Toxic air pollutants, also known as air toxics, are pollutants that, at sufficient concentrations and exposure, are known or suspected to cause cancer or other serious health effects, such as reproductive effects or birth defects, or to cause adverse environmental effects.

Generally, the toxic air pollutants of greatest concern are those released to the air in amounts large enough to create a risk to human health or where many people are likely to be exposed.

There are three main types of air toxics:

  • Gases including benzene, toluene and xylenes (all found in gasoline)
  • Liquid aerosols such as perchloroethylene (used in dry cleaning) and methylene chloride (an industrial solvent)
  • Particles including heavy metals - cadmium, chromium, lead and mercury, for example - and polycyclic aromatic hydrocarbons formed when fossil fuels and waste materials are burned

Some air toxics are released from natural sources, including volcanic eruptions and forest fires, but most of these pollutants come from human activity:

  • On-Road Mobile Sources including cars, trucks and buses
  • Off-Road Mobile Sources such as outdoor power equipment, recreational vehicles, farm and construction equipment, boats, and locomotives
  • Stationary Sources including factories, refineries and power plants
  • Indoor Sources such as building materials and cleaning solvents

Air toxics tend to be highly transportable, arriving in Massachusetts on prevailing winds from other states.

In all, the U.S. Environmental Protection Agency (EPA) lists 188 toxic air pollutants that can have significant impacts on air quality and human health even when emissions of these toxics are controlled through available technology.

Health Effects of Air Toxics & Who is Most at Risk

Short-term exposures can include effects such as eye irritation, nausea or difficulty in breathing. Long-term exposures may result in damage to the respiratory, nervous or reproductive systems, birth and developmental defects, and other serious health problems.

While everyone is at risk of health problems from exposure to air toxics, many factors determine the extent to which different pollutants affect the health of any individual or at-risk population, including the level, duration and frequency of exposure, the toxicity of the pollutant, overall health, and level of resistance or susceptibility.

The U.S. Environmental Protection Agency (EPA) maintains a Health Effects Notebook on the 188 air toxics it lists as being of most significant concern.

Environmental Effects of Air Toxics

Some toxic air pollutants, such as mercury, can deposit onto soils or surface waters, where they are taken up by plants, ingested by animals and fish, and work their way up the food chain. Like humans, animals may experience health problems if exposed to sufficient quantities of air toxics over time.

Air Toxics Standards

EPA has established health-based National Emission Standards for Hazardous Air Pollutants (NESHAPs) addressing 188 air toxics. MassDEP, meanwhile, has developed recommended Ambient Air Exposure Limits for Chemicals which, in the some instances, are more stringent than NESHAPs.

Long-Term Air Toxics Trends

MassDEP uses photochemical analyzers to seasonally monitor for certain air toxics at six locations across Massachusetts. The figure below shows a steady decline in air concentrations of four target pollutants at the agency's Lynn monitor over an eight-year period.


Lynn Toxics Summary 1994-2007 24-hour samples


Since 2003, MassDEP also has operated a National Air Toxics Trends Station (NATTS) in Boston that is designed to collect and quantify a number of toxic air pollutants including aldehydes, black carbon, metals and volatile organic compounds (VOCs). Data from this site are compared with information collected by a network of similar sites positioned across the country to identify transport, trends and site-specific characteristics of these pollutants.

EPA conducts a periodic national air toxics assessment (NATA) focused on air toxics that are of greatest concern to public health. This assessment has included modeling of emissions, concentrations, exposure and risk for 33 air toxics that present significant health threats in numerous urban areas.

According to NATA, air toxics found at occasionally unhealthy levels in specific Massachusetts locations include benzene, 1,3-butadiene, dioxin and perchloroethylene. There also are unhealthy levels of mercury in fish in Massachusetts lakes and ponds, primarily due to deposition of mercury in the air from sources both within and outside of our state.

Specific results for Massachusetts can be viewed and mapped by state and county at the NATA web site. Information about air, water and land releases of toxic chemicals from manufacturing facilities is available from EPA's Toxics Release Inventory database, which is searchable by Zip Code. Information about toxic chemical use, as well as releases, is available from the Massachusetts Toxics Use Reduction Act database , which is searchable by community.