Toxic Substances

Toxic compounds are instantly a concern in any laboratory working with chemicals.

First, toxicity is relative to the dose and the exposure for each chemical. Any substance, even water, can be toxic, but it is the amount of substance required to cause toxic effects on the body that is important.

LD50 Values

A common feature of MSDS sheets is the LD50 which stands for the dose at which the mortality rate for a given organism is 50%.  LD50's are listed as mg/kg, which means mg of substance (dose) divided by the mass of the organism in kg. The lower the LD50, the more toxic a substance is.LD50's for common substances for rats are listed below:



Vitamin C (mg/kg) 11900 Water 180000
Ethanol 10600 Nicotine 50
Sodium Chloride 3000 DDT 113

It should also be noted that theLD50 can vary based on the route of exposure, with skin exposure usually requiring a higher dose.

LD50's however, do not indicate anything about non-lethal toxic effects and therefore should be used carefully. Use in comparing the toxicity of different compounds as well as determining the level of protection to use are some areas the LD50 can assist in.

Permissible Exposure Limits

Permissible Exposure Limits (PEL's) are regulatory limits from OSHA that limit the amount of a substance in the air in which individuals are working. PEL's are based on an adult person with an eight hour time weighted average (TWA) but are some times given as a short term exposure limit (STEL) for 15-30 minute working times.

PEL's often refer to airborne concentrations that may be inhaled and they are typically listed in units of parts per million (ppm).

Working with substances with low PEL's and those that volatilize easily require the use of engineering controls, such as fume hoods to minimize exposure.

PEL's in ppm for common substances are listed below:

  • Acetone - 750ppm
  • Benzene - 10ppm
  • Chloroform - 50ppm
  • Diethyl Ether - 400ppm
  • Formaldehyde - 0.75ppm
  • Toluene - 300ppm
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