Until recently, I had not given any thought to hair extensions, although I was once in a salon where I was amazed at the variety of these products. They were to be found in every possible length, shade and style, made either of real hair or of various synthetic fibres.

I recently came across a paper in the peer-reviewed journal Environment and Health called Identifying Chemicals of Health Concern in Hair Extensions. The researchers subjected 44 samples to analysis and detected 169 chemicals, including 48 that appeared on hazard lists. These included dye components, phthalates that make synthetic fibres soft and pliable, tin compounds that stabilize polyvinyl chloride (PVC) polymers as well as some pesticide residues. The researchers suggest that hair extensions may pose a risk because they are in constant contact with the skin, present possible hand-to-mouth contact and their volatile components can be potentially inhaled.

What the researchers did not do was test for any of these chemicals in the blood or urine in the people using hair extensions.

Also, critically, there is the difference between hazard and risk. “Hazard” is the potential to cause harm; “risk” is the likelihood that harm will occur after dose, mode of exposure (inhalation, ingestion, skin contact) and extent of exposure are taken into account. Carbon monoxide, for example, is deadly when inhaled in a closed garage but is quickly diluted outdoors. Alcohol is toxic when consumed at a high dose but not in a glass of wine. Apple seeds contain cyanogenic glycosides that can release cyanide but swallowing a few when eating an apple presents no risk. (It would be unadvisable to consume a cup of apple seeds.)

It is difficult to estimate the number of chemicals to which we are exposed every day, but it........

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