"Car-free days" were first introduced in France in 1998 in a bid to build awareness of the pollution generated by motor vehicles. The event is also an opportunity for real-life tests of some alternatives as entire urban areas are barred to traffic.

This year's car-free day should be especially welcome in Paris where, as in most of Europe, the past three months' hot, sunny weather have been causing dangerous levels of ozone concentration.

Although stratospheric ozone, high in the earth's atmosphere, is a vital shield against the sun's ultraviolet radiation, the gas can be harmful to human health when it builds up at ground level as tropospheric ozone.

High concentrations are formed from reactions between nitrogen dioxide and hydrocarbon emissions from cars, especially in bright sunlight and warm weather.

The impact of those unusually high concentrations was compounded by the unusual duration of the heatwave. According to Airparif, the agency monitoring the quality of the atmosphere in Paris, the resulting high degree of pollution certainly had a role in the unusual number of deaths recorded in the capital and elsewhere in France during and immediately after the heatwave.

The early August heatwave officially killed some 15,000 people in France, 4,175 in Italy and between 3,500 and 7,000 in Germany.

France's ozone levels for August were the highest since records began in 1991. At one point in Paris, ozone concentration topped 200 microgrammes per cubic metre -- 65 percent above the 120 microgrammes considered "safe" for humans.

Road traffic obviously bears a fair amount of responsibility for air pollution. In France, motor vehicles produce 50 percent of the nitrogen dioxide that contributes to ozone concentration at ground level. Cars and trucks also generate 25 percent of the greenhouse gas emissions blamed for climactic changes.

Faced with the mounting challenge of pollution, public authorities' response is "hardly effective", said Philippe Lameloise, who heads the Airparif agency.

In Paris, public notice boards urge drivers to reduce speed by 20 kilometers (12.5 miles) per hour during pollution peaks.

But even if every single driver abided by the recommendation, this would reduce noxious emissions by a mere three percent, according to Airparif experts.

One alternative is to allow free parking in a bid to encourage drivers to switch to public transport. But the impact has never been quantified.

Paris tried another method, known as alternate traffic, whereby only vehicles with odd numbers are allowed on the road on odd-number dates.

Those vehicles with more environmentally friendly engines are exempt from alternate traffic restrictions. But then up to 85 percent of vehicles in France now meet the standards required for exemption. Therefore the impact of the scheme is dubious at best.

This year's auto-free day may be an opportunity to look beyond local action on the ground and concentrate on the wider context.

"We must all act together at the north European level," said Lameloise.

Ozone knows no borders as two thirds of the ozone in Paris are "imported" from northern Europe. Therefore northern European countries should put together an "ozone plan" before every summer, the typical high season for this type of pollution, said Lameloise.

Such a plan should also look beyond pollution by motor vehicles. On top of making catalytic converters mandatory, a comprehensive plan would also look to restrict industrial gas emissions, change the ingredients in fuels and industrial paints and encourage both rail freight and combined rail-road freight transport.

French authorities made one step in the right direction when they increased taxes on gas oil to finance rail freight development. But the tax only hits cars, trucks are exempt.

In the shorter term, Lameloise is satisfied that existing monitoring and warning procedures effectively alert vulnerable individuals to dangerous levels of pollution.

But here again, more can be done, he added. Weather bulletins should include pollution forecasts for the day, since nowadays sunny days come hand in hand with pollution peaks.

TERRA.WIRE