Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. The more toxic a material, the smaller the amount necessary to be absorbed before harmful effects are caused. The lower the toxicity, the greater the quantity necessary to be absorbed. Generally, the toxicity of a chemical is determined through experiments on animals (quite often rats), and measured in terms of the amount of material necessary to cause death in 50% of the test animals. These values are called LD50 (lethal dose) or LC50 (lethal concentration), and are usually given in weight of material per kg of body weight or airborne concentration of material per set time period respectively.

Toxic Effects

Once a toxic substance has contacted the body it may have either acute (immediate) or chronic (long-term) toxicity.

  • Acute toxicity refers to those adverse effects of a substance that result either from a single exposure or from multiple exposures in a short period (usually less than 24 hours). 
  • Chronic toxicity refers to those adverse effects as the result of long-term exposure to a toxicant, often measured in months or years. It can cause irreversible side effects.

Evaluating Toxicity Data

While exact toxic effects of a chemical on test animals cannot necessarily be directly correlated with toxic effects on humans, the LD50 and LC50 can give a good indication of the toxicity of a chemical, particularly in comparison to another chemical. LD50 and LC50 values are often found in safety data sheets, and even if specific values are not quoted, OSHA/GHS hazard categories for acutely toxic materials should be given. Categories of acute toxicity as defined by OSHA are listed below.

Note: Substances in Categories 1 and 2 are considered to have a “high degree of acute toxicity.” These substances are also referred to as “Highly Acutely Toxic Substances.”

Exposure Route

Category 1

Category 2

Category 3

Category 4


(mg/kg bodyweight)

≤ 5

> 5 and

≤ 50

> 50 and

≤ 300

> 300 and

≤ 2000


(mg/kg bodyweight)

≤ 50

> 50 and

≤ 200

> 200 and

≤ 1000

> 1000 and

≤ 2000

Inhalation – gases

(ppm by volume)

≤ 100

> 100 and

≤ 500

> 500 and

≤ 2500

> 2500 and

≤ 20000

Inhalation – vapors


≤ 0.5

> 0.5 and

≤ 2.0

> 2.0 and

≤ 10.0

> 10.0 and

≤ 20.0

Inhalation - dusts and mists


≤ 0.05

> 0.05 and

≤ 0.5

> 0.5 and

≤ 1.0

> 1.0 and

≤ 5.