A NASA satellite image showed a yellowish-brown plume stretching north from Western Sahara and inland to Mali.

Residents of Mauritania's seaside capital Nouakchott covered their faces with scarves or stayed indoors while drivers used headlights at midday to cut through the haze as the storm, which began on February 6, left thick layers of dust on sidewalks and shop windows.

"I have been eating sand for the past two days," Lea Polony, an entrepreneur in Nouakchott, said. "The places I go to for work have been deserted by workers and clients."

In neighbouring Senegal, Leopold Sedar Senghor airport cancelled more than a dozen flights on Tuesday, including Delta's direct flight from New York, after dust particles obscured the sun and cut visibility to below 400 metres.

A meteorologist said the dust came mostly from the northwest of the Sahara, kicked up by the so-called Harmattan, a seasonal wind that occurs during the northern hemisphere winter.

"This is an annual phenomenon during the Harmattan, but the last time we saw a dust plume like this was in 2010," said Mamina Kamara of Senegal's Meteorology Agency. "We're hoping it starts to lift tomorrow."

"This normally happens in the last four days of February and the first three days of March," said Sid Ahmed, resident of the small Mauritanian town of Akjoujt.

Senegal's Environment Ministry said on Wednesday air quality remained "bad" in Dakar, an urban sprawl of some 4 million people, posing a public health risk.

"The dust contains many microbes which can infect people's food or cause respiratory problems," Kamara said. "In the past, we have also seen an uptick in meningitis cases after the passing of dust clouds."

(Additional reporting by Laurent Prieur in Nouakchott and Diadie Ba in Dakar; Editing by Janet Lawrence)

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