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There are two types of OEL, Occupational Exposure Standards (OESs) and Maximum Exposure Limits (MELs). Both refer to concentrations of a substance in air, averaged over 8 hours and/or 15 minutes, but there are very important differences between them. They are set in different ways, and employers have different legal duties with respect to them.

The HSE says, "An OES is set at a level that (based on current scientific knowledge) will not damage the health of workers exposed to it by inhalation day after day." In contrast, "MELs are set for substances which may cause the most serious health effects, such as cancer and occupational asthma, and for which 'safe' levels of exposure cannot be determined or for substances for which safe levels may exist but control to those levels is not reasonably practicable."

Where an OES applies, workers must not be exposed to levels above it, whereas an MEL requires employers not only to keep exposures below the MEL, but also to reduce exposure to a level "as low as reasonably practicable". According to EH40, "In assessing reasonable practicability, the nature of the risk presented by the substance should be weighed against the cost and the effort involved in taking measures to reduce the risk."

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