Introduction to Health and Safety at Work

Chemical and biological health hazards and control
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should be reviewed on a regular basis, particularly when 
there are changes in work process or substances or when 
adverse ill-health is reported.
14.8.2 Workplace exposure limits
One of the main purposes of a COSHH assessment is 
to adequately control the exposure of employees and 
others to hazardous substances. This means that such 
substances should be reduced to levels which do not 
pose a health threat to those exposed to them day after 
day at work. Under the 2005 amendments to the COSHH 
Regulations 2002, the Health and Safety Commission 
has assigned workplace exposure limits (WELs) to a 
large number of hazardous substances and publishes 
an annual update in a publication called ‘Occupational 
Exposure limits’ EH40. The workplace exposure limit is 
related to the concentration of airborne hazardous sub-
stances that people breathe over a specifi ed period of 
time – known as ‘time weighted average’. Before the 
introduction of WELs, there were two types of exposure 
limits published – the maximum exposure limit (MEL) 
and the occupational exposure standard (OES).
The COSHH (Amendment) Regulations 2005 
replaced the OES/MEL system with a single Workplace 
Exposure Limit (WEL). THis removed the concern of 
HSC that the OES was seen as a ‘safe’ limit rather than 
a ‘likely safe’ limit. Hence, the Workplace Exposure Limit 
(WEL) must 
not be
exceeded. Hazardous substances 
which have been assigned a WEL fall into two groups:
1. Substance which are carcinogenic or mutagenic 
(having a risk phase R45, R46 or R49) or could cause 
occupational asthma (having a risk phase R42, or 
R42/43 or listed in section C of the HSE publication 
‘Asthmagen? Critical assessment for the agents impli-
cated in occupational asthma’ as updated from time 
to time) or are listed in Schedule 1 of the COSHH 
Regulations. These are substances which were assig-
ned a maximum exposure limit (MEL) before 2005. 
The level of exposure to these substances should be 
reduced as far as is reasonably practicable.
2. All other hazardous substances which have been 
assigned a WEL. Exposure to these substances by 
inhalation must be controlled adequately to ensure 
that the WEL is not exceeded. These substances 
were previously assigned an occupational exposure 
standard (OES) before 2005. For these substances, 
employers should achieve adequate control of 
exposure by inhalation by applying the principles 
of good practice outlined in the approved code 
of practice and listed in section 14.9.1. The impli-
cation of these principles is discussed later in this 
chapter.
The workplace exposure limits are subject to time-
weighted averaging. There are two such time-weighted 
averages (TWA): the long-term exposure limit (LTEL) or 8 
hour reference period, and the short-term exposure limit 
(STEL) or 15 minute reference period. The 8 hour TWA is 
the maximum exposure allowed over an 8 hour period 
so that if the exposure period was less than 8 hours the 
workplace exposure limit is increased accordingly with 
the proviso that exposure above the LTEL value con-
tinues for no longer than 1 hour. Table 14.1 shows some 
typical workplace exposure limits for various hazardous 
substances.