The concept of Time Weighted Average (TWA) exposure limits is simple; it is a daily average concentration limit. The health concept is that if the exposure does not exceed this exposure limit, nearly all exposed workers will have no adverse effects. For the purpose of setting these limits, TWAs are usually defined as exposures for an 8-hour day, part of a 40-hour work week for an entire working lifetime. In terms of industrial hygiene measurement, air is drawn through a filter at a known rate for an 8-hour day. The contaminant on the filter or tube is measured, by a laboratory using established procedures, and this amount is divided by the volume of air drawn through the filter/tube. The lab reports the contaminant on the filter in milligrams (mg) and the volume is measured in cubic meters (m3). For sample tubes, results are generally reported in parts per million (ppm). Sample results can be compared to published exposure limits that use these units.
Notice that this takes in account long term effects. If the exposure is kept below this limit, then the belief of those who set these limits is that workers will not develop some disease later that was not evident earlier. Shorter term adverse effects will not occur either.
You may be thinking when reading this, what if the concentration level is high during part of the day? Haber’s law in inhalation toxicology attempts to address this. This law maintains that in terms of toxicology, concentration (C) x time (T) = a constant. That is, there is a value of concentration and time (CT) that produces a biological effect. The concentration could be high and the time low or the time could be high and the concentration low. If the CT value is equal to or above the one that produces biological effect, it is likely to happen. In an 8-hour day this idea would look like this.
C1T1 + C2T2 + C3T3 …CnTn = C8-hourT8-hour
So, if you prevent the 8-hour value from reaching the CT that causes adverse effect then the variations of concentration or time that might happen during a day are taken to account in the TWA.
This concept is for chronic, or long-term, hazards, generally if higher concentrations are not more dangerous than lower ones. However, we know this is not true for all contaminants. Another toxicological idea is that the “dose makes the poison”. Even things that are innocuous at low concentrations are likely be damaging at high concentrations. So, the TWA exposure limit idea is generally only valid for a range of concentrations. If a health effect can be experienced at higher concentration, then toxicologists have found it necessary to establish exposure guidelines for shorter periods of time such as 15-minute time-weighted averages or even ceiling levels that should not be exceeded. For the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values (TLV) these are TLV-STEL (short-term exposure limit) and TLV-C (ceiling). Not all chemicals have a TLV-STEL or TLV-C. In these cases, the ACGIH has recommended a rule of thumb that 3 time the TLV-TWA only be allowed for short periods of time during the workday and exposures over 5 times the TLV-TWA not be allowed.
