Half-Life
1. The time required for a pollutant to lose half its affect on the environment. For example, the biochemical half-life of DDT in the environment is 15 years of Radium. 1,580 years. 2. The time required for half of the atoms of a radioactive element to undergo self-transmutation or decay. 3.The time required for the elimination of one half a total dose from the body.

Halon
Bromine-containing compounds with long atmospheric lifetimes whose breakdown in the stratosphere causes depletion of ozone. Halons are used in fire-fighting.

Hazard
A situation or activity that presents a risk.

Hazardous Waste Landfill
An excavated or engineered site where hazardous waste is deposited and covered.

Hazardous Waste
By-products of society that can pose a substantial or potential hazard to human health or the environment when improperly managed.

Headspace
The vapour mixture trapped above a solid or liquid in a sealed vessel.

Heavy Metals
Metallic elements with high atomic weights; (e.g., mercury, chromium, cadmium, arsenic, and lead); can damage living things at low concentrations and tend to accumulate in the food chain.

Herbicide
A chemical pesticide designed to control or destroy plants, weeds, or grasses.
High-Density Polyethylene: A material used to make plastic bottles and other products that produces toxic fumes when burned.

HSE
Health and Safety Executive.

Hydrocarbons (HC)
Chemical compounds that consist entirely of carbon and hydrogen.

Hydrogen Sulphide (H2S)
Gas emitted during organic decomposition. Also a by-product of oil refining and combustion. Smells like rotten eggs and, in heavy concentration, can kill or cause illness.

Hydrology
The science dealing with the properties, distribution, and circulation of water.

Hydrophilic
Having a strong affinity for water.

Hydrophobic
Having a strong aversion for water.