In an article published on March 25, 2020, news provider Chemical Watch informed about the results of an expert workshop examining the hazards and risks of poorly soluble low toxicity (PSLT) particles. One such PSLT is titanium dioxide, which was recently classified as a carcinogen in the EU (FPF reported). Since this classification, the discussions surrounding PSLT substances have been growing, and this industry-funded workshop aimed “to document agreements and differences amongst a panel of highly experienced scientists and regulators” in regard to the toxicology of such substances. Participants are described as including expert scientists from academic and regulatory institutions, as well as industry and consultancies. An additional group of observers was also in attendance. The workshop resulted in an agreement by participating experts on a functional definition and set of criteria for PSLT substances, which is presented in the report using multiple and detailed sub-definitions of “poorly soluble particles” and “low toxicity.”

Participants also agreed that (i) rats can serve as “a sensitive model for PSLT inhalation toxicology,” (ii) lung inflammation is a critical endpoint for the risk assessment, (iii) “rat lung cancer occurring only under conditions of lung particle overload does not imply a cancer hazard for humans under non-overloading exposures,” and (iv) PSLT substances should not be considered as human lung carcinogens using data from rats alone without additional supporting data from other species.

Given these outcomes, the authors of the workshop report recommend that regulators reassess the guidelines for evaluation and classification of PSLT substances and prior regulatory decisions be reviewed to determine if they are still appropriate.

However, many civil society groups would disagree and have instead shown consistently strong support for the classification of TiO2 as a carcinogen. Consortiums of non-governmental organizations have even called on the European Chemicals Agency (ECHA) to further expand the current carcinogenic classification beyond just powder forms to include all forms of TiO2 (FPF reported).

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Andrew Turley (March 25, 2020). “Preliminary definition developed for titanium dioxide-type substances.”


Driscoll, K. and Borm, P. (March 9, 2020). “Expert workshop on the hazards and risks of poorly soluble low toxicity particles.” Inhalation Toxicity; DOI: 10.1080/08958378.2020.1735581