Introduction to Health and Safety at Work

For example
, If a person was exposed to a hazard-
ous substance with a WEL of 100 
mg/m
3
(8 hour 
TWA) for 4 hours, no action would be required until 
an exposure level of 200 mg/m
3
was reached (further 
exposure at levels between 100 and 200 mg/m
3
should 
be restricted to 1 hour).
If, however, the substance has an STEL of 150 mg/m
3
, 
then action would be required when the exposure level 
rose above 150 mg/m
3
for more than 15 minutes.
The STEL always takes precedence over the LTEL. 
When a STEL is not given, it should be assumed that it 
is three times the LTEL value.
The publication EH40 is a valuable document for the 
health and safety professional since it contains much 
additional advice on hazardous substances for use 
during the assessment of health risks, particularly where 
new medical information has been made public. The 
HSE are constantly revising WELs and introducing new 
ones and it is important to refer to the latest publication 
of EH40.
It is important to stress that if a WEL from Group 
1 is exceeded, the process and use of the substance 
should cease immediately and employees removed 
from the immediate area until it can be made safe. In the 
longer term the process and the control and monitoring 
measures should be reviewed and health surveillance of 
the affected employees considered.
Table 14.1
Examples of Workplace Exposure Limits 
(WELs)
Group 1 WELs 
LTEL (8 hr TWA) 
STEL (15 min)
All isocyanates 
0.02 mg/m
3
0.07 
mg/m
3
Styrene 430 mg/m
3
1080 
mg/m
3
Group 2 WELs
Ammonia 18 
mg/m
3
25 
mg/m
3
Toluene 191 
mg/m
3
574 
mg/m
3

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