Where there’s smoke…07 March 2019

Recently-published HSE guidance has set the first UK limits for workplace exposure of nitrogen dioxide and nitrogen monoxide, as well as lowering the carbon monoxide value: that means that employers whose workers come into contact to diesel engine exhaust emissions could need to start monitoring exposures

From 21 August 2018, the third edition of HSE’s guidance document ‘EH40/2005 Workplace Exposure Limits’ came into force, introducing new and revised workplace exposure limits (WELs) for 31 substances. Understanding the updated WELs will enable organisations to take appropriate actions to ensure compliance with the changes to legislation and safeguard the health of employees.

The updates include recommendations from the European Agency for Safety and Health at Work in its fourth list of indicative occupational exposure limit values of chemical agents in the workplace. That list draws on the most recent data available, and takes into account the availability of reliable measurement techniques.

The HSE has translated those figures to WELs, outlining the highest acceptable concentration of an airborne hazardous substance in the workplace. The amended EH40 document publishes, for the first time, WEL entries for nitrogen dioxide and nitrogen monoxide, as well as reducing the WEL for carbon monoxide. Therefore, they affect any company assessing exposure to diesel engine exhaust emissions (DEEEs).

However, so great is the impact of these rules expected to be on the European mining and tunnelling industries, that they have been given a five-year grace period, during which time member states are allowed to apply national limits for CO, NO and NO2 (see table). EH40 states: “The European Commission recognises there may be technological challenges and associated costs for underground mining/tunnelling industries in complying with the proposed limit values for nitrogen monoxide, nitrogen dioxide and carbon monoxide.”

Other compounds that have been revised include the new short-term exposure limit for sulphur dioxide, which is present in emissions from motor vehicles (although modern low-sulphur fuels normally have minimal levels). Diacetyl, a chemical used in food and manufacturing industries, has also been revised.


With diesel used as a major source of fuel for internal combustion engine-powered vehicles, equipment and power generators, monitoring DEEE exposure is fundamental – and a requirement under COSHH, where employees may be exposed to the inhalation of harmful DEEEs (should the risk assessment indicate this monitoring is required).

DEEEs contain a complex mixture of combustion products, including gases, vapours, aerosols and particulates. The main components are oxides of nitrogen, carbon dioxide and carbon monoxide – among other substances – and the inhalation of these by-product compounds can be hazardous to human health. Trace quantities of unburned or partially oxidised diesel fuel may also result in the production of aldehydes, volatile organic compounds (VOCs), and polycyclic aromatic hydrocarbons (PAHs). Some of these compounds are classed as blood poisons and, in high enough concentrations, can starve the body of oxygen, leading to asphyxiation. Chronic exposure to DEEE can also have long-term health impacts, with diesel engine exhaust classed as a Group 1 carcinogen (“carcinogenic to humans”) by the International Agency for Research on Cancer (IARC).

The primary health effect associated with exposure to DEEEs is irritation of the eyes and respiratory tract. This is particularly notable wherever there are high levels of white smoke present in the workplace, though exposure to blue/black smoke can also lead to coughing and breathlessness.


WELs are important in managing activities which use or generate hazardous substances, and for undertaking risk assessments. As per Control of Substances Hazardous to Health (COSHH) Regulation 10, exposure monitoring via personal air sampling may be required to determine acceptability of exposure levels.

It’s important to keep in mind that WELs are not fine lines between safe and unsafe; these were never intended to be absolute standards. In all cases, the principles of good practice for the control of exposure to hazardous substances must be met (COSHH Schedule 2A). Where an exposure threshold for safe use can’t be established (which is the case for some carcinogens and sensitisers), then the question for adequate control cannot be conclusively addressed by compliance with an exposure limit. In this case, an ALARP approach should be taken – ensuring exposure is ‘as low as reasonably practicable’.

Keep in mind also that many WELs apply to inhaled substances, although exposure is also possible through ingestion, skin absorption or eye contact. All routes must be considered when assessing control of exposure to a hazardous substance.


Knowing if you’re above/below the outlined WEL can only be done through personal exposure monitoring – but DEEEs are complex. As COSHH doesn’t state a specific overall exposure limit for DEEE, firms must rely on the WELs in EH40/2005 for individual DEEE component chemicals.

Personal exposure monitoring must be undertaken regularly to ensure that WELs are not exceeded. For DEEE, this can be carried out via real-time measurements and active long-term sampling (normally for several hours within the working day). Real-time monitoring, while engines are in operation, can identify spikes in the levels of combustion gases to determine whether remedial actions are needed to lower exposure levels. Laboratory analysis of personal exposure samples can detect the presence of other hazardous substances present in DEEE via long-term sampling.

The carbon particles in diesel exhaust are produced during combustion. Many substances from the combustion process can be adsorbed on to these carbon particles. Diesel exhaust particulate contains two main components: organic carbon (OC) and elemental carbon (EC). OC is associated with adsorbed organic substances, and EC with the soot cores.

When assessing DEEE exposure, determining EC concentration is considered an appropriate method. This substance acts as a surrogate, as it’s very fine and acts like other DEEE gases and vapours, so an elevated EC concentration indicates high concentrations of these other components.

One well-recognised method for sampling diesel particulates is using a cyclone sampler with quartz filter and subsequent analysis for respirable carbon. There is no WEL for EC in the UK, but guidance values are available. When monitoring DEEEs, Socotec applies the more cautious lower limit for EC exposure at 0.1 mg/m3.

It is crucial to note, especially with the updates to EH40/2005, there is not a direct correlation between EC particulates and the concentration of carbon monoxide or oxides of nitrogen (see also www.is.gd/azikuf). The exposure to these gases should therefore be measured using other methods, such as real-time data-logging gas measurement instruments.

To monitor DEEE exposure, Socotec normally proposes sampling of respirable carbon and combustion gases including carbon monoxide, nitrogen monoxide and nitrogen dioxide. Carbon dioxide levels may also be determined as a ventilation indicator. Should the DEEE exposure level be expected to be elevated – and/or ventilation expected to be poor – then it would also be recommended that the suite of analysis extends to include PAHs, VOCs, and aldehydes.

For companies operating in industries using and/or generating hazardous substances, understanding the new WELs is fundamental to ensure appropriate changes can be made to improve compliance and safeguard worker health.

Failing to comply may expose workers to hazardous substances at consequential levels. Any subsequent illness results in productivity loss, and the risk of prosecutions and hefty fines.

Mary Cameron, occupational hygienist, testing consultancy Socotec

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