"The situation is very alarming," Swiss transport, energy and environment minister Moritz Leuenberger said at a press conference.

Potentially carcinogenic fine particle pollution, mainly from diesel engines, reached more than double the permitted levels, as the seasonal cold and climatic conditions trapped pollution, authorities said.

In 10 out of 13 recording stations, fine particle pollution exceeded the maximum of 50 microgrammes per cubic metre on Monday, according to data from the federal environment office.

The highest level was recorded in the capital, Bern, with 133 on Sunday, followed by a motorway junction in the centre of the country (124) and the city of Zurich (118).

Only sparsely populated mountain areas have escaped the high concentrations of particles or soot, which is also caused by wood fires or heating and has been increasing in recent years.

Leuenberger said he wanted to make exhaust soot filters obligatory on new diesel-engined cars, and introduce tax incentives for trucks and buses equipped with filters, by 2010.

The measures need to be approved by his fellow government ministers.

In the shorter term, the transport ministry announced emissions labelling for cars, stricter requirements for buses, new standards for wood-fuelled heating units and stricter emission rules for industry.

Cantonal authorities in Zurich urged the federal government to implement tougher standards last week, saying they could only ask people to reduce their use of motor vehicles in a bid to slow the pollution build-up.

Since measurements of fine particle pollution began in 1997, soot limits have been exceeded about 30 to 80 days each year in Switzerland. But this winter's recordings were the highest ever, Zurich cantonal authorities said.

During winter, most central and western areas of the country suffer from "temperature inversion", with still, cold air trapped under a layer of warmer air at higher altitude.

The most visible impact is lingering low cloud or mist in the most inhabited parts of the country, sometimes for days on end, which also hems in heavier types of air pollution and allows concentrations of noxious particles to build up.

Some 3,700 deaths each year are associated with fine particle pollution, according to the Swiss transport and environment ministry. High concentrations of soot can cause cancer, as well as other breathing or heart ailments.

Road haulage associations and a leading car club voiced concern about the minister's proposals for trucks and cars, saying they did not feel compulsory measures were necessary.

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