UK Workplace Exposure Limits (EH40) for Common Gases: A Gas Detection Reference

Workplace exposure limits (WELs) are the legal limits on the concentration of a hazardous substance in workplace air in Great Britain. They are published by the Health and Safety Executive in EH40/2005 Workplace exposure limits, last revised in its fourth edition in January 2020, and apply under COSHH. Gas detector alarm levels for toxic gases are normally set with these limits in mind.

Two limits: 8-hour TWA and 15-minute STEL

Each WEL can have two values. The long-term limit is an 8-hour time-weighted average (TWA): the average exposure across a working shift. The short-term exposure limit (STEL) is a 15-minute average, which controls brief peaks. A personal gas monitor typically shows both, alongside instant low and high alarms.

EH40 limits for common gases (ppm)

• Ammonia (NH3): 25 ppm TWA, 35 ppm STEL

• Carbon dioxide (CO2): 5,000 ppm TWA, 15,000 ppm STEL

• Carbon monoxide (CO): 20 ppm TWA, 100 ppm STEL

• Chlorine (Cl2): 0.5 ppm STEL (no 8-hour limit)

• Chlorine dioxide (ClO2): 0.1 ppm TWA, 0.3 ppm STEL

• Hydrogen chloride (HCl): 1 ppm TWA, 5 ppm STEL

• Hydrogen cyanide (HCN): 0.9 ppm TWA, 4.5 ppm STEL

• Hydrogen fluoride (HF): 1.8 ppm TWA, 3 ppm STEL

• Hydrogen peroxide (H2O2): 1 ppm TWA, 2 ppm STEL

• Hydrogen sulphide (H2S): 5 ppm TWA, 10 ppm STEL

• Nitric oxide (NO): 2 ppm TWA

• Nitrogen dioxide (NO2): 0.5 ppm TWA, 1 ppm STEL

• Ozone (O3): 0.2 ppm STEL

• Phosphine (PH3): 0.1 ppm TWA, 0.2 ppm STEL

• Sulphur dioxide (SO2): 0.5 ppm TWA, 1 ppm STEL

• Benzene: 1 ppm TWA (carcinogen; exposure must be kept as low as reasonably practicable)

Always check the current edition of EH40 on the HSE website before relying on a figure for compliance, as limits are revised from time to time.

Oxygen and flammable gases are different

Oxygen and flammable gases do not have WELs. Oxygen monitors alarm on deficiency and enrichment, typically below 19.5% and above 23.5% by volume (normal air is 20.9%). Flammable gas is measured as a percentage of the lower explosive limit (LEL), with alarms commonly at 10% and 20% LEL.

What this means for your gas detector

• Set toxic alarms at or below the relevant STEL and TWA, and use the monitor's TWA and STEL alarms where it has them.

• Choose sensors with a range and resolution suited to low limits: chlorine dioxide, phosphine and nitrogen dioxide need low-range sensors.

• Bump test before each day's use and calibrate at the manufacturer's interval, typically every six months.

• Record exposure data where your COSHH assessment requires monitoring.

7 Solutions supplies, hires and calibrates portable and fixed gas detectors with alarms set to UK limits. Call 01489 326031, or see gas detector hire and calibration and service.