|Occupational Health and Safety
TLV's (Threshold Limit Values)
TLV’s (Threshold Limit Values) are U.S. standards used for limiting worker exposure to various airborne contaminants which can
adversely affect workers’ health.
Most Canadian health and safety jurisdictions simply adopt the TLV’s as legal limits. They are then known as TLV’s Permissible
Concentrations (PC’s) or Maximum Allowable Concentrations (MAC’s)
These TLV’s, PC’s or MAC’s are often found in government regulations as an appendix. There are TLV’s for more than 500 airborne
According to the U.S. TLV booklet, the Time Weighted Average (TWA) is the average amount of a substance a worker can be
exposed to without suffering ill health effects. In other words the level a worker is allowed to be exposed to over the work day or
work week can greatly exceed the TLV at various times as long as there are enough lower concentrations so that, on average, the
TLV is not exceeded.
The Threshold Limit Value - Ceiling (TLV-C) is the concentration that should not be exceeded during any part of the workday.
TLV’s are measured in ppm (parts per million) of the substance in question (usually a gas) or in mg/m3 (milligrams per cubic meter)
of the substance in question (usually a dust or fume).
Amounts of the substance can be measured with a dosimeter worn by the worker for the entire shift. This device determines if the
TWA (Time Weighted Average) has been exceeded or not.
Grab samples will give a reading of the current level of the contaminant. Such samples can be taken with drager or gas tech tubes.
They are easy to use but give relatively inaccurate readings. There are other, more accurate measuring instruments as well as some
that are suitable for measuring substances by putting them in a fixed location.
A U.S. organization called the American Conference of Governmental Industrial Hygienists (ACGIH) publishes its TLV list each year.
The ACFIH TLV Committee is a private, not government body. Since 1970 the TLV Committee has included corporate representatives
from companies such as Dow Chemical and DuPont as active participants.
Many of the TLV's are set as a result of information received from corporations. This information is often unpublished.
Prior to 1989, meetings of the TLV Committee were not open to the public and business interests were allowed to make
presentations to the Committee in private.
Since the TLV's have been developed in large part by company representatives, can we trust the standards to protect workers'
The TLV booklet states that "These limits are not fine lines between safe and dangerous concentrations...". In other words, even
the ACGIH recognizes that ill health can occur at exposure levels below the TLV.
The TLV's are suspect. The U.S. Government organization, NIOSH (National Institute for Occupational Safety and Health) has
recommended stricter limits for 68 of the substances in the TLV list. Many European countries have even stricter limits for more
What Should Be Done?
Canadians should review published health study results to develop new, more stringent standards. The standards should not just
prevent overt ill health effects but also prevent early warning signs that show that the human body is being affected by the
In the meantime:
1. we should treat TLV’s as maximum allowable concentrations or ceilings not to be exceeded at any time.
2. we should follow the most stringent of the U.S. Government’s OSHA (Occupational Safety and Health Administration), or the U.S.
Government’s NIOSH (National Institute for Occupational Safety and Health) recommendations wherever they have a limit which is
stricter than the ACHIH TLV.
Source: CAW Health, Safety & Environment Department