Page 239 - IGC BOOK

There are often limits to how far above the WEL an exposure may be allowed to go, which are
set by short-term exposure limits, say no longer than 5 minutes above 25 ppm. These short-
term exposure limits should never be exceeded.

Enclosure of Hazards, Segregation of Process and People

The control measures we have looked at are all based on either preventing the risk or reducing
it in some way to an insignificant level. If this approach is not possible then we have to
consider physical controls which enclose the hazard and segregate people from the process
involving it.

Total enclosure or containment of the hazard is the best form of control since no one can then
be exposed to it; for example, total enclosure of a process which generates dust or fumes will
prevent the escape of airborne contaminants which could be inhaled by operators nearby.
When an area has been totally isolated, it may still be necessary to access equipment or
material within that area; for example, remote controlled robot handling systems may be
used, allowing access without physically entering the enclosed danger area.

Where isolation of the source is difficult it may be more practical to enclose the workers to
ensure that they remain segregated from the hazard.

There will always be situations where it is not possible to totally enclose the process or the
workers at all times; for example, when cleaning or maintenance work has to be carried out,
or access is necessary to introduce raw materials or remove the product. Special measures
will then be necessary to prevent any escape of the substance during periods when the
security of the enclosure will be broken; for example, through the use of ventilation systems
to carry away any airborne contaminants.

Local Exhaust Ventilation

Local exhaust ventilation (LEV) is the standard control measure for dealing with dusts, vapours
and fumes which are generated from a point source. The harmful contaminant is extracted at
the point of generation using engineered systems to ensure that the direction of the
ventilation flow is away from the breathing zone of any operators.

LEV is a mechanical system of ventilation designed to remove a specific contaminant from its
source, e.g. welding fume. It consists of the following: a receptor or capture hood; ducting to
remove the contaminant; an air cleaning device, e.g. filter to remove the contaminant from
the air; a fan to ensure an adequate ventilation rate for the contaminant. This will be
dependent on the size of the particles and the size of the LEV system; and a discharge point.

ENSIGN| Unit IG2 – Element 7 – Chemical and Biological 34
Agents