Interpreting Workplace Exposure Limits
Workplace exposure limits (WELs) confuse me. It is great that Australia is reviewing its exposure standards, most of which haven’t been touched since 1995.
This post won’t debate the merits of “health-based” versus “practical” exposure standards or look at the underlying science at all. Instead, I have open questions on how WELs are intended to be interpreted I’ve yet to figure out.
The Law
In Australia’s model WHS legislation, the two most relevant sections are:
“A [PCBU] must ensure, so far as is reasonably practicable, the health and safety of workers…”
“A [PCBU] … must ensure that no person … exceeds the exposure standard.”
It appears that the change to WELs won’t come with a change in legislation other than perhaps updating the name.
But even the current difference between the primary duty of care (s19) and preventing exceedance (s49) is interesting to me.
It seems that compliance with s19 doesn’t necessarily mean compliance with s49, and vice versa. So could you fulfill your “reasonably practicable” obligations and still experience a at least one exceedance? Does s49 then fit in a risk based model?
Regardless, s49 is clear. Any exceedance is a breach of the regulation. No confusion.
The Name
The change from “workplace exposure standard” (WES) to “workplace exposure limit” (WEL) was intended to align with international conventions—but not entirely. While “limit” is widely used, only the UK uses WEL specifically. Why not adopt ISO’s term, “occupational exposure limit value,” if international consistency is the goal?
The second reason was to reinforce that it is a hard limit. No exceedance is ever acceptable. Well… at least no measured exceedance.
The Intention
Here’s where I get confused. The name and regulation, and the WEL document seem to send mix messages. WEL are referred to as “the maximum level of an airborne contaminant that most (but not all) people can be exposed to without harm to their health”. They are derived from international “health-based limits”.
This seems to conflict with another statement in the same document - “WEL for some chemicals have been modified to reflect the health-based recommendations … however, most limits remain unchanged.” (Emphases added).
So, are WELs truly health-based? Many of the original 1995 WES values were adopted from ACGIH TLVs. Perhaps the review concluded these values remain relevant. However, their application under the regulation is inconsistent with the idea of health-based limits.
Chronic diseases typically correlate with accumulative exposure; that is, the work life average exposure * working years. So by extension, ensuring the average exposure of the worker being lower than the standard will mean the risk is lower than that used to derive the standard (e.g. <1 in 1,000 risk of COPD).
I could be convinced that this metric is suitable when considering health risk and what is “reasonably practicable”. But it’s clearly inappropriate under the regulation as it allows for quite higher rates of exceedance (almost 50% in extreme cases).
That said, any parametric (i.e. using a distribution like normal or log-normal) criteria will never categorically demonstrate compliance. Statistically, there is always some probability of an exceedance occurring, even when exposures are really well controlled.
This leaves compliance monitoring (direct comparison). This problematic because:
the chances of getting a compliant result in a poorly control environment is high
every additional sample increases the probability of getting an exceedance which incentivises doing as little (or no) sampling as possible.
Non-Threshold Limit Values
I’m not smart enough to comment on the threshold discussions. But for chemicals deemed to have thresholds (NTGC), the WELs have been withdrawn. The logic being that NTGC can cause cancer at any level. Instead, exposures must be eliminated or reduced as low as responsibly practicable.
This confuses me for a few reasons:
It’s already a requirement to manage risk SFARP, even with exposure standards
Other carcinogens with WEL are not set at their threshold which suggests an acceptable risk of cancer
The cancer risk of NTGC is still proportional to the exposure. At some point we are talking 1:1 million, 1:1 billion risk. Is there a level of risk we can agree is tolerable?
Additionally, section 50 of the regulation requires exposure monitoring if you are unsure if you comply to an exposure standard, or if needed to determine whether there is a risk to health. There is no WEL, and NTGC always have a risk to health (no matter how small). Does that mean there’s no requirement to assess exposures under s50?
The Documentation
I haven’t seen detailed documentation supporting the new WELs. Surely, this exists from the review process. Have I missed it? This seems important to understand what the WEL is intended to protect against.
Mixtures
Consideration of mixtures is a requirement of the regulations (s49). The guidance document for WESs explain the varies types of mixtures but only give specific guidance for additive effects. I still don’t know how to properly identify and account for synergistic effects. That’s my fault, but I don’t think I’m the only one.
Conclusion
The differences between how the legislation is written and how supporting documents communicates ways of compliance seems important. A company or a hygienist may think they are meeting their obligations when they are not.
This leaves me with some questions:
What does it mean for a WEL to be health-based given how it will be enforced?
Is it possible to meaningfully demonstrate legal compliance and effective control from an OH perspective? (Exceedances fractions verses single sample comparisons)
Does s49 incentivise doing as little (or no) monitoring as possible? Are business led to question “Why create evidence that can be used against you willingly?”
Should there be more specific guidance on how you to demonstrate compliance to s49? (E.g. assessment strategies)
Is the logic of removing WEL for NTGC sound?
I don’t have the answers. I’d love to hear your thoughts.