Lethal Dose and Lethal Concentration. Every naturally occurring or manufactured chemi- cal is toxic to living organisms if they receive excessive exposure or consume large enough quantities of the substance. This is as true for everyday compounds such as wa- ter, salt, sugar, aspirin, and every other material in the environment as it is for poisonous chemicals, such as pesticides. The difference, of course, is the amount, or dose, of a substance that the organism receives. Toxicologists commonly use the expression “the dose makes the poison.”
For lethal dose or lethal concentration studies, researchers divide laboratory animals into several groups and examine the effects from various routes of exposure. They rate a pesticide’s toxicity by determining the amount, or lethal dose (LD) that kills 50% of a test population. Researchers use the term LD50 to signify this dose. The unit of measurement for the LD50 is milligrams of pesticide per kilogram of the test animal’s body weight (mg/kg). When dealing with pesticide vapors or dusts, researchers measure concentrations of the material in the air that will cause death in 50% of test animal populations. They make similar measurements for a pesticide dissolved in a volume of water. This is the lethal concentration or LC50. They express LC50 as micrograms – one one-millionth of a gram (1/1,000,000 g) – per liter of air or water (µg/l).
Higher LD50 or LC50 values indicate lower hazards, while lower LD50 or LC50 values indicate higher hazards. These are guidelines and not absolute indicators of hazard because different species of animals react differently to the same toxic chemicals. Even animals of the same species sometimes respond differently. Differences relate to genetic makeup as well as age, life stage, health, or influences of environmental surroundings. Humans also respond differently to many pesticides due to these same factors.
The more toxic pesticides present higher hazards, or risks of injury, to people and the environment. Besides toxicity, pesticide hazards also vary for people accord- ing to the way they enter the body. Normal routes of entry include the mouth, skin, eyes, and lungs. Researchers usually determine the LD50 of a pesticide for skin applied (dermal) and swallowed (oral) exposures, based on extrapolation from animal studies.
Lethal dose or lethal concentration classifications do not provide information about chronic, long-term toxic effects. A pesticide that has a high LD50 value (relatively nontoxic) may not necessarily be harmless. Low exposure to some pesticides may cause health problems that only appear months or even years later.
No Observable Effect Level. The no observable effect level (NOEL) is the maxi- mum dose or exposure level of a pesticide that produces no noticeable toxic effect on test animals. NOEL is a guide for establishing maximum exposure levels for people and for establishing residue tolerance levels on pesticide-treated produce. Usually, the U.S. EPA sets exposure levels and residue tolerances at 100 to 1,000 times less than the NOEL, providing a wide margin of safety.
Threshold Limit Value. The threshold limit value (TLV) for a chemical is its air- borne concentration measured in parts-per-million (ppm) that produces no adverse effects over an exposure time. The TLV applies to pesticides used as fumigants, such
WHAT ARE PESTICIDES?