TLVs Explained

Threshold Limit Values or TLVs are air quality standards developed by the American Conference of Governmental Industrial Hygienists. They are the model for many other air quality limits such as OSHA’s permissible exposure limits (PELs).

TLVs are the amounts of chemicals in the air that almost all healthy adult workers are predicted to be able to tolerate without adverse effects.

There are three types:

  • TLV-TWA (TLV-Time-Weighted Average) which is averaged over the normal eight-hour day/forty-hour work week. (Most TLVs.)
  • TLV-STEL or Short Term Exposure Limits are 15 minute exposures that should not be exceeded for even an instant. It is not a stand alone value but is accompanied by the TLV-TWA. It indicates a higher exposure that can be tolerated for a short time without adverse effect as long as the total time weighted average is not exceded.
  • TLV-C or Ceiling limits are the concentration that should not be exceeded during any part of the working exposure. Unless the initials “STEL” or “C” appear after “TLV”, it is the eight-hour TLV-TWA that is meant.

TLVs are NOT “safe” limits. Even a few “healthy adult workers” will be unable to tolerate concentrations at the TLV.

TLVs also do not apply to people with certain health problems, allergies, people taking certain medications or drugs, people who work longer than eight hours per day, children or the fetus.

TLVs are not static – they change. They represent the current best estimates based on research. Make sure you have an up-to-date MSDS. Check the revision date.

TLVs are not available for all chemicals (There are ~700 TLVs. OSHA estimates there are over 650,000 chemicals in the workplace. Many have never been studied. Chemicals without TLVs or other exposure limits should NOT be considered safe.)

TLVs are used to assess workplace exposure by providing a comparison point for testing and to assist in product selection. When deciding which product to use, choose the chemical with the higher TLV so long as the chemical characteristics match your needs.

A lower TLV means less is allowed in the workplace air, and less you should inhale.

If you must use a chemical with a low TLV, make sure you plan adequate control measures (e.g., use in a chemical fume hood or with adequate local exhaust.) Make sure you arrange with EH&S for air monitoring if you suspect your exposure is not being adequately controlled.

TLV Measurement Units Contaminant Units Unit definition Dust, Fumes or Mist (solid particles)mg/m3Milligrams of contaminant per cubic meter of air. Gases and vaporsppmParts of contaminant per million parts of air.