A thick cloud of dust from the Sahara Desert yesterday enveloped the country, leading to the cancellation of dozens of domestic flights and sending many asthma-sufferers to hospital with aggravated symptoms. Weather experts said that the arrival of the thick fog, which was brought on by southerly winds, is a normal phenomenon for this time of year. But the concentration of dust particles in Attica was denser this time than it has been in previous years, according to Dimitris Ziakopoulos, the head of the forecasting department of the National Meteorological Service (EMY), who said that the fog would probably linger for a couple more days and may reappear on Tuesday. The Environment Ministry yesterday confirmed that there had been an increase in the rate of dust particles in the atmosphere. Meanwhile, dozens of domestic flights were canceled due to a dangerous lack of visibility above airports, especially in the southern Aegean. Athens International Airport has the technology to allow aircraft to land safely even when there is low visibility but many Athens-bound flights could not leave yesterday due to thick fog and, in the case of Rhodes, gale-force winds. One of the regions worst hit by the dust cloud was Crete, where hospitals saw a marked increase in patients with breathing problems. Those suffering from asthma or lung-related complaints should try to remain indoors as much as possible for a couple of days, the president of the Hellenic Thoracic Society, Nikos Siafakas, said yesterday.