(Created 12/02)
UNL Environmental Health and Safety · (402) 472-4925 · http://ehs.unl.edu
Time-Weighted Average (TWA). This exposure is determined by averaging the concentrations
of the exposure with each concentration weighted based on the duration of exposure. For
example, an exposure to acetone at the following concentrations and durations:
1000 ppm for 3 hours
500 ppm for 2 hours
200 ppm for 3 hours
would have an 8-hour, TWA exposure of:
[(3hrs)(1000ppm) + (2hrs)(500ppm) + (3hrs)(200ppm)] / 8 hrs
= [3000 ppm + 1000 ppm + 600 ppm] / 8
= 575 ppm
This exposure would be compared to an 8-hour TWA
exposure limit.
A TWA can be the average concentration over any period of time. However, most TWAs are the
average concentration of a chemical most workers can be exposed to during a 40-hour
week and
a normal 8-hour work day without showing any toxic effects. NIOSH TWA recommendations,
on the other hand, can also be based on exposures up to 10 hours.
The time-weighted average
permits exposure to concentrations above the limit, when they are compensated by equal
exposure below the TWA. (
Graph 3) shows an example that illustrates
this point for a chemical
with a TWA exposure limit of 750 ppm.
Graph 3
Example of an Exposure Compared to a TWA Exposure Limit
(Created 12/02)
UNL Environmental Health and Safety · (402) 472-4925 · http://ehs.unl.edu
Short-Term Exposure Limit (STEL). The excursions allowed by the TWA could involve very
high concentrations
and cause an adverse effect, but still be within the allowable average.
Therefore, some organizations felt there was a need for a limit to these excursions. In 1976,
ACGIH added STELs to its TLVs. The STEL is a 15 minute, TWA exposure. Excursions to the
STEL should be at least 60 minutes apart, no longer than 15 minutes in duration and should not
be repeated more than 4 times per day. Because the excursions are calculated into the 8-hour
TWA, the exposure must be limited to avoid exceeding the TWA.
Graph 4 illustrates an
exposure that exceeds the 15 minute limit for an STEL of 1000 ppm.
The STEL supplements the TWA. It reflects an exposure limit that protects
against acute effects
from a substance which primarily exhibits chronic toxic effects. This concentration is set at a
level to protect workers against irritation, narcosis, and irreversible tissue damage.
OSHA added
STELs to its PELs with the 1989 revisions.
AIHA has some short-term TWAs similar to the STELs. The times used vary from 1 to 30
minutes. These short-term TWAs are used in conjunction with, or in place of, the 8-hour TWA.
There is no limitation on the number of these excursions or the
rest period between each
excursion.
