The London Underground is key to the functioning of England’s capital city. Roughly 2 million people use it each day. But it is polluted with small particulate matter from heavy metals, including iron oxide, that may be damaging to human health.
These particles range in size, but so-called PM2.5 particles are typically less than two and a half micrometres (2,500 nanometres) in diameter and can cause asthma, lung cancer, cardiovascular diseases and neurological problems. If it was classified as an outdoor environment, concentrations of particulate matter on the underground would exceed the air quality limits set by the World Health Organisation.
