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habitat: The place where a plant or animal species naturally lives and grows.
hazard class: One of 9 categories of hazardous materials used in DOT placards (DOT hazard label). The hazard class indicates the most important hazard of a given material (e.g., Explosives or Poison Gas). While some materials meet the criteria for more than one class, each material is assigned just one class.
hazardous chemical: Any chemical that is a physical or health hazard as defined in 29 CFR § 1910.1200(c).
hazardous material: Any substance or material in a quantity or form that may be harmful to humans, animals, crops, water systems, or other elements of the environment, if accidentally or intentionally released. Hazardous materials include: explosives, gases (compressed, liquefied, or dissolved), flammable and combustible liquids, flammable solids or substances, oxidizing substances, poisonous and infectious substances, radioactive materials, and corrosives.
Hazardous Materials Response Division (HAZMAT): See Emergency Response Division (ERD).
hazardous substances: Substances designated as hazardous under CERCLA. CERCLA includes substances listed under the Clean Water Act, the Clean Air Act, the Resource Conservation and Recovery Act (RCRA), and the Toxic Substances Control Act (TSCA) Section 7. See also CERCLA.
HAZMAT: See Emergency Response Division (ERD).
heavy crude: Crude oil with a high specific gravity and a low API gravity due to the presence of a high proportion of heavy hydrocarbon fractions and metallic content. Contrast with light crude. See also crude oil.
hectare: One hectare is equal to 10,000 square meters, or 2.471 acres.
herding agent: A chemical agent that confines or controls the spread of a floating oil film by increasing its viscosity.
high explosive: Or primary explosive. An explosive that is readily detonated by heat, friction, or shock. High explosives vary widely in their sensitivity to these forms of initiating energy.
high tide (high water): The highest level reached by the water during one tidal cycle. Contrast with low tide.
highly flammable: Designation applying to (a) substances with flash points below 100° F, and (b) mixtures that include substances with flash points below 100° F. Materials designated as highly flammable include pyrophoric solids and substances that present an exceptional flammability hazard, in that they may suddenly and dangerously increase the intensity of a fire.
humidity: At a given temperature, the ratio of water vapor in the air to the maximum amount that can be held in the air at that temperature.
hydraulic dispersion: One of various shoreline cleanup techniques which utilizes a water stream at either low or high pressure to remove stranded oil. These techniques are most suited to removal of oil from coarse sediments, rocks and man-made structures, although care must be taken to avoid damage to intertidal flora and fauna.
hydrocarbons: A large class of molecules containing only carbon and hydrogen. Common in petroleum products and other oils. The largest source of hydrocarbons is petroleum crude oil.
hydrophilic: Lit. "Water-loving." Refers to substances that attract and retain water, and to wettable solids whose surfaces readily attract water. Contrast with hydrophobic.
hydrophobic: Lit. "Water hating." Refers to substances that repel water and are not easily wetted or emulsified (fats, waxes, oils, metal powders, and many inorganic compounds). Contrast with hydrophilic. To be useful in combating oil spills, sorbents need to be both oleophilic (oil-attracting) and hydrophobic (water-repellent).

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