Understanding Workplace Exposure Standards

The Health and Safety at Work Act 2015 states that a person conducting a business or undertaking (PCBU) must:

  • Identify hazards that might give rise to risks to health and safety
  • Eliminate risks to health and safety
  • Minimise risks that are not reasonably practicable to eliminate
  • Ensure the safe use, handling, and storage of substances
  • Provide the information, training, instructions, or supervision necessary to protect all persons from risks arising from work carried out as a part of the conduction of the business or undertaking
  • Provide and maintain a work environment that is without risks to health and safety

These aims can only be achieved by a combination of identifying, measuring, and then eliminating or minimising.

If you decide to measure how much of a substance is present, then you need to have standards and limits, and that is where workplace exposure standards (WES) come into place.

BUT (and it is a big “BUT”), due to insufficient data, not all substances have a WES.

If a substance doesn’t have a WES, this should not be taken to mean that it is safe under all conditions.

For example, the gas that we use for domestic cooking is propane. We know it is hazardous, but it has no WES. In contrast, flour dust has a workplace exposure standard – TWA of 1 mg/m³.

What does "workplace exposure standard" actually mean?

A workplace exposure standard is a value that refers to the airborne concentration of substances at which it is believed that nearly all workers can be repeatedly exposed day after day without coming to harm.

The values are normally calculated on work schedules of five shifts of eight hours' duration over a 40-hour work week.

Because measuring for a substance eight hours a day, five days a week is onerous and often expensive, you could decide to measure over four hours, and that would be just as valid provided work practices are uniform.

This would give you a workplace exposure standard time weighted average (WES – TWA) where you have monitored and measured over the duration of a shift or partial shift.

We only work with some hazardous substances for a short period, and these will often have workplace exposure standard short-term exposure limits (WES – STEL). These are typically measured over a 15-minute period.

There is also a WES ceiling. This gives an absolute limit that the workplace must not exceed, whereas a WES limit gives a value that the worker must not exceed over an 8-hour period.

Here are some examples

Methanol has a TWA of 200 ppm but a STEL of 250 ppm

If you are working with methanol for eight hours a day, the average measurement over the eight hours should not exceed 200ppm. If you are working with methanol for a short period (15 minutes), the average measurement should not exceed 250ppm.

Iodine has a TWA of 0.01 ppm but a WES ceiling of 0.1ppm

For iodine your average eight hour readings should not exceed 0.01 ppm, and at no point when you are working with iodine should the levels exceed 0.1 ppm.