The haze covering the Greek capital comes from southerly winds that blow dust from North Africa across the eastern Mediterranean from March to April, the head of the Greek meteorological service Theodoros Kolydas said on X, formerly Twitter.

"A typical sandstorm with a range of 200 kilometres (about 120 miles) carries 20 to 30 million tonnes of dust and sometimes as much as 100 million," he wrote.

The city regularly experiences such sandstorms but the current one is accompanied by unusually high spring temperatures, heightening the choking effect.

Wednesday saw the highest March temperature in central Athens since 2009 -- 25.3 degrees Celsius (77.5 Fahrenheit).

Further south on the island of Crete, the temperature reached 32 Celsius.

The Greek Pulmonologists' Union advised asthma sufferers and other vulnerable people to avoid "unnecessary movements and outdoor sports during the times of highest dust concentration".

It warned that the dust can be mixed with pollen, bacteria and fungi, "a highly toxic mixture that is dangerous for the human body, particularly the respiratory system".

Scientists say climate change caused by human burning of fossil fuels is raising the risk of extreme weather events, including heatwaves and droughts.

The weather service said the dust clouds were expected to dissipate from Friday.