Page 238 - IGC BOOK

• Vacuuming, rather than sweeping up (which pushes dust into the air), reduces dust levels.
• Damping substance during mixing or when clearing up also reduces dust levels.

In general, the aim is to identify the particular element of the process or work activity which
is responsible for the harmful agent and try to replace it with one with less potential for harm.
The opportunities for this may be limited by practicability from a production point-of-view.

Reduced Time Exposure

The ill-health effects arising from hazardous substances and agents in the workplace are often
related to the length of time of exposure as well as the severity (the concentration of the
contaminant) of the hazard. So reduction of exposure can be used as a means of minimising
possible ill-health effects.

As a general principle, when a hazard exists from a substance or a physical agent, the
cumulative dose should be reduced to as low a level as possible by organising the work
pattern to provide periods of zero exposure. There are two methods of achieving this, based
on establishing safe exposure time limits:

• Providing for regular breaks away from contact with the hazardous substance.
• Job rotation, where the exposure of any particular individual is reduced by sharing the dose
with other workers, such as having a number of workers performing a task in rotation, with
strict control over length of time of exposure in order to ensure that dose limits are not
exceeded.

There is a close relationship between exposure and time. At a fixed level of contamination, the
effect will be proportional to the time exposed. Concentrations of airborne substances
averaged over a period of time are known as a time weighted average (TWA). This is the basis
of workplace exposure limits, including long-term exposure limits (8 hours’ time weighted
average value) and short-term exposure limits (15 minutes time weighted average value).
Long-term exposure limits are used to help protect against chronic ill-health effects and short-
term exposure limits are to protect against acute ill-health effects such as eye irritation, which
may happen after minutes or even seconds of exposure.

The working time limits will be based on an assessment of the average exposure over a period
of time, whether it is 8 hours or 15 minutes. Within this period, variations in the exposure
levels above the average are allowed provided there are equivalent exposures below so that
the overall average is not exceeded. For instance, the limit set may be 20 parts per million
(ppm) for an airborne substance; so there may be periods where the levels are as high as, say,
25 ppm, provided there is an equal time where levels are 15 ppm, ensuring that the time
weighted average meets the 20 ppm level.

ENSIGN| Unit IG2 – Element 7 – Chemical and Biological 33
Agents