Page 232 - IGC BOOK

The gaseous state (vapours and gases) is measured in ppm and refers to the number of parts
of vapour or gas of a substance in a million parts of air by volume, measured at a standard
temperature and pressure (usually 25°C and 760 mm Hg, respectively). Particulate matter in
3
dusts, fumes, etc. is measured in mg/m , which refers to the milligrams of the substance per
cubic metre of air.

One further unit of measurement is used in relation to fibres (such as asbestos).
Concentrations of fibres are expressed in:
-1
• Fibres per millilitre of air (fibres ml ).

Why Time-Weighted Averages Are Used

A worker can be exposed to different levels of inhalation of a hazardous substance throughout
the working day.

At some times, they may be exposed to high levels of contaminant; at other times, the
exposure level may be low. In many cases, it will not be practical to measure an individual's
exposure for all of the working period.

A time-weighted average is equal to the sum of the part of each time period, which is
multiplied by the exposure level of the contaminant in that time period. It is then divided by
the hours in the working day (usually eight hours) and the level indicated as a time-weighted
average (as seen above). That is, the average of all the total exposures in the working day.
This can be significant where the concentration of the chemical changes through the day, or
the time exposure varies.

ACGIH defines three categories of threshold limit values:

Threshold Limit Value – Time-Weighted Average (TLV-TWA): The concentration of a hazardous
substance in the air averaged over an 8-hour workday and a 40-hour workweek to which it is
believed that workers may be repeatedly exposed, day after day, for a working lifetime without
adverse effects.

Threshold Limit Value – Short-term exposure (TLV-STEL): A 15-minute time weighted average
exposure that should not be exceeded at any time during a workday, even if the overall 8-hour TLV-
TWA is below the TLV-TWA. Workers should not be exposed more than four times per day to
concentrations between TLV-TWA and TLV-STEL. There should be at least a 60-minute interval
between exposures. The short-term exposure threshold has been adopted to account for the acute
effects of substances that have primarily chronic affects.

Threshold Limit Value – Ceiling (TLV-C): This is the concentration that should not be exceeded
during any part of the working exposure. Peak exposures should be always controlled. For

ENSIGN| Unit IG2 – Element 7 – Chemical and Biological 27
Agents