Fine particles, known as PM10 (particulate matter, smaller than 10 thousandths of a millimeter, or one micrometer), are a growing cause of concern. They are of both natural and man-made origin, and of very diverse composition.

On the one hand, it appears that they affect the climate, and on the other they have an impact on health.

These particles come from a variety of sources: factory emissions, exhaust from road and rail traffic and from farm and construction machinery. Furthermore, vehicles not only produce exhaust fumes, they also stir up particles into the air.

A secondary source of PM10s is chemical reactions in the atmosphere under the influence of solar radiation. These are known as secondary organic aerosols, or SOAs.

The Swiss Federal Agency for the Environment believes about half the fine particle pollution is carried in from neighbouring countries. The government is pressing the European Union to step up measures to combat these emissions.

Swiss experts believe that some 3,700 people die prematurely in Switzerland every year as a consequence of fine particles in the air.

PM10s are not the smallest particles; even finer ones exist: PM2.5, with a diameter of less than 2.5 micrometers. The World Health Organisation announced strict new permitted levels for PM2.5s in 2006, but they have yet to be introduced in Switzerland. Switzerland currently has few measuring stations able to monitor them.