Now we come to the exposure limits. Exposure limits are airborne concentration levels of a hazardous substance, which if not exceeded; it is believed that is not likely to cause any adverse effect. Exposure limits are most likely to be expressed as parts per million (ppm) or sometimes parts per billion (ppb). Another method of expressing airborne concentration levels that may be used is milligrams per cubic meter (mg/m3).
The Permissible Exposure Limits (PEL) is established or adopted by OSHA and carries the weight OSHA regulation. The Threshold Limit Values (TLV) is an exposure limit that is established by the American Conference of Governmental Industrial Hygienists152 (ACGIH).
Both of these exposure limits are Time Weighted Averages (TWA) or the average value of exposure over the course of an 8-hour work shift. What this means in plain language is as long as the Time Weighted Average for the entire day is below the PEL or TLV, a worker may be exposed to a level higher than the PEL or TLV for part of the day. These values take into account, if known, the adverse effects of chronic exposures by assuming an eight a day, forty hours a week, over a twenty or thirty year exposure period.
So, let us say you have to use Tcat Unreal Kleen and the ventilation in the area of use is less than ideal. You find that your exposure to n-Hexane is 100 ppm, twice the PEL. By using this product for during four hours of an eight-hour workday, your TWA would be 50 ppm. At an exposure-level of 200 ppm for two hours would still be only 50 ppm TWA. Now, what if you are assigned to work that has an exposure experience of 1600 ppm for just ½ hour a day? Now, your boss tells you that its only okay as this exposure will not exceed the TWA. Do your calculations and you will detect indeed it averages out to 50 ppm. So there is no problem, right?
It is possible to exceed the threshold of an acute exposure level while maintaining a TWA exposure at or below the PEL or TLV. Additional limits are often established restricting at how much higher and for how long the TWA limits may be exceeded. The Short Term Exposure Limit (STEL) sets a level that cannot be exceeded for periods longer then 15 minutes, no more then four times a day with at least 60 minutes between exposures. The Ceiling limit sets a level that should be never exceeded (often denoted with a capital C). In the above example, n-Hexane has a 1000 ppm STEL, so even though 1600 ppm does not exceed the TWA, it does exceed the STEL.
Many chemicals do not have established STELs or ceilings. Additionally, many MSDSs do not list the STELs or Ceiling. As a general rule of thumb, when not stated, figure on the STEL as being approximately one and a half (in any case, never more then two times) the PEL or TLV
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