Pollution of the atmosphere (ie the air we breathe) by vehicle exhausts. A comparison of the emission factors between road and rail shows the following general relativity of emissions in grammes per tonne kilometre: Road (g/tkm) Rail (g/tkm) Carbon dioxide 183.00 30.00 Carbon monoxide 2.08 0.05 Nitrogen oxides 3.86 0.55 Hydrocarbons 3.60 0.07 Sulphur oxides 0.26 0.23 Respirable particles 0.67 0.11 (Source: Railtrack plc, ‘A Guide to Railfreight’ April 1999)
