30
Time History Graph
Ceiling
2
.
Time Weighted Average (TWA)
:
The maximum average concentration to
which an unprotected worker may be
exposed over an eight hour working day.
During this time, STEL and ceiling
concentration limits may not be
exceeded.
Time History Graph
Ceiling
TWA
(8 hour)
3
.
Short Term Exposure Limits
(STEL):
Toxic substances may have short term
exposure limits which are higher than the
eight hour TWA. The STEL is the
maximum average concentration to which
an unprotected worker may be exposed in
any fifteen minute interval during the day.
During this time, neither the eight hour
TWA or the ceiling concentration may be
exceeded.
Any fifteen minute periods in which the
average STEL concentration exceeds the
permissible eight hour TWA must be
separated from each other by at least one
hour. A maximum of four of these periods
are allowed per eight hour shift.
Time History Graph
Ceiling
STEL
TWA
15 Minutes
