The National Ambient Air Quality Standards (NAAQS, also referred to as "air quality standards") are Federal standards, established through extensive scientific review, that set allowable concentrations and exposure limits for certain pollutants. Primary standards are intended to protect public health, while secondary standards protect public welfare (i.e., damage to crops, vegetation, buildings). Air quality standards have been established for the following six criteria pollutants: ozone (or smog), carbon monoxide, particulate matter, nitrogen dioxide, lead, and sulfur dioxide. If monitored levels of any of these pollutants violate the NAAQS, then EPA in cooperation with the State will designate the contributing area as "nonattainment."
Transportation contributes to four of the six criteria pollutants: ozone, carbon monoxide, particulate matter, and nitrogen dioxide. New standards for ozone and particulate matter have been established by EPA that will also impact transportation planning and programs in the future.
Volatile organic compounds (VOCs) and nitrogen oxides (NOx) react in the presence of sunlight to form ozone. For mobile sources, VOCs come primarily from vehicle exhaust and fuel evaporation. 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. Ozone 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. A number of VOCs are also toxic, with the potential to cause cancer. NOx also contributes to the formation of acid rain and degrades visibility due to its brownish color.
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 impairs visual perception, manual dexterity, learning ability, and performance of complex tasks.
Particulate matter can be released from combustion of fuels, industrial processes, construction sites, farm fields, and transportation activities. PM is tiny particles of dust that cause irritation and damage to the respiratory system. This can result in difficulty breathing, induce bronchitis, aggravate existing respiratory disease, and exposure to PM impacts individuals with chronic pulmonary or cardiovascular disease, people with influenza or asthma, children and elderly persons. PM aggravates breathing difficulties, damages lung tissue, alters the body's defense against foreign materials, and can lead to premature mortality.
Nitrogen dioxide is a reddish brown, highly reactive gas that is formed by combustion sources such as in automobiles and power plants. Exposure to NO2 concentrations may lead to changes in airway responsiveness and lung function in individuals with pre-existing respiratory illnesses and increases respiratory illnesses in children, as well as leading to increased susceptibility to respiratory infection and alterations in the lung. The Los Angeles area is the only area of the country that was nonattainment for nitrogen dioxide.