Choking on the trees

Scientists at York University have found that trees in Britain can help to increase levels of ozone in the air in hot temperatures.

When temperatures reach 35 degrees celsius, as they did last summer in parts of the UK, deciduous trees produce significant amounts of isoprene, which acts as a catalyst to convert nitrogen oxide from exhaust fumes into ozone. While ozone in the upper atmosphere protects the earth's surface from solar radiation, at ground level it can be harmful to people with sensitive lungs, like asthmatics and the elderly. 'This phenomenon has been seen before in California and the Mediterranean but, of course, we hardly ever get temperatures like that in the UK and northern Europe,' explained Dr Alastair Lewis to BBC News. The suggestion is that if average temperatures continue to rise, the phenomenon will become more common. How ironic that trees, generally characterised as cleaning up the air, can actually make it harder to breathe in hot weather. Perhaps someone will call for a ban on trees in areas of heavy traffic - a proposal that is no more ridiculous than the usual equation that 'natural' inevitably equals 'good'.

Heatwaves can make trees pollute, BBC News, 10 May 2004