Nearly a third of the 22 regions in mainland France on Wednesday were on a maximum pollution alert including a swathe of the country's north, centre-east and the Paris region. The prevalance of particles with a diameter of less than 10 microns were reported by regional air monitoring centres in the affected areas. A lack of wind during the anticyclonic period and cold nights followed by balmy days triggered the phenomenon, they said. In addition, there were emissions from cars and from heating in buildings and factories. In general, the smaller and lighter a particle is the longer it stays in the atmosphere and can remain suspended for weeks. Particles less than 2.5 microns in diameter are also the most dangerous to health as they can pierce the lungs and the blood system. They can also lead to asthma, allergies and respiratory ailments. Alerts have been declared in Haute Normandie, Pas de Calais and Calvados in the north, the Ile-de-France region where Paris is located and the centre-east including some Alpine areas and in some parts of the west. Alerts are issued in France when the concentration of small particles reaches 80 micrograms per cubic metre. Measures to fight the advanced pollution level include orders to drive vehicles slowly, a ban on chimney fires and an advice to residents to eschew physical activity either indoors or outdoors. Airparif, an official monitor and a federation of regional watchdog for air pollution in France, said the pollution threshold would be breached Thursday in the Paris region. The levels of PM 10 -- or small particles which easily penetrate the lungs -- were reported at 50 micrograms per cubic metre in several areas such as Brittany, the Loire region and parts of wine-producing Burgundy.