Tiny elements suspended in the air, fine particles, are the main culprits. Invisible to the naked eye, they measure less than 10 micrometers – so they are even tinier than a grain of sand, finer than the diameter of a hair.
Part of the problem is that polluted air from the surface enters the underground network. Added to this are tiny metal particles from the wear of railway equipment: the brakes, but also friction between the wheels and the rails. All this in a confined and poorly ventilated space. According to the National Health Security Agency, levels of toxic particles in the Paris metro are at least three times higher than outside.
In the absence of health studies on the subject, there are no European or international standards concerning air quality in railway enclosures. In contrast, it has been shown that fine particles in the air can penetrate deep into the respiratory tract and be associated with multiple health problems, such as asthma and cardiovascular disease.
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