Forest Trail


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Hazardous Air Pollutant (HAP)

An air pollutant listed under section 112 (b) of the Federal Clean Air Act as particularly hazardous to health. Emission sources of hazardous air pollutants are identified by U.S. EPA and emission standards are set accordingly. For more information, visit our Title III website area.

Haze (Hazy)

A phenomenon that results in reduced visibility due to the scattering of light caused by aerosols. Haze is caused in large part by man-made air pollutants.

Health Risk Assessment (HRA)

A document that identifies the risks and quantities of possible adverse health effects that may result from exposure to emissions of toxic air contaminants. A health risk assessment cannot predict specific health effects; it only describes the increased possibility of adverse health effects based on the best scientific information available.

Health-Based Standard (Primary Standard)

A dosage of air pollution scientifically determined to protect against human health effects such as asthma, emphysema and cancer.

Heavy-Duty Vehicle Inspection Program (HDVIP)

This regulation authorizes random roadside smoke opacity testing of heavy-duty diesel trucks and buses. The opacity of exhaust emitted from these engines must not exceed 40 percent (1991 and newer engine model years) or 55 percent (all pre-1991 engines). Gasoline and diesel trucks and buses are also inspected for tampering and for engine certification label compliance.

Hot Spot

(See toxic hot spot.)

Hybrid Electric Vehicle (HEV)

A vehicle that combines an internal combustion engine with a battery and electric motor. This combination offers the range and refueling capabilities of a conventional vehicle, while providing improved fuel economy and lower emissions.


Compounds containing various combinations of hydrogen and carbon atoms. They may be emitted into the air by natural sources (e.g., trees) and as a result of fossil and vegetative fuel combustion, fuel volatilization and solvent use. Hydrocarbons are a major contributor to smog. (See also Reactive Organic Gases).

Hydrogen Sulfide (H2S)

A colorless, flammable, poisonous compound having a characteristic rotten-egg odor. It is used in industrial processes and may be emitted into the air.