WEL stands for Workplace Exposure Limit, a guideline used to help protect workers from harmful exposure to airborne substances such as gases, vapours, aerosols, or dust. These limits are set to reduce the risk of harm when substances are inhaled, ingested, or absorbed into the body over a period of time during work activities.

Each WEL is based on a concentration value, which is then averaged over a reference period (either short-term or long-term) to assess whether exposure levels are within safe boundaries. WELs can apply to individual chemicals or groups of similar substances and are detailed in the UK Health and Safety Executive (HSE) publication EH40.

If an assessment shows that a chemical exceeds its WEL — for example, if Xylene is measured at 300ppm in the workplace and the WEL is 50ppm — the Required Protection Factor (RPF) is calculated by dividing the measured value by the WEL:

300ppm ÷ 50ppm = 6 (RPF)

This means a protection factor of 6 is required. In the UK, Assigned Protection Factors (APFs) are set at standard levels such as 4, 10, 20, etc. Therefore, the next available APF above the RPF (in this case, 10) should be selected.

To protect against xylene at these levels, a half mask fitted with an A1 organic gas vapour filter would be appropriate, as it meets the APF of 10.

Key points:

• WEL = Workplace Exposure Limit, used to control exposure to hazardous substances.
• Exposure levels are compared to the WEL using a reference period.
• If exposure exceeds the WEL, calculate the Required Protection Factor (RPF).
• Match the RPF with the next highest Assigned Protection Factor (APF).
• Use appropriate respiratory protection, such as a half mask with the correct filter type, to meet the APF.

If you need further support, please contact our Technical Support Team using the email: technical@jspsafety.com or contact us via our WhatsApp.