Hair used as evidence of exposure to certain chemicals

Certain chemicals to which we are exposed via our environment or food – such as bisphenols, phthalates and pesticides – are eliminated by our bodies in just a few hours. This, coupled with frequent re-exposure, can be an obstacle to assessing long-term exposure to these pollutants using blood or urine analyses. "We were faced with this problem during an expert appraisal on bisphenol A", explains Claire Beausoleil, a toxicologist in ANSES's Risk Assessment Department. The results of certain scientific studies concluded that there was no correlation between the effects studied in humans and the concentrations of bisphenol A measured in urine. Did this mean that the study participants had not been exposed to doses high enough to be detected in their urine? Or did these measurements fail to reflect exposure, mainly due to the substance being eliminated too quickly and the resulting high variability in urine concentration?

It is essential to be able to trace internal exposure to a chemical in order to determine its long-term toxicity, because the health effects of a substance depend on its concentration in the body.

A successful study on rat hair

To find out which analysis method was the most appropriate for each substance, a study was designed by ANSES, the Luxembourg Institute of Health (LIH) and the French Institute for Industrial Environment and Risks (INERIS). It was co-funded by ANSES and the LIH. The scientists mainly wanted to know whether it was possible to use hair to measure exposure to certain pollutants, as it is easier to collect hair samples than blood or urine and this may better reflect the actual exposure of individuals.

Rats used as models were exposed by ingestion to a mixture of 17 pollutants: pesticides, phthalates, bisphenols and DINCH, another plasticiser. Hair and urine samples were then collected to measure the concentrations of metabolites produced from the transformation of these substances in the body.

The result was that for 14 of the 17 substances to which the animals had been exposed, a strong correlation was observed between the ingestion exposure dose and the concentration of metabolites measured in hair. This concentration was also proportional to that found in urine, indicating that the substances had become incorporated in hair after being transported by the blood. 

Hair, a good indicator of long-term exposure to most chemicals

For substances whose concentration in animal or human hair gives a good indication of actual exposure, this measurement could even be more representative than one taken in blood. This is because the substance may have already been eliminated from the blood at the time of sampling, whereas hair retains traces of a pollutant for longer once it has attached itself to the keratin. Hair analysis therefore reflects exposure over a longer period of time and is not subject to the short-term variations usually measured in blood or urine.

The incorporation of substances into hair depends on parameters – such as the absorption and elimination time of each compound – that vary from one species to another. In order to extrapolate these data from the rat study to humans, a further adjustment is needed to take account of each species' unique metabolic characteristics.

Find out more

Faÿs François, Paul Palazzi, Florence Zeman, Emilie M. Hardy, Charline Schaeffer, Christophe Rousselle, Claire Beausoleil, Brice M. R. Appenzeller, Incorporation of Fast-Elimination Chemicals in Hair Is Governed by Pharmacokinetics–Implications for Exposure Assessment, Environmental Science & Technology 2023 57 (19), 7336-7345, doi: 10.1021/acs.est.2c06777