Science Terms Definitions - L

Land Farming (of waste) - A disposal process in which hazardous waste deposited on or in the soil is degraded naturally by microbes.

Landfills - Sanitary landfills are disposal sites for non-hazardous solid wastes spread in layers, compacted to the smallest practical volume, and covered by material applied at the end of each operating day. Secure chemical landfills are disposal sites for hazardous waste, selected and designed to minimize the chance of release of hazardous substances into the environment.

Latency Period - The period of time from exposure to an agent to the onset of a health effect.

LC50 - Lethal concentration fifty. A calculated concentration [in air] which when administered by the respiratory route is expected to kill 50% of a population of experimental animals during an exposure of four hours. Ambient concentration is expressed in milligrams per liter. A calculated concentration in water which is expected to kill 50% of a population of aquatic organisms after a specified time of exposure.

LD50 - Lethal dose fifty. A calculated dose of a chemical substance which is expected to kill 50% of a population of experimental animals exposed through a route other than respiration. Dose is expressed in milligrams per kilogram of body weight.

Leachate - Liquid that has percolated through solid waste and has extracted dissolved or suspended materials from it.

Leaching - The process by which nutrient chemicals or contaminants are dissolved and carried away by water, or are moved into a lower layer of soil.

Lethal dose - (Lethal dose 50/30) The dose of radiation expected to cause death to an exposed population within 30 days to 50 percent (LD 50/30) of those exposed. Typically, the LD 50/30 is in the range from 400 to 450 rem (4 to 5 sieverts) received over a very short period of time.

LEU (low enriched uranium) - Most nuclear reactors run on LEU, which is usually 3%-5% uranium-235. LEU cannot be used in nuclear weapons.

Level of Concern - The concentration in air of an extremely hazardous substance above which there may be serious immediate health effects to anyone exposed to it for short periods.

Lifetime Exposure - Total amount of exposure to a substance that a human would receive in a lifetime (usually assumed to be 70 years).

Liquid Waste - Any waste material that is determined to contain "free liquids" as defined by Method 9095 (Paint and Filter Liquid Test), as described in "Test Methods for Evaluating Solid Wastes, Physical/Chemical Methods," Environmental Protection Agency (EPA), Pub. No. SW-846, November 1986. Liquid waste may then be classified as controlled, uncontrolled, or sanitary.

Listed Waste - Wastes listed as hazardous under RCRA but which have not been subjected to the Toxic Characteristics Listing Process because the dangers they present are considered self-evident.

LLW (low level waste) - Is radioactive waste that is more easily described by what it is not. This is even true of the Policies Acts and Regulations that define LLW as radioactive material that is not TRU, HLW, spent nuclear fuel, or by product material from uranium, plutonium, or thorium mines (Atomic Energy Act section 11(e)2 by-product material). LLW can consist of short-lived and long-lived isotopes.

Log-probit Model - A dose-response model which assumes that each animal has its own threshold dose, below which no response occurs and above which a tumor [or other effect] is produced by exposure to a chemical.

Logit Model - A dose-response model which, like the probit model, leads to an S-shaped dose-response curve, symmetrical about the 50% response point. The logit model leads to lower "very safe doses" than the probit model even when both models are equally descriptive of the data in the observable range.

Lowest-Observed-Adverse-Effect-Level (LOAEL) - A dose-response model which, like the probit model, leads to an S-shaped dose-response curve, symmetrical about the 50% response point. The logit model leads to lower "very safe doses" than the probit model even when both models are equally descriptive of the data in the observable range.