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Use and Limitation of Dilute Ventilation

Dilution ventilation operates simply by diluting the contaminant concentration in the general
atmosphere to an acceptable level. This is achieved by changing the air efficiently in the
workplace over a given period of time; for example, a number of complete changes every
hour.

The workplace air will be extracted by the use of fans set in the walls or roof, with fresh air
being pumped in.

The system is intended to remove gas contaminants (sometimes fumes) and keep the overall
concentration of any contaminant to below the WEL and/or the concentration of a flammable
substance to below its lower explosive limit. Where both a harmful and flammable substance
is encountered, such as propanone (acetone), then control of the first objective will usually
control the second.

Dilution ventilation has fairly limited use as an effective control strategy in occupational
hygiene. It can, however, be used with reasonable success provided the contaminants
conform, where applicable, to the following descriptions:

• The WEL of the harmful substance is high.
• The vapour pressure of a liquid is low, that is it has a low evaporation rate.
• The rate of formation of the gas product is slow.
• Operators are not in close contact with the contamination generation point.
• Any hazardous substance is carried swiftly away from the operator.

Two important criteria have to be considered when contaminants are to be removed from a
workplace using dilution ventilation:

The rate of contaminant generation. This condition the number of air changes per hour
required. Relevant factors in respect of the generation of contaminated vapour from liquids
include:
• The vapour pressure and potential to evaporate at the operating temperature of the
system.
• The surface area of the liquid in contact with the workplace air, including the potential
increase in surface area from spreading; for example, contact adhesives generate vapour
at a much greater rate after they have been spread over a surface, and complex metal parts
may have quite an extensive covering of solvent after they have been removed from a
degreasing bath.

ENSIGN| Unit IG2 – Element 7 – Chemical and Biological 38
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