The Boston Harbor Islands are in a class II area as defined by the Clean Air Act. The state may permit a moderate amount of new air pollution (sulfur dioxide, particulate matter, and nitrogen oxides) as long as neither national ambient air quality standards nor the maximum allowable increases (increments) over established baseline concentrations are exceeded. The islands are part of the Boston Metropolitan Air Quality Region within which there are several large stationary and mobile sources of contaminants, including Mystic and Salem Generating Stations. Air quality in this region is also affected by air pollution transported into the region. The Boston Metropolitan Air Quality Region does not meet EPA standards for ozone or carbon monoxide.

The major pollutants originating from the park are boat emissions (primary hydrocarbons, carbon monoxide, nitrogen oxides) and particulates, most of which are generated during the summer months. A variety of particulates (salt from seawater, diatoms and other plankton, silt and clay particles from erosion and wave action) are produced as part of natural processes in the marine environment. Minor additional amounts of particulates are produced by human activities on the islands. No monitoring of air quality or visibility is conducted at the park, and there is no assessment of air quality–related impacts on island resources. The state Department of Environmental Protection monitors air quality in the region. The National Park Service does monitor ozone and acid deposition at Cape Cod National Seashore 60 miles to the southeast of Boston.

The Boston Harbor Islands are in a class II area as defined by the Clean Air Act. The state may permit a moderate amount of new air pollution (sulfur dioxide, particulate matter, and nitrogen oxides) as long as neither national ambient air quality standards nor the maximum allowable increases (increments) over established baseline concentrations are exceeded. The islands are part of the Boston Metropolitan Air Quality Region within which there are several large stationary and mobile sources of contaminants, including Mystic and Salem Generating Stations. Air quality in this region is also affected by air pollution transported into the region. The Boston Metropolitan Air Quality Region does not meet EPA standards for ozone or carbon monoxide.

The major pollutants originating from the park are boat emissions (primary hydrocarbons, carbon monoxide, nitrogen oxides) and particulates, most of which are generated during the summer months. A variety of particulates (salt from seawater, diatoms and other plankton, silt and clay particles from erosion and wave action) are produced as part of natural processes in the marine environment. Minor additional amounts of particulates are produced by human activities on the islands. No monitoring of air quality or visibility is conducted at the park, and there is no assessment of air quality–related impacts on island resources. The state Department of Environmental Protection monitors air quality in the region. The National Park Service does monitor ozone and acid deposition at Cape Cod National Seashore 60 miles to the southeast of Boston.