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Toxicology Nomenclature

LD and LC

LD50 is lethal dose 50. This is the single dose of a toxic substance administered by any route (other than by inhalation) that causes the death of 50% of an animal population. LDLo is lethal dose low—the lowest dose of a substance reported to have caused death in humans or animals.

LC50 is lethal concentration 50. It is the concentration of a material in air that kills 50% of a group of test animals when administered as a single exposure in a specific time period (usually 1 hour). Note that lethal concentration values are used when the route of administration is by inhalation (rather than oral, dermal, etc.) whereas LD values are used for all routes except inhalation. LC values are often expressed as parts of material per million parts of air (ppm).

    The route of administration used in the test is designated as:

    • inhalation
    • oral (taken by mouth, sometimes abbreviated)
    • dermal (on the skin)
    • iv (intravenous)
    • im (intramuscular)
    • ip (intraperitoneal, or injected into the abdominal cavity)
    • subcutaneous (injected just under the surface of the skin).

    Approximate toxicities in everyday values:

    LD50 (animal) correlation to ingestion by a 150 pound adult human toxicity value in NFPA or HMIG
    0-50 mg/kg 0-1 teaspoon extreme 4
    50-500 mg/kg 1 t to 1 oz very 3
    500-5000 mg/kg 1 oz to 1 pt moderate 2
    5-15 g/kg 1 pt to 1 qt slight 1
    over 15 g/kg more than 1 qt practically non-toxic 0

TD and TC

TDLo is toxic dose low. It is the lowest dose of a substance reported to produce any toxic effect in humans or to produce tumorigenic or reproductive effects in animals or humans. Like LD, it is used for all routes of administration except inhalation. The same subscripts, routes of administration, and species tested abbreviations are used as with LD and LC. TD50 is toxic dose 50, and means that 50% of the test subjects suffered from one or more toxic effects.

TCLo is toxic concentration low. It is the lowest concentration of a substance in air to which humans (or animals) have been exposed for any given period of time that has produced any toxic effect or produced a tumorigenic or reproductive effect one or more members of the group of subjects. Toxic concentration values are for the inhalation toxicity of a compound—they are not used for any other route of exposure. TC values are of great importance to industrial workers and chemists exposed to fumes and dust.


TLV and PEL are exposure limits.and are used by agencies to establish permissible exposure limits in workplaces. TLV is threshold limit value and is used by the American Conference of Governmental Industrial Hygienists (ACGIH) to express the maximum airborne concentration of a material to which most workers can be exposed during a normal daily and weekly work schedule without adverse effects. PEL is the permissible exposure limit and is used by the Occupational Health and Safety Administration (OSHA). Values for TLV and PEL are usually equivalent; each is the average concentration of a chemical in the air to which most people can be exposed and show no ill effect. These values are sometimes derived from TCLo values. A PEL or TLV value refers only to inhalation toxicity, not to skin or eye contact or to ingestion. Not all TLV’s have been established by scientific experimentation. Many are estimates, based on experience with the chemical or based on known information about similar chemicals.

TLV’s are expressed in three ways:

  • TWA is the allowable time-weighted average concentration for a normal 8-hour workday or 40-hour week to which a person can be repeatedly exposed for 8 hours a day, day after day, without adverse effect.
  • TLV-STEL is the short-term exposure limit or maximum concentration for a continuous exposure period of 15 minutes (with a maximum of four such periods per day, with at least 60 minutes between exposure periods, and provided that the daily TLV-TWA is not exceeded).
  • TLV-C is the ceiling concentration—the value not to exceeded at any time.

    term denominators definition usage
    Lo or 50
    lethal dose exposure by any route except inhalation
    LC Lo or 50 lethal concentration exposure by inhalation
    TD Lo or 50 toxic dose exposure by any route except inhalation
    TC Lo or 50 toxic concentration exposure by inhalation
    TLV TWA, STEL, C threshold limit value airborne concentration allowed in a normal work schedule
    PEL TWA, STEL, C threshold limit value airborne concentration allowed in a normal work schedule


A carcinogen is a substance that causes cancer. Officially, a compound must be labeled as a carcinogen if it is identified as such in the latest edition of one of three lists:

    National Toxicology Program (NTP) Annual Report on Carcinogens

    OSHA - OSHA 29 CFR1910, Subpart Z, Toxic and Hazardous Substances. The link below takes you to the OSHA page which then takes you to the NTP site for the list (thus, the NTP list and the OSHA list are one and the same). The OSHA page itself has some good information.

Informally, compounds are called carcinogens if they are on the NTP or IARC lists of known or suspected carcinogens, or on the "California List" or on the list of the EPA Carcinogen Assessment Group.

Other Hazards

A teratogen is a compound that can cause fetal damage. Allergens cause allergic reactions. A lachrymator causes tearing of the eyes.

A term used both on bottle labels and in MSDS’s that is of particular use to laboratory workers is the contact hazard. It is usually expressed in terms of descriptive word form. Irritant means that it causes a reversible inflammatory effect on living tissue. Corrosive means that the chemical causes visible destruction of tissue if it comes into contact with skin. Sensitizer means that with repeated exposures it causes a substantial proportion of exposed people to develop an allergic reaction. A contact hazard description might include the affected body part, such as “highly irritating to the eyes” or “causes irritation of the respiratory tract.”