Cycling to work may seem the healthy option, but a study has shown that people riding in cities inhale tens of millions of toxic nanoparticles with every breath, at least five times more than drivers or pedestrians. The research showed that urban concentrations of nanoparticles, which measure just a few millionths of a millimetre, could reach several hundred thousand in a cubic centimetre of air. The particles, when inhaled, have been linked to heart disease and respiratory problems. Because they are exerting themselves, cyclists breathe harder and faster than other road users. The study found that they suck in about 1,000 cubic cm with each breath, meaning they may inhale tens of millions of the particles each time they fill their lungs, and billions during a whole journey.