WEL stands for Workplace Exposure Limit and they have been derived to safeguard workers during tasks. A WEL is a concentration limit of a contaminant aligned to gas, vapour, aerosol or dust that is airborne.

The hazard concentration is measured and then averaged over a reference period of which workers can be exposed by inhalation, ingestion or absorption into the body. WELs can apply to individual substances or classes/groups of substance(s). WELs will identify the required protection value. WELs can be found in the UK HSE publication EH40.

For example, let’s take the substance: ‘Xylene’ (an organic chemical giving off gas vapours). Xylene has been measured in the workplace at a concentration of 300ppm. The WEL is 50ppm (as identified in the Health and Safety Executive (HSE) document EH40/2005), the necessary protection factor is, therefore: 300ppm divided by 50ppm = 6. This is called the Required Protection Factor (RPF). In the UK assigned protection factors (APF) are set at 4, 10, 20 and higher, in this case, an APF of 10 is needed to offer appropriate protection. Therefore a half mask with an A1 organic gas vapour filter fitted is suitable for the task.