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Transportation Air Quality Selected Facts and Figures


Pollutant Effects

Above certain concentrations, ozone (O3), particulate matter (PM), and carbon monoxide (CO) can cause or exacerbate health problems and/or increase mortality rates, making their control an important goal under the Clean Air Act.

Ozone (O3)

Ground-level ozone is the major component of smog. While ozone in the upper atmosphere (the 'ozone layer') occurs naturally and protects life on earth from harmful ultraviolet radiation, ozone at ground level is a noxious pollutant. Ground-level ozone is not directly emitted, but is formed by the reaction of oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight.

Ozone is a severe irritant, responsible for the choking, coughing, and stinging eyes associated with smog. Ozone damages lung tissue, aggravates respiratory disease, and makes people more susceptible to respiratory infections. Children are especially vulnerable to ozone's harmful effects, as are adults with existing disease. Even healthy individuals may experience impaired lung function from breathing ozone-polluted air. In addition to affecting human health, ozone harms vegetation, resulting in reduced agricultural and commercial forest yields, increased tree and plant susceptibility to disease and other environmental stresses, and potential long-term effects on forests and ecosystems. Peak concentrations typically occur in summer.

In April 2004, EPA promulgated a new, more stringent ozone standard. This new '8-hour' standard requires ozone levels to be lowered to .08 parts per million (ppm) rather than .12 ppm, and applies the standards to an 8-hour average concentration rather than a 1-hour average.

Particulate Matter (PM10 and PM2.5)

Particulate matter is the term used for a mixture of solid particles and liquid droplets found in the air. These particles come in a wide range of sizes and can remain suspended in the air for extended periods. PM can be emitted directly by a source or formed in the atmosphere by the transformation of gaseous emissions, such as SO2, NOx, and VOC.

Fine particles, under 2.5 microns in diameter (PM2.5), result from fuel combustion by motor vehicles and other sources, as well as transformation of gaseous emissions. Coarser particles up to 10 microns in diameter (PM10) generally consist of windblown dust, and are released from agriculture, and crushing and grinding operations.

Particulate matter irritates the membranes of the respiratory system, causing increased respiratory problems and disease, decreased lung function, alterations of the body's defense systems, and premature mortality. Sensitive groups include the elderly, individuals with cardiopulmonary disease such as asthma, and children. In addition to health problems, airborne particles cause soiling and damage to materials and reduce visibility in many parts of the United States. There are daily (24-hour) and annual PM NAAQS.

In December 2004, the EPA designated new PM standards. Two new PM2.5 standards (annual and 24-hour) were added to the existing standards for PM10. These standards focus on fine particles under 2.5 microns in diameter, which are believed to be the most closely associated with acute health effects.

Carbon Monoxide (CO)

Carbon monoxide is an odorless, colorless, gas that interferes with the delivery of oxygen to the body's organs and tissues. Effects of CO include dizziness, headaches, fatigue, visual impairment, reduced work capacity, reduced manual dexterity, and poor learning ability. The health effects of CO vary depending on the length and intensity of exposure and the health of the individual, and are most serious for those who suffer from cardiovascular disease. CO has both a one-hour and eight-hour standard.

The incomplete burning of carbon in fuels such as gasoline produces CO. High concentrations of CO occur along side roads with heavy traffic, particularly at major intersections, and in enclosed areas, such as garages and poorly ventilated tunnels. Peak concentrations typically occur during the colder months of the year when vehicular emissions of CO are greater and night-time inversion conditions are more frequent.

Air Quality Standards

National Ambient Air Quality Standards (NAAQS) are set by EPA to protect public health and welfare. Primary standards are designed to protect against adverse health effects, while secondary standards protect against welfare effects, such as damage to crops, vegetation, buildings, and decreased visibility.

An area is in violation of a standard if it exceeds the concentration level for its evaluation time frames. For example, for an area to attain the 8-hour ozone standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at each monitor within the year must not exceed 0.08 ppm.

Primary Air Quality Standards for Transportation-related Pollutants
PollutantType of AverageConcentration
CO 8-hour
9 ppm (10mg/m3)
35 ppm (40 mg/m3)
03 8-hour
0.08 ppm (157 mg/m3)
0.12 ppm (235mg/m3)
PM2.5 Annual
15 μg/m3
65 μg/m3
PM10 Annual
50 μg/m3
150 μg/m3

ppm = parts per million.
mg/m3 = milligram per meter cubed
μg/m3 = micrograms per meter cubed.

Updated: 3/17/2015
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