From a BBC News magazine article posted today:
it does seem that diesel is getting an increasingly bad press.But my biggest surprise was on my train journey. Diesel-powered trains like the one I commute on are found on many major routes across the UK. The East Coast Main Line north of Edinburgh, the Great Western route through to Cornwall, and London services to Sheffield and Nottingham are just some examples.
The average reading I got on-board my air conditioned train was 8.5. A researcher from King's College conducted an experiment to mirror mine on his train journey from London to Exeter and came out with similar results.
My time spent standing on the station concourse at London St Pancras, waiting for my train, produced a reading of 13.2.
So it turns out that during the 80 minutes I spend sitting still on a train every day I am being exposed to more diesel fumes than when I'm walking or cycling down a street full of traffic in London. On the day I took an electric train instead, my reading was only 2.4.