Beyond its purely aesthetic function, hair does have a use. They are a first barrier against the sun, keep us warm, and more generally, reflect our state of health. For example, unusual hair loss may be due to a hormonal imbalance, stress or even excessive weight loss. But the potential of hair is much greater. They are also markers of our daily exposure to various pollutants.
In June 2023, fourteen environmentalist deputies had, for example, had their own strands of hair analyzed. The results caused a stir in the media: all the samples contained more or less high concentrations of PFAS, industrial pollutants dangerous to health. Which then suggested that the deputies had all been contaminated at some point in their lives without knowing it.
Long before this story, numerous studies had already detected the presence of pollutants on human hair or in the coat of animals. Based on one of the latest studies, carried out by researchers from the Luxembourg Health Institute (LIH) in May 2023, ANSES took stock in mid-November of the value of hair for measuring an individual's exposure to various toxic substances.
Attached by its root to the scalp, the hair extracts the nutrients essential for its growth directly into the blood system. In the process, it absorbs tiny concentrations of pollutants contained in the blood and which generally come from our food or the particles we inhale. Living cells in the bulb trap these compounds. They then become permanently fixed in the hair's keratin, the fibrous protein which gives it its structure.
Throughout its growth, until it falls or is cut, hair therefore retains a memory of the pollutants to which we have been exposed. “Traditionally, we use blood or urine tests but the disadvantage of these methods is that they are only reliable in the short term since the chemical substances contained in these fluids are eliminated within a few hours by our body,” explains Mathilde Body -Malapel, research engineer at the Institute for Translational Research on Inflammation, in Lille. The other problem is that the measured concentrations vary greatly over the course of a day. “For a urine sample, the dosage sometimes varies by a factor of 10 to 100 between the morning and the evening, which is not very representative of our exposure,” adds Brice Appenzeller, head of the biomonitoring research unit. human at the Luxembourg Health Institute.
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Several studies have already measured, in human hair or animal (rodent) hair, the presence of endocrine disruptors (bisphenols, phthalates, etc.). However, the toxicity of these compounds is well known. They are particularly suspected of promoting fertility disorders, certain cancers and even chronic pathologies such as obesity and diabetes. “We also know that prenatal exposure is harmful to the fetus because certain compounds cross the placental barrier,” says Mathilde Body-Malapel. This results in an increased risk of developing certain diseases (allergy, asthma), or even neurodevelopmental disorders. A French study also established, in 2019, a link between the birth weight of newborns and traces of 19 pesticides measured in their mother's hair.
Unfortunately, it is almost impossible to protect yourself from it. In addition to pesticides, endocrine disruptors are present in packaging, certain cosmetics, clothing and children's toys. According to the European Environment Agency (EEA) report published in 2023, bisphenol A, a plastic additive, flows in the veins of 92% of adults tested in eleven European countries. Even if it has been banned since 2018, in France, in certain products such as baby bottles, this compound is found for example in microplastics present suspended in the air that we inhale.
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The hairs are analyzed using mass spectrometry techniques which allow chemical compounds to be characterized and quantified very precisely. Even if the concentrations measured are very tiny (of the order of a picogram), they remain representative of the dose having penetrated into an organism and therefore of its level of exposure. “Hair growth is on average 1 centimeter per month. Each centimeter analyzed therefore provides the chemical fingerprint over a month. If we find a concentration of pollutants of 2 picograms in the centimeter of hair closest to the root compared to 1 picogram in the following section, we can conclude that our exposure has doubled over the last month,” analyzes Brice Appenzeller.
Do the levels of pollutants trapped in the hair then provide more precise indications of the importance of this exposure? Given the specificity of metabolites, their incorporation strongly depends on certain parameters such as the time of absorption and elimination by the body, which can influence the concentrations found in the hair. It therefore remains very difficult to make generalizations.
Blood alcohol concentration. Hair testing is commonly used in the forensic field. These tests make it possible, for example, to carry out blood alcohol checks in order to guide legal decisions (child custody, license withdrawal, etc.). “In the consensus, we take measurements on 3 centimeters of hair in order to have information over three months, which traditional blood alcohol tests do not allow us to obtain,” explains Professor Appenzeller.
Smoking and narcotics. The method is also used to detect illegal drug use and more frequently to estimate passive exposure to tobacco smoke, for example in children who are a population at risk. “Passive smoking is a significant public health problem. But it remains very difficult to quantify this exposure, underlines the researcher. Using simple hair samples, it is now possible to do this by measuring cotinine, a metabolite of nicotine.”