From BBC News online...

quote
Pollution filters fitted to all London buses as part of a £30m project
are to be modified, after it emerged they were pumping out other
harmful toxins.

"Particulate traps" to catch tiny bits of dust and dirt were fitted at
the end of 2005 to reduce the environmental impact of the expanded bus
fleet.

But a chemical reaction caused by the filters created five times the
amount of nitrogen dioxide (NO2) emissions.

Transport for London is now testing new filters which would screen out
NO2.
/quote

Full story at:
http://news.bbc.co.uk/1/hi/england/london/5364510.stm


It appears London Buses' attempt to do the right thing encountered
unforseen unintended consequences. My basic opinion is at least TfL is
attempting to minimise negative environmental impacts from
transportation - we are at pretty early days in this effort, and there
will be an element of learning from mistakes such as this. Perhaps
London Buses should've trialled the particulate traps more thoroughly -
I'm sure the pretty thorough Transport Committee at the London Assembly
will take a good look into this.

I've been pretty impressed with the experimental hydrogen fuel cell
buses that operate on the RV1 route, which emit only water vapour [1].
I wonder if that's not the way forward - obviously it would involve an
enormous change of infrastructure, but perhaps there could be a more
comprehensive trial of fuel cell buses in London. Obviously the cost is
a major issue - but TfL has a considerable level of indirect buying
power in that it can specifiy to bus operators what type of vehicles
they use.

[1] http://www.tfl.gov.uk/buses/fuel-cell-buses.asp