(1) Threshold limit value-time weighed average . The threshold limit value-
time weighed average (TLV-TWA) is the time weighted average for a normal 8-hour
workday and a 40-hour workweek to which nearly all workers may be repeatedly
exposed day after day without adverse health effects.
(2) Threshold limit value short-term exposure limits. The threshold limit
value short-term exposure limits (TLV─STEL) is the concentration to which workers can
be exposed continuously for a short period of time. A STEL is defined as a l5 minute
time weighted average exposure, which should not be exceeded at any time during a
work day even if the 8-hour time-weighted average is within the TLV. Exposure at the
STEL should not exceed l5 minutes and should not be repeated more than 4 times a
day. There should be at least 60 minutes between successive exposures at the STEL.
(3) Threshold limit value-ceiling. The Threshold limit value-ceiling (TLV-C)is
the concentration that should not be exceeded even instantaneously. Indicated by a "C"
in the TLV listing.
(4) "Skin" notation. Listed substances followed by the designation "skin"
refer to the potential contribution to overall exposure by the coetaneous route including
the mucous membranes and eyes.
(5) "Ceiling" values. There are certain toxic substances for which time-
weighted average concentrations are inappropriate. For example, there are substances
that are predominantly fast acting, and whose threshold limit is based on this response.
Substances of this type are assigned a "ceiling" limit ("C") that should not be exceeded
under any circumstances. Appendix E contains examples of substances to which "C"
limits have been assigned.
c. Computation of a Time-Weighed Average. Exposures must be averaged
for the time period of the standard 8 hours or l5 minutes, and because sampling periods
may differ, a simple arithmetic average will not yield the true average. The following
formula is used to average results from sampling for atmospheric contaminants.
T = Time
C = Concentration
TWA = time weighed average
Hrs = hours
(T1 x C1)
(T2 x C2)
(Tn x Cn)