A 2000 study reported that air quality was seventy-three times worse than at street level, with a passenger breathing the same mass of particulates during a twenty-minute journey on the Northern line as when smoking a cigarette.
En analyse fra 2000 rapporterede, at luftkvaliteten var 73 gange værre end på gadeniveau, og at en passager indånder samme mængde partikler under en 20 minutters rejse på Northern line, som ved at ryge en cigaret.