Page 240 - IGC BOOK

Local Exhaust Ventilation.

Examples of LEVs include:
Glove boxes: total enclosures, often used in laboratories, which are accessed through flexible
gloves and kept under negative air pressure to prevent any release of contaminant.

Fume cupboards: partial enclosures, again often used in laboratories, which are accessed
through a vertical sliding sash, with the enclosure again being kept under negative pressure
so that the air flow is through the sash into the hood to prevent any release of contaminant.

Captor hoods: movable ventilators which can be positioned as near as possible to the hazard
and capture contaminants by a negative air flow into the hood before they reach the
operator, as are used to extract woodworking dust.

Receptor hoods: large structures designed to capture contaminants which have been
directed into the hood by thermal draughts, directional movement, or by local generation.
An example of a receptor hood is a chimney in an incinerator.

To be effective the LEV must be properly designed and located close to the source of
contamination so that the system can extract all or at least sufficient of the contaminant to
prevent exposure above the WEL. Capture and extraction may be through engineered natural
air flows, such as pressurised systems, or by the use of fans or pumps to suck the air away.
Some systems are very noisy and this in itself may represent a hazard.

The contaminant must be carried away by secure ducting to an exhaust outlet. There will
usually be some form of filter fitted between the capture hood and the outlet to remove as
much of the contaminant as possible before venting.

The positioning of the outlet itself is important. The exhausted air must exit from the system

ENSIGN| Unit IG2 – Element 7 – Chemical and Biological 35
Agents