the sum of environmental conditions in a specific place that is occupied by an organism, population or community.


Health Assessment Document.


the time taken by certain materials to lose half their strength. For example, the half life of DDT is 15 years; of radium 1,580 years. The time required for half of the atoms of a radioactive element to undergo decay. The time required for the elimination of one half a total dose from the body.


any of a group of 5 chemically-related nonmetallic elements that includes bromine, fluorine, chlorine, iodine, and astatine.


bromine containing compounds with long atmospheric life- times whose breakdown in the stratosphere cause depletion of ozone. Halons are used in fire fighting.


a high-speed machine that uses hammers and cutters to crush, grind, chip, or shred solid wastes.


hazardous air pollutant; any of 190 air toxics identified for regulation under the CAA Amendments.


Hazardous Air Pollutant Enforcement Management System.


Hazardous Air Pollutant Prioritization System.


Hydrogeological Assessment Report.


a characteristic of water, imparted by salts of calcium, magnesium and iron, such as bicarbonates, carbonates, sulfates, chlorides and nitrates, that cause curdling of soap, deposition of scale, damage in some industrial processes and sometimes objectionable taste. It may be determined by a standard laboratory procedure or computed from the amounts of calcium and magnesium as well as iron, aluminum, manganese, barium, strontium and zinc, and is expressed as equivalent calcium carbonate.

hard water

alkaline water containing dissolved mineral salts, that interfere with some industrial processes and prevent soap from lathering.


Hazardous and Trace Emissions System.

hazard communication rule

see OSH Act. Requires chemical manufacturers and importers to assess the hazards associated with the materials in their workplace (29 CFR 1910, 1200).Material safety data sheets, labeling, and training are all results of this law. You are urged to acquire and become familiar with these regulations. Contact your local OSHA office.

hazard warning

Defined by OSHA as any words, pictures, symbols, or combination thereof appearing on a label or other appropriate form of warning which convey the hazard(s) of the chemical(s) in the container(s).

hazardous air pollutants

air pollutants which are not covered by ambient air quality standards but which, as defined in the Clean Air Act, may reasonably be expected to cause or contribute to irreversible illness or death. Such pollutants include asbestos, beryllium, mercury, benzene, coke oven emissions, radionuclides, and vinyl chloride.

hazardous chemical, material

in a broad sense, any substance or mixture of substances having properties capable of producing adverse effects on the health or safety of a human. In 1971 OSHA adopted the following definition in regulations affecting employers in operations subject to the Federal Longshoremen's and Harbor Worker's Compensation Act. "The term Hazardous Material means a material which has one or more of the following characteristics; (1) Has a flash point below 140 degrees F, closed cup, or is subject to spontaneous heating; (2) has a threshold limit value below 500 ppm for gases and vapors, below 500 mg/m3 for fumes, and below 25 mppcf (million particles per cubic foot) for dust; (3) Has a single dose oral LD50 below 50 mg/kg; (4) Is subject to polymerization with the release of large amounts of energy; (5) Is a strong oxidizing or reducing agent; (6) Causes first degree burns to skin (from a) short time exposure, or is systemically toxic by skin contact; or (7) In the course of normal operations may produce dusts, gases, fumes, vapors, mists, or smokes which have one or more of the above characteristics." Also included are substances that are carcinogens, toxic, irritants, corrosives, sensitizers, and agents which damage the lungs, skin, eyes, or mucous membranes.

hazardous decomposition

a breaking down or separation of a substance into its constituent parts, elements, or into simpler compounds accompanied by the release of heat, gas, or hazardous materials.

hazardous decomposition products

some materials give off hazardous materials when they decompose or burn.

hazard quotient

the ratio of a single substance exposure level over a specified time period (e.g., subchronic) to a reference dose for that substance derived from a similar exposure period).

hazardous air pollution

substances covered by Air Quality Criteria, which may cause or contribute to illness or death; asbestos, beryllium, mercury, and vinyl chloride.

Hazardous Ranking System

the principle screening tool used by EPA to evaluate risks to public health and the environment associated with abandoned or uncontrolled hazardous waste sites. The HRS calculates a score based on the potential of hazardous substances spreading from the site through the air, surface water, or ground water and on other factors such as nearby population. This score is the primary factor in deciding if the site should be on the National Priorities List and, if so, what ranking it should have compared to other sites on the list.

hazardous substance

1. any material that poses a threat to human health and/or the environment. Typical hazardous substances are toxic, corrosive, ignitable, explosive or chemically reactive. 2. any substance designated by EPA to be reported if a designated quantity of the substance is spilled in the waters of the United States or if otherwise emitted to the environment. As defined by section 101(14) of CERCLA, means: Any substance designated pursuant to section 311(b)(2)(A) of the CWA; any element, compound, mixture, solution, or substance designated pursuant to section 102 of CERCLA; any hazardous waste having the characteristics identified under or listed pursuant to section 3001 of the Solid Waste Disposal Act (but not including any waste the regulation of which under the Solid Waste Disposal Act has been suspended by Act of Congress); any toxic pollutant listed under section 307(a) of the CWA; any hazardous air pollutant listed under section 112 of the Clean Air Act; and any imminently hazardous chemical substance or mixture with respect to which the EPA Administrator has taken action pursuant to section 7 of the Toxic Substances Control Act.The term does not include petroleum, including crude oil or any fraction thereof which is not otherwise specifically listed or designated as a hazardous substance in the first sentence of this paragraph, and the term does not include natural gas, natural gas liquids, liquefied natural gas, or synthetic gas usable for fuel (or mixtures of natural gas and such synthetic gas).

hazardous substance UST system

means an underground storage tank system that contains a hazardous substance defined in section 101(14) of the Comprehensive Environmental Response, Compensation and Liability Act of 1980 (but not including any substance regulated as a hazardous waste under subtitle C) or any mixture of such substances and petroleum, and which is not a petroleum UST system.

hazardous waste

under RCRA, any solid, liquid or combination of solid or liquid wastes, which, because of its physical chemical or infectious characteristics, may pose a hazard when improperly managed.

hazardous waste number

an identification number assigned by the EPA, per the RCRA Law, to identify and track wastes. (40 CFR 261.33, 40 CFR 302.4).

hazards analysis

the procedures involved in: (1) identifying potential sources of release of hazardous materials from fixed facilities or transportation accidents; (2) determining the vulnerability of a geographical area to a release of hazardous materials; and (3) comparing hazards to determine which present greater or lesser risks to a community.

hazards identification

1. Providing information on which facilities have extremely hazardous substances, what those chemicals are, and how much there is at each facility. The process also provides information on how the chemicals are stored and whether they are used at high temperatures.2. The process of determining whether exposure to an agent can cause an increase in the incidence of a particular adverse health effect (e.g., cancer, birth defect) and whether the adverse health effect is likely to occur in humans.


Hazardous Material.


Hazard and Operability Study.


Hazardous Waste Operations and Emergency Response regulations; OSHA standards applying to hazardous waste site workers and emergency response personnel; covered in 29 CFR 1910.120.


Hispanic and Black Employment Programs.


Hazardous Constituents.


Hydrochlorofluorocarbon; family of substances temporarily allowed as CFC substitutes in refrigerants and industrial applications.


Hypothermal Coal Process.


Hazard Communication Standard; OSHA standard (29 CFR 1910.1200) requiring communication of hazardous materials risks to workers in regulated facilities.


Heavy Duty Vehicle.

head loss

the drop in pressure of a water flow through a resin bed.

health hazard

A chemical for which there is statistically significant evidence based on at least one study conducted in accordance with established scientific Principles that acute or chronic health effect may occur in exposed employees. The term "health hazard" includes chemicals which are carcinogens, toxic or highly toxic agents, reproductive toxins, irritants, corrosives, sensitizers, hepatotoxins, nephrotoxins, neurotoxins, agents which act on the hematopoietic system, and agents which damage the lungs, skin, eyes, or mucous membranes.

heat island effect

a haze dome created in cities by pollutants combining with the heat trapped in the spaces between tall buildings. This haze prevents natural cooling of air, and in the absence of strong winds can hold high concentrations of pollutants in one place.

heating oil

means petroleum that is No. 1, No. 2, No. 4--light, No. 4--heavy, No. 5--light, No. 5--heavy, and No. 6 technical grades of fuel oil; other residual fuel oils (including Navy Special Fuel Oil and Bunker C); and other fuels when used as substitutes for one of these fuel oils. Heating oil is typically used in the operation of heating equipment, boilers, or furnaces.

heating season

the coldest months of the year, when pollution increases in some areas because people burn fossil fuels to keep warm.

heat rate

an expression of heat-conversion to power, given in Btu/kWh. Theoretical conversion is 3413 Btu/kWh.

heavy metals

metals which can be precipitated by hydrogen sulfide in acid solution, e.g., lead, silver, gold, mercury, bismuth, copper, nickel, iron, chromium, zinc, cadmium and tin.


Human Exposure Modeling.


blood in the urine.

hemolysis, hemolytic

separation of the hemoglobin from red blood corpuscles.

Henry's law

an expression for calculating the solubility of a gas in a fluid based on temperature and partial pressure.


high-efficiency particulate air filter. Also called "absolute". Has a 99.97% removal efficiency for .03-micron particles.


pertaining to the liver.


an insecticide that was banned on some food products in 1975 and all of them 1978. It was allowed for use in seed treatment until in 1983. More recently is was found in milk and other dairy products in Arkansas and Missouri, as a result of illegally feeding treated seed to dairy cattle.


Prefix meaning the liver.


Chemicals which produce liver damage. EXAMPLE LAY LANGUAGE: (Potential) Liver toxins, may cause liver damage.


a chemical that controls or destroys undesirable plants.


an animal that feeds on plants.

heterotrophic organism

consumers such as humans and animals, and decomposers - chiefly bacteria and fungi - that are dependent on organic matter for food.




Fluoride Removal-Chelate (ETUS).


Highly Hazardous Chemical.


U.S. Department of Health and Human Services.


Higher Heating Value.


Household Hazardous Waste.


Hazard Index.

high density polyethylene

a material used to make plastic bottles that produces toxic fumes when burned.

high level radioactive waste (HLW)

waste generated in the fuel of a nuclear reactor, found at nuclear reactors or nuclear fuel reprocessing plants. It is a serious threat to anyone who comes near the wastes without shielding. (See Low-Level Radioactive Waste).

highly toxic chemical (poison)

A chemical falling within any of the following categories:(1) A chemical that has a median lethal dose (LD50) of 50 milligrams or less per kilogram of body weight when administered orally to albino rats weighing between 200 and 300 grams each.(2) A chemical that has a median lethal dose (LD50) of 200 milligrams or less per kilogram of body weight when administered by continuous contact for 24 hours (or less, if death occurs within 24 hours) with the bare skin of albino rabbits weighing between 2 and 3 kilograms each.(3) A chemical that has a median lethal concentration (LC50) of gas or vapor in air of 200 parts per million (ppm) or less by volume, or 2 milligrams per liter or less of mist, fume, or dust, when administered by continuous inhalation for 1 hour (or less, if death occurs within 1 hour) to albino rats weighing between 200 and 300 grams each, provided such concentration or condition, or both, are likely to be encountered by man when the chemical is used in any reasonably foreseeable manner.(4) A chemical that is a liquid having a saturated vapor concentration (ppm) at 68¿F (20 ¿C) equal to or greater than ten times it LC50 (vapor) value (ppm), if the LC50 value 1000 parts per million (ppm) or less when administered by continuous inhalation for 1 hour) to albino rats weighing between 200 and 300 grams each, provided such concentration, or condition, or both, are likely to be encountered by man when the chemical is used in any reasonably foreseeable manner.

hindered settling

a stage of settling where the accumulated settled solids have compacted to an extent that egress of water from the mass in hindered and, therefore, settling is slowed.

hi-volume samples

a device used to measure and analyze suspended particulate pollution.


High-Level Radioactive Waste.


Hazardous Materials Control Research Institute.


Hazardous Materials-Exercise Evaluation Methodology


the hazardous materials identification system, developed by NPCA to provide information on the acute health hazards, reactivity, and flammability encountered in the workplace at room temperatures.A number is assigned to a material indicating the degree of hazard, from 0 for the least up to 4 for the most severe. Letters designate personal protective equipment. (Details available from Labelmaster, 5742 N. Pulaski Rd., Chicago, IL 60646; [312] 478-0900). See NPCA.


Hazardous Materials Regulations; administered and enforced by various agencies of DOT governing transportation of hazardous materials by air, highway, rail, water and intermodal means.


Hazardous Materials Storage Ordinance.


Hazardous Materials Transportation Act (1974, 1990); federal law assigning authority to various agencies to enforce hazardous materials transportation regulations.


Hazardous Materials Transportation Regulations.


Hazardous Materials Transportation Uniform Safety Act (199); amendments to HMTA including requirements for performance-oriented packaging.


Halogenated Organic Compounds.


Hazardous Organic Constituents.

holding pond

a pond or reservoir usually made of earth built to store polluted runoff.


Hazardous Organic NESHAP.

hood capture efficiency

the emissions from a process which are captured by hood and directed into the control device, expressed as a percent of all emissions.


(1) in genetics, the organism, typically a bacterium, into which a gene from another organism is transplanted. (2) in medicine, an animal infected by or parasitized by another organism.


slang for radioactive material.

Hot Zone

refers to the area immediately surrounding a hazardous materials incident/accident that extends to such a distance as to prevent adverse effects from the release to personnel located outside of this zone. Also referred to as the exclusion zone of restricted zone.


High Occupancy Vehicle.


High Performance Liquid Chromatrography.


High Priority Violator.


Hazard Ranking System.


Hazardous Substance Data Base.


Hazardous Substance List.


Health and Safety Plan.


Hazardous and Solid Waste Amendments (1984); amendments to RCRA establishing a timetable for landfill bans and more stringent UST requirements.


Hypothermally Treated.


High Temperature and Pressure.


Housing and Urban Development.


the addition of water vapor to air.


decomposed organic material.


Heating, Ventilation, and Air Conditioning.


High Volume Industrial Organics.


Hazardous Waste Action Coalition.


Hazardous Waste Control Law.


Hazardous Waste Data Management System.


Hazardous Waste Engineering Research Laboratory.


Hazardous Waste Ground Water Task Force.


Hazardous Waste Ground Water Test Facility.


Hazardous Waste Information System.


Hazardous Waste Land Treatment.


Hazardous Waste Management.


Hazardous Waste Restrictions Task Force.


Hazardous Waste Treatment Council.


a cell or organism resulting from a cross between two unlike plant or animal cells or organisms.


a hybrid cell that produces monoclonal antibodies in large quantities.


the chemical combination of water into a substance.

hydraulic lift tank

means a tank holding hydraulic fluid for a closed-loop mechanical system that uses compressed air or hydraulic fluid to operate lifts, elevators, and other similar devices.


a movement or action resulting from liquid flow.


compounds found in fossil fuels, that contain carbon and hydrogen and may be carcinogenic.

hydrogen sulfide

the gas emitted during organic decomposition that smells like rotten eggs. It is also a byproduct of oil refining and burning and can cause illness in heavy concentrations.


the geology of ground water, with particular emphasis on the chemistry and movement of water.

hydrologic cycle

the water cycle, including precipitation of water from the atmosphere as rain or snow, flow of water over or through the earth and evaporation or transpiration to water vapor in the atmosphere.


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


the splitting of a salt and water into its ions and formation of a weak acid or base, or both. As in ion exchange: R-NH4+OH- -> R-H+ + NH4+OH- where R- is a WAC resin.


a device to measure specific gravity of fluids.


having an affinity for water. Its opposite, non- water-wettable, is hydrophobic. Describing materials having large molecules that tend to absorb and retain water, causing them to swell and frequently to become gels. See Deliquescent.


the anion of water or OH-, also present in all hydroxides.


readily adsorbing available moisture in any form.See Deliquescent.


congestion of blood in a body part.


self-igniting upon contact of its components without a spark or external aid; especially rocket fuel or a propellant that consists of combinations of fuels and oxidizers.


calcium deficiency of the blood.


insufficient oxygen, especially applied to body cells.See anoxia.




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