Most chemical exposure in the workplace occurs when workers inhale vapours, fumes, dusts, and gases. This blog will help you understand chemical exposure limits set by Safe Work Australia and defined in the legal document Workplace Exposure Standard for Airborne Contaminants. We’ll outline what they are in real-speak (ie, sentences you can actually understand); and how to apply the standards to the chemicals at your worksite.
LEGISLATION: A person conducting a business or undertaking at a workplace must ensure that no person at the workplace is exposed to a substance or mixture in an airborne concentration that exceeds the exposure standard for the substance or mixture. Model WHS Regulation.
What are chemical exposure standards
When chemicals are used in the workplace they release potentially harmful contaminants into the air. These take the form of fumes, dusts, mists, vapours etc. Because the substances are not naturally present in the air, if they occur in a high enough concentration (and are inhaled or ingested) they could cause your workers to become sick, die, or develop chronic illness like asthma and cancer. These mists and vapours etc are more commonly known as airborne contaminants.
The Workplace Exposure Standards define the acceptable concentration levels of more than 700 airborne contaminants and set limits which must not be exceeded. These chemical exposure levels are established in three different ways:
8 hour time weighted average (daily exposure levels based on an 8 hour working day x 5 days per week)
Short term exposure limit (STEL) (short term exposure levels for periods of up to 15 minutes x 4 times per day)
Peak limitation (exposure limits which must never be exceeded at any time)
REMEMBER: Exposure standards are not like the NSW/QLD border — there is no definitive safe side. Workers may become ill even when working within the set exposure limits as people can metabolise chemicals differently (body weight, BMI, age, allergies are all factors).
Understanding the three types of exposure standards
Your workplace must not exceed any of the three types of exposure standards. Let’s take a closer look at them below:
1. Eight-hour time-weighted average (TWA) exposure standards
This exposure standard set the chemical exposure limits which are averaged over a 40 hour working week — and based on an 8 hour day x 5 days working every week. If your workers have shifts longer than 8 hours, or the working week is variable (ie, blocks of more than 40 hours could be worked in one week) you will need to adjust the standard. The variation to the standard must compensate for the decreased recovery times between shifts.
2. Short term exposure limits (STEL)
While keeping the normal work environment within the 8 hour TWA exposure standards, there are also short term exposure limits which may occur up to 4 times in any 8 hour work day. These short term exposure limits are the maximum airborne concentration levels permitted over a period of 15 minutes with at least 60 minutes between exposures.
The STELs address the acute effects of some chemicals which become intolerable after brief exposure and contribute to irritation, tissue damage, and narcosis (which impairs a worker’s ability to carry out their job tasks safely). The STEL’s must never be exceeded.
3. Peak limitation exposure standards
Some chemicals in the exposure standards are rapidly acting substances and are too dangerous for prolonged exposure. These chemicals produce adverse effects after even brief exposure to high concentration so it is NOT appropriate to set an average concentration over an 8 hour shift. Workers should only contact these chemicals for the briefest periods possible (never more than 15 minutes) and the peak limit is the maximum concentration in the air at any time. It must never be exceeded at any time.
BEST PRACTICE: Aim to keep chemical exposure limits below the 8-hour TWA exposure standard at all times.
Demonstrating exposure levels
Let’s look at two different chemicals to demonstrate each of the three types of exposure standards.
Acetic Acid (used when manufacturing paints and adhesives) and has a TWA exposure limit of 25 (milligrams of substance per cubic metre of air). It also has a STEL of 37 (milligrams p/cubic metre). This means that a contractor working a 40 hour week (5 x 8 hour shifts) and continually breathing air with 25 milligrams of acetic acid in every cubic metre, could also have 4 short term exposures where the acetic acid content in the air increased to 37 mg (per cubic metre) for up to 15 minutes each time.
Potassium hydroxide (used to make liquid fertilisers and soaps) has a peak limit of 2 milligrams per cubic metre of air. This means workers should never be exposed to the chemical for more than 15 minutes and the maximum exposure limit (of that 15 minute period) is 2 milligrams per cubic metre of air.
CAUTION: Be careful when applying exposure standards to workplace environments that contain chemical mixtures of different airborne contaminants. The interaction of different substances may need to be assessed by a toxicologist, occupational hygienist or physician.
Understanding the nature of airborne contaminants requires specialised knowledge of measurements statistics and how to interpret those results. Our free eBook How to manage the risk of Hazardous Chemicals in the workplace won’t give you this knowledge but it will get help you implement a chemical risk management methodology so you can decide on the best hazard control measures and determine whether you need to engage an occupational hygienist to work out an air monitoring strategy. Download and read it today.