The Technical Framework Behind COSHH Compliance Measurement
Adequate control of workplace chemical exposure under the Control of Substances Hazardous to Health Regulations 2002 (COSHH) is not simply a matter of installing ventilation and declaring compliance. The law requires that exposure is measured and demonstrated to be below applicable limits — or that the controls in place are inherently sufficient without measurement. Understanding the distinction between the different types of limit, and when biological monitoring replaces or supplements air monitoring, is essential technical knowledge for any safety professional or responsible employer.
Workplace Exposure Limits (WELs) — The Current Standard
A Workplace Exposure Limit (WEL) is the maximum concentration of an airborne substance to which a worker may be exposed, expressed as a time-weighted average (TWA). WELs are set by the HSE and published in EH40 Workplace Exposure Limits (currently 4th edition, 2020, incorporating 2023 amendments). They are given statutory effect by COSHH Regulation 7(7)(a) — exposure must not exceed the WEL.
WELs are expressed as two values:
- Long-term WEL (8-hr TWA): The concentration averaged over an 8-hour working shift. This is the primary control limit for chronic health effects — respiratory sensitisation, carcinogenicity, organ toxicity.
- Short-term WEL (15-minute TWA): The concentration averaged over any 15-minute period within the shift. This protects against acute effects — narcosis, acute irritation, immediate organ damage. Short-term WELs are not separate limits that replace the 8-hr TWA; both apply simultaneously.
The Old OES System — Why It Still Appears in Documentation
Before 2005, the UK used Occupational Exposure Standards (OESs) and Maximum Exposure Limits (MELs) — a two-tier system. OESs were 'good practice' benchmarks; MELs were absolute limits for the most dangerous substances. The 2005 amendment to COSHH abolished this distinction and replaced it with a single WEL system. If you encounter legacy risk assessments or SDSs referring to OES values, they must be updated — the OES values are not current legal limits. Many older assessments still carry OES references, which creates compliance uncertainty.
WEL Types and Examples
| Substance | 8-hr TWA WEL | 15-min WEL | Notation |
|---|---|---|---|
| Acetone | 500 ppm / 1210 mg/m³ | 1500 ppm / 3620 mg/m³ | — |
| Toluene | 50 ppm / 191 mg/m³ | 100 ppm / 384 mg/m³ | Sk (skin absorption) |
| Isocyanates (as NCO) | 0.02 mg/m³ | 0.07 mg/m³ | Sen (respiratory sensitiser) |
| Benzene | 1 ppm / 3.25 mg/m³ | — | Sk, BMGV (carcinogen) |
| Hydrogen chloride | 1 ppm / 2 mg/m³ | 5 ppm / 8 mg/m³ | — |
| Silica (respirable crystalline) | 0.1 mg/m³ | — | Carc |
The 'Sk' Notation — Skin Absorption and Secondary Containment
Substances marked 'Sk' in EH40 can be absorbed through intact skin to produce systemic toxicity. For these substances, controlling airborne exposure below the WEL is not sufficient — there must also be controls preventing skin contact. This is where secondary liquid containment (spill pallets, bunded work surfaces, chemical-resistant PPE) becomes directly relevant to COSHH compliance, not just to environmental protection. Examples include organophosphate pesticides, phenol, aniline, and nitrobenzene.
Biological Monitoring (BM) and Biological Monitoring Guidance Values (BMGVs)
For substances where skin absorption or variable exposure patterns make air monitoring insufficient to demonstrate adequate control, COSHH Regulation 10 requires biological monitoring. Biological monitoring measures exposure via body fluids (blood, urine, exhaled air) by testing for the substance itself or its metabolites.
HSE publishes Biological Monitoring Guidance Values (BMGVs) in EH40 Table 2 for substances where biological monitoring is practicable and validated. BMGVs are not health-based limits in the same sense as WELs — they are surveillance guidance values. Exceeding a BMGV does not in itself indicate that a worker has been harmed, but it indicates that the control system needs review.
Substances with BMGVs include:
- Benzene (S-phenylmercapturic acid in urine; trans,trans-muconic acid in urine)
- Toluene (toluene in blood; o-cresol in urine)
- Lead (blood lead — separately regulated under COSHH Approved Code of Practice for Lead)
- Mercury (mercury in urine and blood)
- 4,4'-Methylenedianiline (4,4'-MDA in urine)
When Atmospheric Monitoring Is Legally Required
COSHH Regulation 10 specifies that atmospheric monitoring is required where:
- It cannot be determined otherwise whether a WEL is being exceeded
- The COSHH ACoP identifies it as necessary (e.g., for Schedule 5 substances including cotton dust, vinyl chloride monomer, rubber fume)
Monitoring must be carried out by a competent person with appropriate analytical support. Results must be recorded and kept for 5 years (40 years for substances posing a risk to health). Monitoring records must include the date, circumstances, results, and analytical method used. A failure to monitor when required is a direct breach of Regulation 10 and a common finding in HSE prosecutions.
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