Transportation infrastructure investment influences emissions of air pollution, primar-ily by affecting the number of vehicle-miles traveled in the region. Nationwide, vehicle sources are responsible for about 75 percent of carbon monoxide pollution and at least half of emitted ozone precursors. Long-term exposure to high levels of air pollution can affect the natural and built environments, as well as harm health by damaging the immune, neurological, reproductive, and respiratory systems.

In response to public health concerns, the federal government has developed air quality standards for specific air pollutants. The 1990 Clean Air Act requires MPOs to perform an air quality conformity determination for any pollutant for which the region has not attained the standard (or has only recently attained it).

How is air quality conformity determined?
To determine conformity, the MPO must show that the changes in emissions expected to result from implementation of its Transportation Plan will not exceed the mobile-source emission “budget” that has been set by the Massachusetts Department of Environmental Protection.

To do this, the MPO forecasts travel behavior and associated emissions using its regional transportation model. Currently, the MPO is required to demonstrate conformity for two ozone precursors—nitrogen oxides and volatile organic compounds—and for carbon monoxide (for which the region has recently attained the standard).

In addition, the MPO must confirm that all appropriate transportation projects and programs outlined in the Commonwealth’s air quality plan are included in the region’s Transportation Plan and show they will be implemented in a timely manner.

Air Pollutants, Air Toxics, and Greenhouse Gases

Air pollutants
The U.S. Environmental Protection Agency (EPA) has set national air quality standards for six common pollutants:

Carbon monoxide (CO) is a colorless, odorless gas that reduces oxygen delivery to the body. As much as 95 percent of the CO in a typical U.S. city comes from mobile sources.

Ozone is not directly emitted from a specific source: it forms when volatile organic compounds chemically react with nitrogen oxides in the presence of sunlight and warm temperatures.

Mobile sources are responsible for more than half of nitrogen oxide (NOx) emissions.

Particulate matter consists of very tiny solid or liquid particles. Diesel-powered engines contribute more than half the mobile-source particulate emissions.

Over 65 percent of the sulfur dioxide (SO2) released to the air comes from electric utilities. Other sources include petroleum refineries and cement factories.

Levels of lead in the air have decreased dramatically since 1978, due to the phase-out of leaded gasoline.

Air toxics
EPA has identified 188 air toxics, which are known or suspected to cause cancer or other serious health effects, or certain adverse environmental effects. The air toxics include benzene (found in gasoline), formaldehyde, dioxin, asbestos, and heavy metals.

Greenhouse gases
Emissions of greenhouse gases—primarily carbon dioxide, methane, and nitrous oxide—enhance the heat-trapping capability of the earth’s atmosphere. Increases in these gases have resulted in the earth’s surface temperature rising by about one degree Fahrenheit in the last century, with accelerated warming over the past two decades.