Hazardous substances: off the trike20 March 2023

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What is the outlook for, and operational safety of, chlorinated substances such as perchloroethylene, a common industry substitute for the outlawed trichloroethylene? by Steed Webzell

Engineers of a certain age will recall the time when a quick dunk in the open-top trichloroethylene tank was a normal part of the daily routine in cleaning grease, oil and grime from metal parts. However, since 2016, the use of trichloroethylene is no longer legal under REACH regulations (known since 1 January 2021 as UK REACH). In light of this development, some manufacturers with a requirement for metal parts cleaning have moved away from chlorinated solvents, largely opting for alcohol- or aqueous-based processes where possible. Yet chlorinated solvents prevail as the preferred cleaning medium for many others, with perchloroethylene proving the most common choice.

As a halogenated solvent (treated or mixed with chlorine), perchloroethylene is not dissimilar to trichloroethylene in its chemical make-up. In some respects this is fortunate, as it has allowed many plants to replace trichloroethylene with little or no modifications to their machines or processes, minimising any cost impact, according to ‘Substitution of trichloroethylene in metal parts cleaning in the European Union’ published in 2021 by the Centre for Future Chemical Risk Assessment and Management Strategies at the University of Gothenburg.

And yet the potential health and safety concerns associated with chlorinated solvents have hardly gone away: perchloroethylene carries a classification as a ‘suspected’ rather than a ‘proven’ carcinogenic substance. Not particularly reassuring.

So, what is the outlook for perchloroethylene (sometimes known as tetrachloroethylene or tetrachloroethene) and other halogenated solvents like dichloromethane (different names include methylene chloride, toluene, xylene, white spirit, acetone or ethyl acetate)? For context, dichloromethane also finds use as a paint stripper and thinner in sectors such as construction.

To determine the outlook, it is useful to know a little more about perchloroethylene, which tends to volatilise quickly when released to surface water or surface soil. The liquid is mobile in soil and therefore has the potential to leach below the soil surface and contaminate groundwater. Perchloroethylene also carries a classification ‘toxic to aquatic life’ with long-lasting effects. Although it has similar hazardous properties as trichloroethylene, it is not subject to the same legal constraints and is not included in the REACH candidate list.

In the US, perchloroethylene has a similar hazard classification, where the Environmental Protection Agency (EPA) says the substance is “likely to be carcinogenic to humans”. As a further point of note, the EPA has recently re-evaluated perchloroethylene, releasing a final chemical risk evaluation in December 2020 with the conclusion that there are “unreasonable risks to workers, occupational non-users, consumers and bystanders from 59 out of 61 uses”. Included in these 59 uses are “industrial and commercial use as solvent for open-top batch vapour degreaser” and “industrial and commercial use as solvent for closed-loop batch vapour degreaser”.


Ironically, research company Fact.MR suggests that the global perchloroethylene market is poised for positive growth through to 2030. The company attributes this growth (up to 4% CAGR) to increased demand in dry cleaning and refrigerant manufacturing applications. However, the reports point out that, due to lack of regularity policies in East Asia, demand for perchloroethylene is set to expand there, while following the identification of perchloroethylene’s hazardous effects on humans and the environment, demand in markets such as the US is declining.

Dichloromethane is a similar case in point. This colourless and volatile liquid with high vapour pressure (pictured above) carries the same classification, namely “suspected to be carcinogenic” (category code Carc. 2) within the EU, and again is not included in the REACH candidate list for authorisation. However, the substance is under evaluation in Italy, which has submitted an intention for re-classification of the substance to a Carc. 1B: “may cause cancer”. In the US, the EPA says dichloromethane is “likely to be carcinogenic to humans”.

Just like perchloroethylene, dichloromethane is receiving attention in the US for risks related to its use. In June 2020, the EPA concluded that there are “unreasonable risks to workers, occupational non-users, consumers and bystanders under 47 out of 53 conditions of use”, again including industrial degreasing.


In May 2020, Minnesota was the first US state to pass a ban on trichloroethylene for manufacturing and cleaning processes, becoming effective on 1 June 2022. The replacement must be a chemical demonstrated to be less toxic to human health.

Jane Paulson, senior engineer at MnTAP (Minnesota Technical Assistance Programme) at the University of Minnesota, says: “The reasons that companies cite when hesitating to replace trichloroethylene include getting reliable information, raising the required investment for new equipment/staff training, and customer notification/requalification – companies worry it might open the door to competitors if they are making a change,” says Paulson.

The steps in MnTAP’s ongoing TCE Alternatives project begin with understanding the current process and cleaning needs, which involves assessing part geometry, component material, contaminants that need removal and the required level of cleaning. The next steps necessitate collecting samples for performance and solubility testing. Subsequently, it should be possible to identify cleaning products and methods that meet process requirements. MnTAP provides technical assistance to support companies through qualification and implementation steps.

“When we look at replacing trichloroethylene, various options are available,” says Paulson. “In order of preference these include: eliminating the process step altogether; substituting with a process that uses non-solvent media, such as CO2 or dry ice; implementing aqueous systems; adopting non-halogenated solvents like alcohols and esters; and, finally, using other halogenated solvents such as perchloroethylene or dichloromethane, which would be the worst option for both worker health and the environment.”


According to Paulson, for common vapour degreasing applications, no safe solvent options are available. Given the EPA findings of neurotoxicity and cancer risks, she queries perchloroethylene’s ongoing selection and use. “A lack of regulation for substances such as perchloroethylene is not necessarily evidence of safety,” she says. “Rather, it often indicates it hasn’t yet been tested to the same levels.”

Ultimately, while there appear to no discernible moves to outlaw substances such as perchloroethylene, the suggestion is clear: it is not good for human health. One notable policy conclusion cited by many researchers is that additional incentives are required to realise the ambition of a fully non-toxic environment.

Steed Webzell

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