In reply to MarkJH:
As far as I can tell, they are the figures for 4 stroke mowers. 2-stroke mowers were something like 40 times worse.
I think the biggest problem is the lack of filtering / catalytic converter plus the lower efficiency.
It's hard to find any real figures behind the newspaper statements, but the figures below which compared cars to leaf blowers (so presumably less powerful than mowers) seem to back it up
NMHC NOx CO
2011 Ford Raptor 0.005 0.005 0.276
2012 Fiat 500 0.016 0.010 0.192
Ryobi 4-stroke leaf blower 0.182 0.031 3.714
Echo 2-stroke leaf blower 1.495 0.010 6.445
Idling - Ryobi 4-stroke leaf blower 0.077 0.002 1.822
Idling - Echo 2-stroke leaf blower 1.367 0.000 2.043
ok, the formatting disappeared when I posted it
Post edited at 14:31