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7.7.2 Evaluating Toxicity Data

SDSs and other chemical resources generally refer to the toxicity of a chemical numerically using the term Lethal Dose 50 (LD50). The LD50 describes the amount of chemical ingested or absorbed by the skin in test animals that causes death in 50% of test animals used during a toxicity test study. Another common term is Lethal Concentration 50 (LC50), which describes the amount of chemical inhaled by test animals that causes death in 50% of test animals used during a toxicity test study. The LD50 and LC50 values are then used to infer what dose is required to show a toxic effect on humans.

As a general rule of thumb, the lower the LD50 or LC50 number, the more toxic the chemical. Note there are other factors (concentration of the chemical, frequency of exposure, etc.) that contribute to the toxicity of a chemical, including other hazards the chemical may possess.

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. For example, when making a decision on what chemical to use in an experiment based on safety for the lab worker, a chemical with a high LD50 or LC50 would be safer to work with, assuming the chemical did not possess multiple hazards and everything else being equal.

In general terms, the resource Prudent Practices in the Laboratory lists the following table for evaluating the relevant toxicity of a chemical:

Relevant Toxicity of a Chemical
Toxicity Class Animal LD50 Probable Lethal Dose for 70 kg Person (150 lbs.) Example
Super Toxic Less than 5 mg/kg A taste (7 drops or less) Botulinum toxin
Extremely Toxic 5 - 50 mg/kg < 1 teaspoonful Arsenic trioxide, Strychnine
Very Toxic 50 - 500 mg/kg < 1 ounce Phenol, Caffeine
Moderately Toxic 0.5 - 5 g/kg < 1 pint Aspirin, Sodium chloride
Slightly Toxic 5 - 15 g/kg < 1 quart Ethyl alcohol, Acetone

In addition to having a toxic effect on the body, some chemicals can be carcinogenic, mutagenic, teratogenic, and acutely toxic. These specific chemical hazards are covered in more detail under the Particularly Hazardous Substances section in this manual.