March 19, 2025

Calcium chromate is a toxic, yellow coloured powdery compound. It is a strong oxidising agent and can be used as a corrosion inhibitor and to depolarise batteries. It is also produced when chromium is present with an oxidant, such as air or water, at a high temperature (~200°C) or more¹.

Calcium chromate contains hexavalent chromium (chromium 6 or CrVI), which is genotoxic, listed as an IARC Group 1 carcinogen and a non-threshold genotoxic carcinogen (NTGC)².

Potential adverse health effects from hexavalent chromium exposure include nasal and skin irritation and ulceration, nasal and sinus cancers, kidney and liver damage and eye damage³.

The hazard associated with the occurrence of calcium chromate was first identified in the gas turbine power generation industry. It was primarily related to the formation of calcium chromate on metal bolts containing chromium on which calcium-containing anti-seize had been used.

But this issue isn’t limited to the gas turbine industry; calcium chromate can form in any instance where there is a source of chromium (such as stainless steel), a source of calcium, an oxidant and heat above ~200°c.

Occurrence of hexavalent chromium in the exhaust systems of mining trucks

The exhaust systems on large mining trucks typically have components made out of stainless steel, which contains chromium in some sections to strengthen the steel.

The exhaust components on trucks, particularly with turbos, can reach a temperature of 400oc or more, and lagging blankets are wrapped around the exhaust system components for insulation. Quite often, these are made of glass fibre.

Glass fibre lagging blankets are produced by melting a number of compounds, one of which is calcium oxide. When this is then combined with the chromium containing steel in the exhaust components at a temperature of ~200°c or more, calcium chromate can be produced.

Lagging blankets have historically been made with asbestos, then RCFs – refractory ceramic fibres or man-made vitreous fibres. These are listed as an IARC Group 2A carcinogen, meaning they are considered probably carcinogenic to humans. So, an alternative was sought to make the lagging blankets, and glass fibre became the substitute as these fibres are bio-soluble.

However, hexavalent chromium contained in calcium chromate has an IARC Group 1 classification, which means it is a confirmed human carcinogen. With the combination of hot, stainless steel exhaust components wrapped in a material that contains calcium, calcium chromate can be formed.

This is a good example of a classic cautionary tale in control of hazards; make sure that when you implement a control (glass fibre insulation) you don’t create a more serious hazard (hexavalent chromium).

We found calcium chromate – what now?

We tested machines, including a haul truck, dozer, water cart, grader, and scraper. We identified the presence of hexavalent chromium where there was yellow discoloration. Interestingly, we also found it on an exhaust section with no lagging. There was no visual evidence of the deposit, and the measured amount was lower than where lagging was used.

In this case study, the chances of calcium chromate exposure during normal operation and maintenance of machines is very low – even during standard maintenance work in the workshop.

This is because maintenance workers generally don’t replace or disturb exhaust lagging blankets that would expose the exhaust components. There is a specific contractor group for the exhaust rebuilds. These workers spend the greatest amount of time pulling the lagging off and putting in new lagging and exhaust components (though still only approximately 1 rebuild per month) and have the highest risk of exposure.

Assessing the risk – our approach

To determine the level of risk for the contractors doing exhaust rebuilds, we undertook monitoring while the contractors removed the exhaust and mufflers and took multiple samples throughout the day.

The contractors wore a new sampling train each time they were doing tasks associated with the exhaust rebuild, and wore a separate sampling pump when they weren’t doing any of the exhaust-related tasks.

This allowed us to see the exposure level by task. Over the whole day, we could also calculate what their time-weighted average would be for full shift results.

Monitoring Results vs Workplace Exposure Standard (WES)

Compared to the current SafeWork Australia WES, we found the readings were reasonably low over a full day.

But it was a different situation when looking at specific tasks. The SafeWork Australia proposed workplace exposure limit for hexavalent chromium was 0.07 micrograms per cubic metre (though we note the updated recommendation for NTGCs is to remove the workplace exposure limit and control to as low as reasonably practicable). The National Institute for Occupational Safety and Health (NIOSH) in the USA also has a health-based recommended exposure limit (REL) of 0.2 micrograms per cubic metre⁴.

We detected results exceeding 1 microgram per cubic metre for the specific tasks. While these results cannot be compared to exposure limits, they do provide an indication that there were moderate levels of airborne hexavalent chromium present.

Reducing the Risk of Hexavalent Chromium Exposure

The risk of exposure to calcium chromate in our case study can be reduced by:

• Preventing the occurrence of calcium chlorate by using insulation that doesn’t include calcium (or another oxide).
• Make sure that exhaust components are not cleaned using power tools or dusty methods.
• Introduce an assessment protocol:
  • Is this an at-risk component? Do I need to put PPE to do the initial assessment?
  • Is there any yellow discolouration or powdery deposit present?
  • Use an initial screening tool – use a hex check colorimetric indicator kit. There are a couple of different types available, for example, a swab kit similar to a lead testing kit. The indicator changes colour if there’s hexavalent chromium present.
  • The outcome of the above steps, points to what further controls may be required.
• Develop a procedure for cleaning and disposal of exhaust components.
• Wear appropriate PPE determined by a risk assessment.

References:

1. https://www.ccj-online.com/wp-content/uploads/2022/06/Chromium-TCL-2021-Rev-1.5-FINAL.pdf
2. https://www.safeworkaustralia.gov.au/doc/workplace-exposure-limits-airborne-contaminants
3. https://www.safework.nsw.gov.au/hazards-a-z/hazardous-chemical/priority-chemicals/chromium
4. https://www.osha.gov/chemicaldata/537