The London Underground was for decades the envy of every capital city in the world. Forty year old trains were powered by electricity from a polluting oil fired power station. Tobacco smoking was permitted at stations and in some carriages. Levels and composition of dust on The Tube had been measured, and found to contain a variety of toxic substances including quartz from brake blocks,¹ but concentrations perhaps did not seem so high compared with other workplace environments and even outdoor urban air at that time.

Today’s trains use their electric motors to decelerate, so brake dust is much reduced. A new power distribution system allows energy to be returned to some sections of track, resulting in less heat and lower electricity consumption. Cleaner sources have replaced the oil fired power station, and smoking is banned. All these measures tackle air pollution at source, especially underground. So why are we increasingly worried about …