heavy metal

Naturally occuring substance; a metal, usually poisonous.

HEPA

Used in vacuums and air purifiers, HEPA filters are highly effective at blocking particles as small as 0.3 microns (a micron is 0.0004 inches), which can irritate airways or cause an allergic reaction. Equipment must be built to accommodate HEPA filters. They cannot be installed on non-HEPA vacuums, air purifiers and air conditioners, as air does not flow easily through them and can cause motors to burn out.

herbicide

A substance that kills plants. Some herbicides target specific types of weeds. Others generally kill all plants.

Hydrocarbon

Any compound consisting only of hydrogen and carbon atoms. Petroleum consists of many different hydrocarbon compounds connected together on a “chain” . Breaking this chain apart separates these various hydrocarbon compounds for use as the basis of synthetic chemicals.

Hydrochloric Acid

(also muriatic acid)

A strong mineral or "inorganic" acid. In high concentrations, it is extremely corrosive.

Found in: Conventional toilet bowl cleaners.

Hydrogen Chloride

(See hydrochloric acid)

Hypochlorite

(See chlorine)