutants (HAPs) that cause serious health effects, and are known to, or suspected of, causing cancer in humans. HAPs also are referred to as toxic air pollutants (or air toxics), although not all air toxics are identified as HAPs.. Most HAPs are emitted from anthropogenic (manmade) sources such as exhaust from aircraft engines powered by fossil fuels and from stationary sources such as boilers and power plants. Examples of HAPs (and their sources) include formaldehyde (aircraft), benzene (gasoline), perchloroethylene (dry cleaning), and methylene chloride (paint stripper).

Question: What are the differences between primary and secondary pollutants?

Answer: Primary air pollutants are emitted directly from a source such as aircraft and GSE engines containing pollutants such as CO, NO_x, SO_x, PM_2.5, and HAPs. However, some pollutants are not emitted directly and form only as a result of complex chemical reactions in the atmosphere involving precursor pollutants. These formed pollutants are referred to as secondary pollutants. For example, ozone is generally not emitted directly from a source but is formed through the photochemical reaction of naturally occurring oxygen in the atmosphere, together with emissions of nitrogen oxides (NO_x) and volatile organic compounds (VOCs) in the presence of sunlight. Fine particulate matter (PM_2.5), a pollutant that has been associated with increased mortality and morbidity in areas where concentrations of the pollutant are elevated, also has significant contributions from secondary formation. Although PM_2.5 can be emitted directly, the pollutant also can be formed through chemical reactions involving NO_x, SO₂, and VOCs, leading to formation of sulfate, nitrate, and organic aerosol particles.

Primary and secondary pollutants are not to be confused with the primary and secondary standards established for the National Ambient Air Quality Standards (NAAQS). The NAAQS establish the maximum allowable concentrations of the criteria air pollutants for the protection of public health (primary standards) and protection of the environment (secondary standards).

Question: What health-related airport air quality studies have been conducted?

Answer: Although the overall literature on this topic is relatively small compared to studies that have been conducted for roadway sources and other industries, the breadth and depth of research in this area has been growing. Chapter 5 provides reviews of selected airport air quality and health studies. Appendix A and B, Literature Review Summary and References, provide a larger list of studies reviewed under this project. The studies range from specific airport health research to more general airport air quality and health studies for other industries.

Question: Why are UFPs from airports a health concern?

Answer: UFPs (defined as PM in the ultrafine range of less than 0.1 µm in diameter) are a health concern because their small size enables them to bypass the body’s filtration defense and penetrate further into the human respiratory system than PM_2.5. Populations living downwind of aircraft runways have been found to have elevated UFP concentrations from aircraft emissions, with characteristics differing from UFP emitted by onroad vehicles. UFPs are more inflammatory than larger PM sizes and translocate to essentially all internal organs. Continued exposure to UFPs can lead to cardiovascular disease, lung inflammation, hypertension, and birth defects, though different studies have provided conflicting results on UFP health impacts. More research is needed in this area.