MetFab - Welding Fume Handbook - Flipbook - Page 14


5 Exposure limits in the UK
Much of the regulatory framework
relevant to welding and allied
processes aims to protect the health
of workers by keeping their exposure
to hazardous substances within
defined limits. These limits are known
generally as exposure limits.
If the concentration of a substance in
the workplace is below the exposure
limit, then that substance is being
adequately controlled. Of course, the
workplace concentration must be
measured in an appropriate way;
the reference here is EN/ISO 10882,
Health and Safety in welding and
allied processes - Sampling of airborne
particles and gases in the operator's
breathing zone, Part 1: Sampling of
airborne particles and Part 2: Sampling
of gases.
In the UK, the levels of pollutants to
which exposure must be controlled
are known as Occupational Exposure
Limits (OELs). Most OELs are expressed
as concentrations in air, measured in
mg/m³.
There are two kinds of OELs:
Occupational Exposure Standards
(OESs) and Maximum Exposure
Limits (MELs). Both are given in HSE
publication GN EH40/02, Occupational
Exposure Limits 2002, and its annual
updates, available from HSE Books
(www.hsebooks.co.uk).
exposure to the substance by inhalation
does not exceed the OES, or, where the
OES is exceeded, the employer identifies
the reason for this and takes steps
to remedy the situation as soon as
reasonably practicable.
MELs are set for substances which
may cause the most serious health
effects, such as cancer and occupational
asthma, and for which safe levels of
exposure either cannot be defined or
cannot reasonably be achieved.
If a substance has an MEL, control is
only considered adequate if the level
of exposure by inhalation is as low as
reasonably practicable, and in any case
below the MEL.
In UK health and safety law, carrying
out a duty "so far as is reasonably
practicable" involves balancing the
degree of risk for an activity against the
time, trouble, cost and physical difficulty
of avoiding or reducing that risk. The
greater the risk, the more likely it is that
it is reasonable to go to the trouble or
expense of preventing or reducing it.
Both OESs and MELs are measured as
average values over defined periods
of time, rather than as instantaneous
values. The resulting average values
are said to be time-weighted averages
(TWAs).
An OES is a level that, based on current
scientific knowledge, will not damage
the health of workers exposed to it, by
inhalation, day after day.
Long term exposure limits (LTELs) are
TWAs measured over 8 hours. This
technique is used to measure and
control substances or concentrations
which cause harmful effects following
chronic exposure.
For any substance that has an OES,
control is considered adequate if
Short term exposure limits (STELs)
are usually averaged over 15 minutes,
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