Central London Tube Railway poster

The Central London Railway began in 1900 and this probably dates from shortly after that.  The "80 million cubic feet of ozone pumped through the tube daily" would cause consternation now: according to wikipedia, "Ozone's odour is sharp, reminiscent of chlorine, and detectable by many people at concentrations of as little as10 ppb in air... ozone [is] a potent respiratory hazard and pollutant near ground level."