Sustaining life in hazardous environments...
Sustaining life in hazardous environments...

What Are The Changes to EH40 Exposure Levels?

1st August 2018

The UK Health & Safety Executive (HSE) will adopt EU Directive 2017/164 which establishes new ‘indicative occupational exposure limit values’ (IOELVs) for a number of toxic substances. This decision complies with Articles 2 and 7 of the Directive requiring Member States to establish the new occupational exposure limit values within national standards by August 21st 2018 (mining and tunnelling applications do not need to comply until 2023). More details can be seen here -

The HSE published WELs (Workplace Exposure Limits) in a document called EH40 which is available on the HSE website.

As well as changes to some of the WELs, the following toxic substances have been re-introduced into EH40 - Nitrogen Monoxide (NO) Nitrogen Dioxide (NO2) and Sulphur Dioxide (SO2).

Most manufacturer’s gas monitors alarm levels are set in accordance with the EU Directive (EH40 in UK) or based on national standards of countries outside of the EU. In gas monitors, these are set as ‘Time Weighted Average’ alarms.

Although there is no specified standard for the instantaneous alarms, these are usually set based on the TWA levels and in some cases higher to reduce the risk of false alarms.

Below is a table which defines the LTEL and STEL limit values for CO, HCN, NO, NO2 & SO2.

A full list of the affected LTEL and STEL limit values can be found here -



TWA Alarms

Instantaneous Alarms

Gas Type



Alarm 1

Alarm 2

Carbon Monoxide (CO)



Defined by manufacturer

Hydrogen Cyanide (HCN)



Defined by manufacturer

Nitrogen Monoxide (NO)



Defined by manufacturer

Nitrogen Dioxide (NO2)



Defined by manufacturer

Sulphur Dioxide (SO2)



Defined by manufacturer


The full EH40 document can be downloaded here – HSE EH40

Any further questions, feel free to speak to one of out tehcnical team on 0844 915 1111.

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