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Transportation Conformity: A Basic Guide for State & Local Officials

Appendix A: Health Effects of Pollutants

EPA has established standards8 for four transportation-related pollutants:

The standards are based upon EPA’s assessment of the health risks associated with each of the pollutants on at-risk populations. These assessments are based upon short- and long-term scientific studies by noted health professionals and medical research institutions. At-risk groups include children, the elderly, persons with respiratory illnesses, and even healthy people who exercise outdoors.

Air pollution often involves a complex set of chemical reactions, including combinations of pollutants and other factors such as weather and geography. Each pollutant plays a different role in the overall air quality in any given geographic area. Below is a brief overview of the key transportation-related pollutants.

Ozone Ozone often irritates the eyes, impairs the lungs, and aggravates respiratory problems. Ozone can cause chest pain, coughing, nausea, pulmonary congestion, and possible long-term lung damage. NOx and VOCs are precursors to ozone formation.

Volatile Organic Compounds VOCs come from vehicle exhaust, paint thinners, solvents, and other petroleum-based products. VOCs and nitrogen oxides react in the presence of sunlight to form ozone. A number of exhaust VOCs are toxic, with the potential to cause cancer.

Nitrogen Oxides Under the high pressure and temperature conditions in an engine, nitrogen and oxygen atoms in the air react to form various nitrogen oxides, collectively known as NOx. NOx, like hydrocarbons, is a precursor to the formation of ozone and also contributes to the formation of acid rain. NOx impacts the respiratory system, causing a high incidence of acute respiratory diseases. Pre-school children are especially at risk. NOx also degrades visibility due to its brownish color and the conversion to nitrate particles.

Carbon Monoxide Carbon monoxide is a product of incomplete combustion and occurs when carbon in the fuel is partially oxidized rather than fully oxidized to carbon dioxide (CO2). Carbon monoxide reduces the flow of oxygen in the bloodstream and is particularly dangerous to persons with heart disease. Exposure to carbon monoxide can impair visual perception, manual dexterity, learning ability, and performance of complex tasks.

Particulate Matter Particulate matter is tiny particles that can cause irritation and damage to the respiratory system, which can result in difficulty breathing, induce bronchitis, and aggravate existing respiratory disease. Exposure to particles may more dramatically impact individuals with chronic pulmonary or cardiovascular disease, people with influenza or asthma, and children and elderly persons. Particles may aggravate breathing difficulties, damage lung tissue, alter the body’s defense against foreign materials, and can lead to premature mortality. There are two PM standards: PM10 and PM2.5. PM10 refers to particles with a diameter of 10 microns (µm) or less, and PM2.5 refers to particles with a diameter of 2.5 µm or less. As a comparison, an average grain of table salt is 100 µm in diameter.

8National Ambient Air Quality Standards (NAAQS)

Updated: 6/5/2017
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