Air pollution doesn’t just cloud the air we breathe: it can also harm our land and water. What goes up must come down, and pollution released into the air—by cars, trucks, gas-powered lawn tools, power plants and other sources—falls back to the earth’s surface, where it can wind up in our waterways. Maintaining the forests that absorb airborne pollutants and enacting regulations to reduce emissions are two ways to reduce air pollution across the watershed.

How does air pollution harm the Chesapeake Bay?

Polluted air can have quite an impact on the health of local waters: scientists estimate that one third of the nitrogen in the Bay comes from the air through a process known as atmospheric deposition. When our cars, power plants or other sources emit air pollution, it can be carried by wind and weather over long distances until it falls onto land or directly into the water.

Atmospheric deposition occurs in several stages.

  • First, pollution is emitted into the air, where wind and weather can carry it over long distances.
  • Eventually, airborne pollution particles fall onto the land or into the water, sometimes in the form of dry particles and sometimes attached to rain, snow or other precipitation.
  • Even pollution that falls onto the land—rather than straight onto the water’s surface—can pollute our water, if it soaks into groundwater or if it is washed off of roofs, streets and sidewalks and into storm drains, rivers and streams.

Even pollution emitted thousands of miles away can eventually end up in our waterways. The area of land over which airborne pollutants can travel to reach the Bay is known as the airshed. The Bay’s airshed is quite large: approximately 570,000 square miles, nine times as large as the watershed itself.

What are the sources of air pollution?

There are four sources of air pollution in the Chesapeake Bay region: stationary and area sources; mobile sources; agricultural sources; and natural sources.

Stationary and area sources

Stationary and area sources are “point sources” of air pollution: they have a fixed location and do not move. Stationary sources are large, consistent sources of air pollution, like power plants or chemical or manufacturing facilities. Area sources are smaller sources of air pollution that are often clustered near each other, like dry cleaners or gas stations.

Mobile sources

Mobile sources of air pollution move. Collectively, these sources—which include cars, trucks and off-road vehicles; boats; airplanes; gas-powered lawn tools; and farm and construction equipment—can produce a significant amount of air pollution.

Agricultural sources

Agricultural sources of air pollution include those farm operations that emit gases, chemicals or particulate matter into the air. Livestock and poultry operations, for instance, often produce ammonia, which is released into the air from animal manure. Exposure to airborne ammonia can irritate our eyes and lungs, and the settling of ammonia onto the earth’s surface can boost nutrient levels on land and in water.

Natural sources

Natural sources of air pollution are those not caused by human activities. These include lightning, dust storms, forest fires and erupting volcanoes.

What airborne pollutants are affecting Bay health?

Nitrogen and chemical contaminants are two kinds of airborne pollutants affecting water quality in the Chesapeake Bay and its tributaries.


Airborne nitrogen is one of the largest sources of pollution affecting the Chesapeake Bay and its tributaries. Excess nitrogen can fuel the growth of algae blooms, which can block sunlight from reaching underwater grasses and create low-oxygen “dead zones” that suffocate marine life.

Scientists estimate that just over one-third of the nitrogen polluting the Bay comes from the air, most often in the form of nitrogen oxides or ammonia:

  • Nitrogen oxides (or NOx) are produced by machines or processes that are powered by gas, coal or oil, like the running of a car or the heating of a building.
  • Ammonia emissions are most often generated by livestock or poultry operations.
  • Pollution from nitrogen oxides is decreasing in response to the Clean Air Act. In 2000, nitrogen oxides accounted for three-quarters of the airborne nitrogen that was polluting the Bay, and they were a big contributor to ground-level ozone pollution. By 2017 nitrogen oxides accounted for half of airborne nitrogen pollution, with ammonia accounting for the remaining half.

Chemical contaminants

The three most common chemical contaminants that are polluting the Chesapeake Bay airshed include mercury, polychlorinated biphenyls (or PCBs) and polycyclic aromatic hydrocarbons (or PAHs):

  • Mercury is a toxic metal that is released into the air when coal, oil, natural gas and hazardous materials are burned.
  • PCBs are a group of chemicals once used as dielectric and coolant fluids in electrical equipment. While the production of PCBs was banned in the United States in 1977, PCBs continue to persist in the environment.
  • PAHs are a group of chemicals that are released into the air when coal, oil, gasoline and other fossil fuels are burned.

Once in the water, these chemical contaminants can bind to sediment and enter the bodies of small, bottom-dwelling organisms like worms, clams or small crustaceans. Through a process known as bioaccumulation, fish that consume contaminated organisms can accumulate these toxins in their tissues. Because humans that eat contaminated fish can also be exposed to these chemicals, fish consumption advisories are issued in areas where chemical contaminants are a concern.

How can we reduce air pollution?

To clean up the Chesapeake Bay, we must also clean up our air. While emissions of airborne pollutants are falling, more reductions are needed to meet pollution-reduction goals for the Bay and its tributaries. Maintaining the forests that absorb airborne pollutants and enacting regulations to reduce emissions from our vehicles and power plants are two ways that we can reduce air pollution across the watershed.

  • Through a process known as attenuation, forests can reduce the amount of pollutants in our air. Trees act like “attenuation machines,” as their roots and leaves and forest soils absorb and trap airborne pollutants. Forests can capture more than 85 percent of the nitrogen that falls onto them from the air. In urban areas, tree cover can lower summertime temperatures and reduce the generation of ozone and other harmful pollutants.
  • New state and federal regulations, as well as new and improved technologies, are reducing emissions from vehicles and power plants. However, emissions of ammonia from agricultural operations have remained constant. As stronger laws take effect and on-the-ground restoration work continues, air pollution rates are expected to decline.

What you can do

To lower air pollution in the Bay watershed, consider walking, biking or taking public transportation when possible; or using electric or manual lawn mowers and yard tools instead of gas-powered machines.