Science Terms Definitions - H

Habitat Indicator - A physical attribute of the environment measured to characterize conditions necessary to support an organism, population, or community in the absence of pollutants, e.g., salinity of esturine waters or substrate type in streams or lakes.

Half-life - The time in which half the atoms of a radioactive substance will have disintegrated, leaving half the original amount. Half of the residue will disintegrate in another equal period of time.

Hazard - A condition or physical situation with a potential for an undesirable consequence, such as harm to life or limb.

Hazard Assessment - An analysis and evaluation of the physical, chemical and biological properties of the hazard.

Hazard Evaluation - A component of risk evaluation that involves gathering and evaluating data on the types of health injury or disease that may be produced by a chemical and on the conditions of exposure under which such health effects are produced.

Hazard Identification - The process of determining whether exposure to an agent can cause an increase in the incidence of a health condition.

Hazard Ranking System (HRS) - 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 groundwater, and on other factors such as density and proximity of human 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 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, radio nuclides, and vinyl chloride.

Hazardous and Solid Waste Amendments (HSWA) - This 1984 Act amended RCRA and required phasing out land disposal of untreated hazardous waste by more stringent hazardous waste management standards (broken down into thirds with a time table for each third). Some of the other mandates of this law include increased enforcement authority for EPA and a program requiring corrective action.

Hazardous Chemical - An EPA designation for any hazardous material requiring an MSDS under OSHA's Hazard Communication Standard. Such substances are capable of producing fires and explosions or adverse health effects like cancer and dermatitis. Hazardous chemicals are distinct from hazardous waste.

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 released into the environment.

Hazardous Waste (HAZ) - HAZ is waste regulated under the Resource Conservation and Recovery Act (RCRA). RCRA regulates solid waste, hazardous waste, and Underground Storage Tanks (USTs) holding petroleum or certain chemicals. Waste that is ignitable, corrosive, reactive, toxic, or contains certain amounts of toxic chemicals is considered hazardous according to the RCRA definition. In Oak Ridge the term Hazardous Waste also included wastes regulated under the Toxic Substances Control Act (TSCA). These are wastes that are contaminated with polychlorobiphenyls (PCB's) or asbestos. When the term Hazardous Waste is used, it implies that the material can be certified NOT to be contaminated with radioactive material, otherwise the term Mixed Waste is used.

Hazards Analysis - Procedures used to (1) identify potential sources of release of hazardous materials from fixed facilities or transportation accidents; (2) determine the vulnerability of a geographical area to a release of hazardous materials; and (3) compare hazards to determine which present greater or lesser risks to a community.

Hazards Identification - Providing information on which facilities have extremely hazardous substances, what those chemicals are, how much there is at each facility, how the chemicals are stored, and whether they are used at high temperatures.

Health Advisory Level - A non-regulatory health-based reference level of chemical traces (usually in ppm) in drinking water at which there are no adverse health risks when ingested over various periods of time. Such levels are established for one day, 10 days, long term and life-time exposure periods. They contain a large margin of safety.

Health and Safety Study - Any study of any effect of a chemical substance or mixture on health or the environment or on both, including underlying data and epidemiological studies, studies of occupational exposure to a chemical substance or mixture, toxicological, clinical, and ecological studies of a chemical substance or mixture, and any test performed pursuant to the Toxic Substances Control Act (TSCA).

Health Assessment - An evaluation of available data on existing or potential risks to human health posed by a Superfund site. The Agency for Toxic Substances and Disease Registry (ATSDR) of the Department of Health and Human Services (DHHS) is required to perform such an assessment at every site on the National Priorities List.

Health Effect - A deviation in the normal function of the human body.

Health Effect Assessment - The component of risk assessment which determines the probability of a health effect given a particular level or range of exposure to a hazard.

Health Hazard (types of) -

1. Acute toxicity: The older term used to describe immediate toxicity. Its former use was associated with toxic effects that were severe (e.g., mortality) in contrast to the term "subacute toxicity" that was associated with toxic effects that were less severe. The term "acute toxicity" is often confused with that of acute exposure.

2. Allergic reaction: Adverse reaction to a chemical resulting from previous sensitization to that chemical or to a structurally similar one.

3. Chronic toxicity: The older term used to describe delayed toxicity. However, the term "chronic toxicity" also refers to effects that persist over a long period of time whether or not they occur immediately or are delayed. The term "chronic toxicity" is often confused with that of chronic exposure.

4. Idiosyncratic reaction: A genetically determined abnormal reactivity to a chemical.

5. Immediate versus delayed toxicity: Immediate effects occur or develop rapidly after a single administration of a substance, while delayed effects are those that occur after the lapse of some time. These effects have also been referred to as acute and chronic, respectively.

6. Reversible versus irreversible toxicity: Reversible toxic effects are those that can be repaired, usually by a specific tissue's ability to regenerate or mend itself after chemical exposure, while irreversible toxic effects are those that cannot be repaired.

7. Local versus systemic toxicity: Local effects refer to those that occur at the site of first contact between the biological system and the toxicant; systemic effects are those that are elicited after absorption and distribution of the toxicant from its entry point to a distant site.

Health Physics - The science concerned with the recognition, evaluation, and control of health hazards which may arise from the use and application of ionizing radiation.

Heavy Metals - Metallic elements like mercury, chromium, cadmium, arsenic, and lead, with high molecular weights. They can damage living things at low concentrations and tend to accumulate in the food chain.

HEU - (highly enriched uranium): An important fissile material that has been used for nuclear weapons, usually defined as uranium whose proportion of uranium-235, the fissile isotope of uranium, has been increased to over 90%.

HI (hazard index) - Potential noncarcinogenic (systemic) effects are characterized by comparing projected intakes of chemicals to toxicity values (i.e, reference doses). The numerical risk or hazard quotient estimates that results is a ratio. The ratio of the intake over the reference dose (hazard index) is compared to unity (1.0). If the quotient is less than 1, then the systemic effects are assumed not to be of concern; if the hazard quotient is greater than 1, then the systemic effects are assumed to be of concern. The hazard index is the sum of hazard quotients.

High Risk Community - A community located within the vicinity of numerous sites or facilities or other potential sources of environmental exposure/health hazards which may result in high levels of exposure to contaminants or pollutants. In determining risk or potential risk, factors such as total weight of toxic contaminants, toxicity, routes of exposure, and other factors may be used.

High-to-Low Dose Extrapolation - The process of prediction of low exposure risk to humans from the measured high exposure-high risk data involving rodents.

HLW (high level waste) - Generally the highly radioactive material resulting from reprocessing of spent nuclear fuel. This includes mainly the liquid wastes remaining from the recovery of uranium and plutonium in a fuel reprocessing plant. This HLW may also be in the form of sludge, calcine, or other products into which such liquid wastes are converted to facilitate their handling and storage. Such waste contains fission products and traces of TRUs that result in the release of considerable decay energy. For this reason, heavy shielding is required to absorb penetrating radiation, and cooling systems are needed to dissipate decay heat from HLW. Currently, no HLW exists on the Oak Ridge Reservation.

Holding Pond - A pond or reservoir, usually made of earth, built to store polluted runoff.

Homeostasis - A tendency to stability in the normal body states of the organism.

Hormesis - The notion that small doses of radiation can be healthful.

Hot - A colloquial term meaning highly radioactive.

Hot Spot - The region in a radiation/ contamination area in which the level of radiation/contamination is significantly greater than in neighboring regions in the area.

Human Equivalent Concentration - Exposure concentration for humans that has been adjusted for dosimetric differences between experimental animal species and humans to be equivalent to the exposure concentration associated with observed effects in the experimental animal species. If occupational human exposures are used for extrapolation, the human equivalent concentration represents the equivalent human exposure concentration adjusted to a continuous basis.

Human Equivalent Dose - A dose which, when administered to humans, produces an effect equal to that produced by a dose in animals.

Human Exposure Evaluation - Describing the nature and size of the population exposed to a substance and the magnitude and duration of their exposure. The evaluation could concern past, current, or anticipated exposures.

Human Health Risk - The likelihood that a given exposure or series of exposures may have or will damage the health of individuals.

Hydrogeologic Cycle - The natural process recycling water from the atmosphere down to (and through) the earth and back to the atmosphere again.

Hydrogeology - The geology of groundwater, with particular emphasis on the chemistry and movement of water.

Hydrologic Cycle - Movement or exchange of water between the atmosphere and the earth.

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