Hazardous air pollutants (HAPs) – chemicals that cause serious health and environmental effects. Health effects include cancer, birth defects, nervous system problems and death due to massive accidental releases such as occurred at the pesticide plant in Bhopal, India. HAPs are released by sources such as chemical plants, dry cleaners, printing plants, and motor vehicles (cars, trucks, buses, etc.)

HDDE – means a heavey-duty diesel engine.  A diesel engine that is used to propel a heavy-duty diesel vehicle.

HDDV – means heavy-duty diesel vehicle. A motor vehicle with a GVWR greater than 14,000 pounds that is equipped with a heavy-duty diesel engine.

HDGV – means heavy-duty gasoline vehicle. A gasoline-fueled motor vehicle that has a GVWR exceeding 8,500 pounds and is designed primarily for transporting persons or property.

HP – means horsepower.

Hydrocarbons (HC) – means any compound or mixture of compounds whose molecules consist of atoms of hydrogen and carbon only.