TLV---Threshold Limit Value:
A term used by ACGIH to express the airborne exposure level to a chemical or physical hazard to which nearly all persons can be exposed day after day without adverse effects.
ACGIH expresses TLVs in three ways:
TLV-TWA: The allowable Time-Weighted Average level for a normal 8-hour workday or 40-hour week.
TLV-STEL: The Short-Term Exposure Limit or maximum level for a continuous exposure period (maximum of four such periods per day, with at least 60 minutes between exposure periods, and provided that the daily TLV-TWA is not exceeded).
TLV-C: The Ceiling Exposure Limit - the level that should not be exceeded even instantaneously.
Three categories of Threshold Limit Values (TLVsŪ) are specified herein, as follows:
1. Threshold Limit Value-Time-Weighted Average (TLV-TWA):
the time-weighted average concentration for a conventional 8-hour workday and a 40-hour workweek, to which it is believed that nearly all workers may be repeatedly exposed, day after day, without adverse effect.
2. Threshold Limit Value-Short-Term Exposure Limit (TLV-STEL):
THE concentration whch it is believed that workers can be exposed continuously for a short period of time without suffering from
2) chronic or irreversible tissue damage, or
3) narcosis of sufficient degree to increase the likelihood of accidental injury, impair self-rescue or materially reduce work efficiency, and provided that the daily TLV-TWA is not exceeded.
It is not a separate independent exposure limit; rather, it supplements the time-weighted average (TWA) limit where there are recognized acute effects from a substance whose toxic effects are primarily of a chronic nature.
TLV-STELs are recommended only where toxic effects have been reported from high short-term exposures in either humans or animals.
A TLV-STEL is defined as a 15-minute TWA exposure which should not be exceeded at any time during a workday even if the 8-hour TWA is within the TLV-TWA.
Exposures above the TLV-TWA up to the TLV-STEL should not be longer than 15 minutes and should not occur more than four times per day.
There should be at least 60 minutes between successive exposures in this range.
An averaging period other than 15 minutes may be recommended when this is warranted by observed biological effects.
3. Threshold Limit Value-Ceiling (TLV-C):
The concentration that should not be exceeded during any part of the working exposure.
In conventional industrial hygiene practice if instantaneous monitoring is not feasible, then the TLV-C can be assessed by sampling over a period that should not exceed 15 minutes, except for those substances that may cause immediate irritation when exposures are short.
For some substances, e.g., irritant gases, only one category, the TLV, may be relevant.
For other substances, one or two categories may be relevant, depending upon their physiologic action.
It is important to observe that if any one of these types of TLVsŪ is exceeded, a potential hazard from that substance is presumed to exist.
Conversion of TLVsŪ in ppm to mg/m3.
TLVsŪ for gases and vapors are usually established in terms of parts per million of substance in air by volume (ppm).
For convenience to the user, these TLVsŪ are also listed with molecular weights.
Where 24.45 = molar volume of air in liters at normal temperature and pressure (NTP) conditions (25°C and 760 torr), the conversion equation for mg/m3 is:
TLV in mg/m3 = (TLV in ppm)(gram molecular weight of substance) / 24.45
Conversely, the equation for converting TLVsŪ in mg/m3 to ppm is:
TLV in ppm = (TLV in mg/m3)(24.45)/
gram molecular weight of substance
The above equation may be used to convert TLVsŪ to any degree of precision desired.
When converting TLVsŪ to mg/m3 for other temperatures and pressures, the reference TLVsŪ should be used as a starting point.
When converting values expressed as an element (e.g., as Fe; as Ni), the molecular value of the element should be used, not that of the entire compound.
In making conversions for substances with variable molecular weights, appropriate molecular weights should be estimated or assumed.
Canadian Center for Occupational Health and Safety