Second the Portuguese newspaper Público, there is a lot of dust in the air, but this time is not caused by industrial or motor vehicles pollution: it's the desert. Portugal is under the influence of a cloud of dust coming from North Africa, which is contributing to high levels of airborne particles.
It's a natural phenomenon that happens with some frequency, according to Francisco Ferreira, a specialist in air pollution and professor of the Faculty of Science and Technology (FCT), Universidade Nova de Lisboa. In recent days, Portugal has been traversed by a mass of warm air transporting dust from North Africa and interior of Spain. Yesterday, in Algarve, in places where it rained, the dust settled on cars, clothes and balconies. If it rains, it doesn't notice - the only sign may be a yellowish stripe on the horizon.
However, the stations of air quality are recognizing the passage of the desert cloud. Today's data published by the Portuguese Environment Agency indicates that in several parts of the country the concentration of particulates (PM10) exceeds the limit value for the protection of human health (50 micrograms per cubic meter, on average daily).
"Part of these values is due to this natural phenomenon, no doubt", said Francisco Ferreira. In 2009, similar situations have affected the air quality in Portugal during 141 days, almost a third of the year. In 2010, so far there have been 40 days of natural pollution influence coming from the drylands of North Africa.
The evolution of this situation is daily followed by the Department of Environmental Sciences and Engineering (DCEA) from FCT, but its weight on the quality of air is not accounted for purposes of compliance with EU law, according to which there can be only 35 days per year with values above the limit for particulate matter.
 Particulates, alternatively referred to as particulate matter (PM) or fine particles, are tiny subdivisions of solid or liquid matter suspended in a gas or liquid. A particle with an aerodynamic diameter of 10 micrometers moves in a gas like a sphere of unit density (1 gram per cubic centimeter) with a diameter of 10 micrometers. The notation PM10 is used to describe particles of 10 micrometers or less in aerodynamic diameter.