Bus filters pump more pollutants

The new hybrid buses are said to be quieter and more fuel efficient

Pollution filters fitted to all London buses as part of a £30m project could 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.

TfL said an assesment on whether to convert all 8,000 London buses - at a cost of £3,000 per bus - would be carried out at the end of the month.

We hope to look at retro-fitting [the equipment] to large numbers of vehicles over the next few years Mike Weston

Mike Weston, of London buses, told the BBC: "These trials have been running for about six months, we have made some modifications to the equipment and it will be running for about another six months.

"Then we hope to look at retro-fitting it to large numbers of vehicles over the next few years."

Professor Mike Pilling, who led a government-funded study into the problem, said NO2 was a pollutant that they were concerned about as it had an impact on human health.

He said places like London were predicted to miss government-set targets for reducing NO2 - which can adversely affect the heart and lungs - over the next few years.

However TfL says the report was misleading by implying that London buses are a "significant source of increasing emissions in the UK".

A spokesman said: "In fact London buses have significantly reduced their emissions and are only a small part of an issue affecting all diesel and larger vehicles."

Low-emission zone

London's bus fleet has expanded rapidly over the past few years, but there was always a concern that introducing more heavy diesel vehicles would create considerable amounts of extra pollution.

Various "eco-friendly" buses have been trialled - such as "hybrid" buses using an electric motor or hydrogen-powered buses which produce nothing but water vapour.

Mr Weston has said he hoped London's bus network would become a UK leader in using innovative technology.

The mayor, Ken Livingstone, is also looking at introducing a "low-emission zone", in which high-polluting vehicles would pay to drive into London.

Nitrogen dioxide irritates and inflames the airways of the lungs causing a worsening of symptoms for those with lung or respiratory diseases.