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Crowcon #4: What is a time-weighted average?

What is a time-weighted average? Chris explains the importance of them when toxic substances are involved

Last week we looked at the toxic gas, Hydrogen sulphide, and briefly went through TWAs, so I thought this week I’d go into more detail about TWAs and the importance of them, especially when coming into contact with toxic gases.

As mentioned last week, toxic gases are considered to be those gases that can cause injury, illness or a reduced length or quality of life. Low concentration may not cause a problem or be noticed, but prolonged exposure may cause chronic illness or even premature death. The regulatory bodies are constantly working on acceptable exposure limits and these must be remembered when considering the purchase of a toxic gas detector.

The Health & Safety Executive’s publication, EH40, lists the occupational exposure limits as defined by the Control of Substances Hazardous to Health (COSHH), and it defines and lists gases which have a workplace exposure limit (WEL).

WELs are concentrations of hazardous substances in the air, averaged over a specific period of time, referred to as a time-weighted average (TWA). TWA values are calculated by taking the sum of exposure during a workday to a particular toxic contaminant in terms of parts-per-million-hours, and dividing by an eight-hour period.

WELs are defined in terms of 2 exposure limits, the long-term exposure limit (LTEL) and short-term exposure limit (STEL).

The LTEL is the maximum exposure allowed over an 8-hour period. When calculating the exposure level, if the exposure period is less than 8 hours, then the exposure limit can be increased providing that exposure above the LTEL value continues for no longer than an hour.

The STEL is a limit value above which exposure to a hazardous substance should not occur and usually relates to a 15-minute reference period. The aim of an STEL is to prevent adverse health effects and other unwanted effects due to peak exposure that may not be controlled by the application of an 8-hour TWA limit.

Exposure limits are in place to control the effects of substances, depending on the nature of the substance and the effects of exposure. Some effects require prolonged exposure, while others may be seen after brief exposures.

Guidance as to how best monitor toxic substances can be found in the HSE document HSG 173. You can also look at the EH40 for guidance. Both LTEL and STEL need to be monitored in order that the correct indication of exposure can be deduced. It is important to recognise that a substance which does not have a WEL is not guaranteed to be safe, 2 examples being NO and NO2. Monitoring and control of exposure within the workplace may still be necessary.

In some countries, but not the UK, some gases also have an instantaneous level that must not be exceeded under any circumstances.

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