Occupational exposure limits explained: what the numbers mean

Short answer: an occupational exposure limit (OEL) is a legal limit on the airborne concentration of a hazardous chemical at work, measured over a defined period. In South Africa, the Minister sets OELs for hazardous chemical agents in Annexure 2 of the Regulations for Hazardous Chemical Agents, 2021. Each one is an eight-hour average, a 15-minute limit or a ceiling, and whether a result complies depends on whether the agent has a maximum limit or a restricted limit.

The regulations were published as Government Notice R. 280 in Government Gazette 44348 on 29 March 2021 and repealed the Regulations for Hazardous Chemical Substances, 1995. Their annexure tables took effect 18 months after promulgation (regulation 18(2)). A monitoring report or risk assessment that still compares results with the 1995 limits is out of date. For the wider duties, see what the Regulations for Hazardous Chemical Agents, 2021 require.

Three ways an OEL is expressed

Regulation 1 allows an OEL to take three forms:

• Eight-hour time-weighted average (TWA): the maximum average airborne concentration calculated over an eight-hour working day, for a five-day working week.

• Short-term exposure limit (STEL): a time-weighted average calculated over a 15-minute period.

• Ceiling (C): a maximum or peak concentration, determined over the shortest analytically practicable period, which does not exceed 15 minutes.

Tables 2 and 3 each have two columns for the OEL and two for the short-term or ceiling value (headed STEL/C): one in ppm (parts per million) and one in mg/m³ (milligrams per cubic metre).

Maximum limits and restricted limits

Regulation 10(1) sets a different test for each table:

• Maximum limit, OEL-ML (Table 2): control is adequate only if exposure is as low as reasonably practicable below the limit. A result under the number is not, on its own, proof of compliance. Temporary excursions need no significant risk, no sign of failed control, a temporarily demarcated area and the protective equipment in regulation 11.

• Restricted limit, OEL-RL (Table 3): control is adequate if exposure is below the limit. Respirators may be used to get below it only in a zoned area, and only after exposure has been reduced as far as reasonably practicable by other means.

Where exposure would exceed the restricted limit without respirators, regulation 8 requires a marked respirator zone, and regulation 11(2) requires respirators capable of controlling exposure to below the OEL.

Reading the tables: four examples

Four entries from the Gazette tables:

• Benzene (Table 2): OEL 1 ppm and STEL/C 5 ppm, with the notations CARC and SKIN.

• Crystalline silica, quartz (Table 2): OEL 0,1 mg/m³, marked (R) for the respirable fraction, with the notation CARC.

• Acetone (Table 3): eight-hour TWA 500 ppm and STEL/C 1 000 ppm.

• Toluene (Table 3): eight-hour TWA 40 ppm, with the notation SKIN.

The notations matter as much as the numbers:

• CARC: carcinogenicity, based on GHS categories 1A and 1B.

• SKIN: danger of absorption through the skin. Overexposure can occur through skin contact even when airborne exposure is at or below the OEL.

• DSEN and RSEN: potential to cause dermal or respiratory sensitisation.

• R, I, T and IFV: the respirable, inhalable or thoracic fraction, or the inhalable fraction and vapour. The sampler must collect the fraction the limit refers to, so a total dust result cannot be compared with a respirable limit.

Two common cases are covered in silica dust at work and welding fume monitoring.

How results are compared with the limits

Regulation 6 requires air monitoring by an approved inspection authority that is representative of employees' exposure. In practice:

• Full-shift personal samples are compared with the eight-hour OEL.

• Short samples taken during peak tasks are compared with the STEL/C value.

• Groups of similarly exposed employees may be sampled instead of everyone, but regulation 6(2)(a) requires the sample size to follow the NIOSH Occupational Exposure Sampling Strategy Manual, chosen for the top 10% of the group at a 95% or 90% confidence level, depending on the type of limit.

• Shift length matters: the TWA assumes an eight-hour day and a five-day week, so results for 12-hour shifts need careful interpretation.

Table 4 of Annexure 2 adds biological exposure indices (BEIs) for biological monitoring, such as end-of-shift urine tests, and exposure to its agents requires medical surveillance (regulation 7(1)(a)).

A quick self-check

• Check that your latest survey report compares results with Annexure 2 of the 2021 Regulations, not the 1995 tables.

• Note whether each agent measured is in Table 2 or Table 3. For Table 2 agents, look for a plan to reduce exposure further, even below the limit.

• For agents marked SKIN, check glove selection and skin contact, not only air results.

• Where an agent has a STEL/C value, check that peak tasks were sampled.

• Where results exceed a restricted limit, check that a respirator zone is marked and the respirators can bring exposure below the OEL.

• Check that the report's shift lengths match the shifts actually worked.

How Qualitas can help

Qualitas Occupational Consulting is a Department of Employment and Labour Approved Inspection Authority and a SANAS-accredited (ISO/IEC 17020) inspection body for asbestos, lead, noise and hazardous chemical agents. Our hazardous chemical agents surveys sample the right fraction for each agent, compare every result with the correct Annexure 2 limit, and explain what the numbers mean for your controls, in a report written for management, SHEQ teams and auditors.

How often the measurements must be repeated is covered in how often hazardous chemical agent monitoring must be done.

Not sure what your site needs? Start with our free Compliance Gap Analysis, request a quote, or call or WhatsApp 082 649 6599.

This article summarises the occupational exposure limits in the Regulations for Hazardous Chemical Agents, 2021 (Government Notice R. 280, Government Gazette 44348, 29 March 2021) for general guidance. It is not legal advice; refer to the regulations for the full requirements and the complete tables.

Christo de Wet is a SAIOH-registered occupational hygienist with 15 years' experience and the founder of Qualitas Occupational Consulting.