Great Britain Workplace Exposure Limits
What are workplace exposure limits in Great Britain?
Workplace exposure limits (WELs) are Great Britain's legal limits on how much of a hazardous substance workers may breathe in. HSE approves them under the Control of Substances Hazardous to Health Regulations 2002 (COSHH) and publishes them in EH40/2005 Workplace exposure limits. Under COSHH regulation 7(7)(b), control of exposure counts as adequate only if "any workplace exposure limit approved for that substance is not exceeded" [1].
Legal definition (COSHH reg. 2(1)) A WEL is "the exposure limit approved by the Health and Safety Executive for that substance in relation to the specified reference period when calculated by a method approved by the Health and Safety Executive, as contained in HSE publication 'EH/40 Workplace Exposure Limits 2005' as updated from time to time" [2]. Any substance with an HSE-approved WEL is a "substance hazardous to health" under COSHH, whether or not it is classified under CLP [2].
How they work (HSE Approved Code of Practice and guidance L5)
- A WEL is "the maximum concentration of an airborne substance averaged over a reference period to which employees may be exposed by inhalation" [3].
- WELs are time-weighted averages over two reference periods: a long-term 8-hour limit and a short-term limit [3]. Short-term exposure limits (STELs) have a 15-minute reference period. They are set for substances with acute effects, to protect against brief exposures [4].
- WELs "should not be considered a hard and fast line between safe and unsafe". How far exposure is kept below the WEL should be proportionate to the health risk [3][5].
- EH40/2005 holds the list of WELs, the approved averaging methods and the definitions of "inhalable" and "respirable" [6]. It also lists which substances count as carcinogens or mutagens for COSHH [7].
- A substance with an "Sk" notation can pass through the skin and cause disease elsewhere in the body, so controlling airborne levels alone is not enough [8].
- Under regulation 10, exposure monitoring is needed where measurement is required to be sure a WEL or an in-house standard is not exceeded [9].
Stricter rules for some substances Meeting the WEL is not enough for these. COSHH reg. 7(7)(c) requires exposure to be reduced "to as low a level as is reasonably practicable" for:
- substances carrying H340, H350 or H350i, or listed in Schedule 1;
- substances carrying H334, those listed in section C of HSE's Asthmagen? publication, or any other substance the risk assessment shows may cause occupational asthma [1].
Substances without a WEL
- Having no WEL "does not mean that it is safe". Employers should control exposure to a level nearly all workers could be exposed to day after day without harm [10].
- Any dust becomes a substance hazardous to health at or above 10 mg/m³ inhalable dust or 4 mg/m³ respirable dust (8-hour time-weighted average) [2].
- For dusts without a WEL that may need lower limits, L5 advises employers to set their own in-house standards [11].
Examples from EH40/2005 Table 1
| Substance | CAS | Long-term (8-hr) | Short-term (15-min) | Comments |
|---|---|---|---|---|
| Benzene | 71-43-2 | 1 ppm / 3.25 mg/m³ | none | Carc, Sk [12] |
| Formaldehyde | 50-00-0 | 2 ppm / 2.5 mg/m³ | 2 ppm / 2.5 mg/m³ | Carc [13] |
| Silica, respirable crystalline (respirable fraction) | none listed | 0.1 mg/m³ | none | Carc (where generated as a result of a work process) [14] |
| Silica, amorphous | none listed | 6 mg/m³ inhalable; 2.4 mg/m³ respirable | none | none [15] |
Approach: I searched the COSHH Regulations 2002 (as amended, from legislation.gov.uk), HSE's Approved Code of Practice and guidance L5, and HSE's EH40/2005 Table 1 list of workplace exposure limits.
I can look up the WEL and notations for any specific substance, or compare GB values with EU, German TRGS 900 or US OSHA limits.
How do I read the HSE exposure limits table?
Read each row of EH40/2005 Table 1 from left to right: substance name, CAS number, then the long-term limit (8-hour TWA) and short-term limit (15-minute), each given in ppm and mg.m-3, then a Comments column with notations such as Sk, Carc, Sen and BMGV. Table 1 is legally binding under COSHH, but the CAS column is not part of the approved list, and the rest of EH40 is guidance (EH40/2005, pp. 2 and 9).
Worked example: toluene
| Column | Value | Meaning |
|---|---|---|
| Substance / CAS | Toluene, 108-88-3 | Names follow IUPAC unless flagged INN or ISO |
| Long-term (8-hr TWA) | 50 ppm / 191 mg.m-3 | Average over the shift |
| Short-term (15-min) | 100 ppm / 384 mg.m-3 | Applies to any 15-minute period in the shift |
| Comments | Sk | Can be absorbed through the skin |
The limit columns
- Long-term (8-hr TWA): this limits total intake by inhalation over one or more shifts. You add up each exposure level multiplied by its duration in any 24-hour period, then divide by 8: (C1T1 + C2T2 + … CnTn) / 8. Time with no exposure counts as zero. In HSE's example, 7 h 20 min at 0.12 mg.m-3 gives 0.11 mg.m-3 as an 8-hour TWA (EH40, pp. 28, 32).
- Short-term (15-min): this is set for effects that can follow a few minutes of exposure, such as eye irritation (EH40, p. 6). If both limits are listed, your measured exposure must be compared against both (EH40, p. 29). Some entries use a different period, and the Comments column says so. For example, acrylic acid's STEL is marked "*STEL in relation to a 1-minute reference period" (EH40, p. 9).
- A dash in the STEL column: "Where no specific short-term exposure limit is listed, a figure three times the long-term exposure limit should be used." HSE presents this as a guideline for controlling short-term peaks. It is not an approved limit (EH40, pp. 8, 28).
- ppm vs mg.m-3: limits for vapours are set in ppm and converted to mg.m-3 as mg.m-3 = ppm × molecular weight / 24.05526, i.e. at 20 °C and 1 atmosphere. Particles (dust, fume) are usually given only in mg.m-3, so the ppm cell shows a dash. MMMF and RCF limits can also be in fibres/ml (EH40, p. 29).
Dust fractions
- Dust limits refer to the inhalable fraction unless the entry says respirable (EH40, pp. 9, 29).
- Inhalable is roughly what enters the nose and mouth.
- Respirable is roughly what reaches the gas-exchange region of the lung (EH40, p. 25).
- Some entries split into two sub-rows, one per fraction. For example, silicon has 10 mg.m-3 inhalable and 4 mg.m-3 respirable [17].
Comments column notations (EH40, p. 8)
- Sk: can be absorbed through the skin, with concern about systemic toxicity. You must also control skin contact, not just airborne levels (EH40, p. 30).
- Carc: capable of causing cancer and/or heritable genetic damage. COSHH reg. 7(7)(c) requires exposure to be reduced as low as is reasonably practicable (ALARP) for H340/H350/H350i or Schedule 1 substances (EH40, p. 26).
- Sen: capable of causing occupational asthma. Exposure must also be reduced ALARP, and health surveillance is appropriate (EH40, p. 26).
- BMGV: a biological monitoring guidance value exists in Table 2. Dichloromethane is one example (BMGV, Sk) [18].
- The notations are not exhaustive (EH40, p. 9). A substance can be a skin hazard, carcinogen or asthmagen without the flag.
Other things the Comments column can carry
- Pointers to definitions: "See paras" in the name or CAS cell sends you to the definitions section. For example, flour dust (10 mg.m-3 long-term, 30 mg.m-3 short-term, Sen) points to para 23 [19].
- Conditions on how the limit applies: for example, the hardwood dust WEL "shall apply to all the wood dusts present in that mixture" [20]. The rubber fume limit "relates to cyclohexane soluble material" [21].
- Sector-specific values: carbon monoxide has a second set of limits marked "applicable to underground mining & tunnelling industries ONLY until 21/8/23" [22]. That date has passed, so the first set of values applies.
Limits of use
- WELs apply to personal exposure of people at work. They are not for neighbourhood or environmental levels.
- They are approved only for atmospheric pressure between 900 and 1100 millibars (EH40, p. 28).
- If a substance is not in the table, that "does not indicate that it is safe" (EH40, p. 6).
- A WEL is not "a hard and fast line between safe and unsafe" [3].
Approach: I used HSE's EH40/2005 Table 1 and the COSHH Approved Code of Practice L5. The explanatory notes come from HSE's EH40/2005 publication (fourth edition, 2020) on hse.gov.uk.