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Clouds of African dust have been sprinkling their contents across the Caribbean for as long as there's been sand in the Sahara Desert. The phenomenon is nevertheless attracting increasing attention from regional scientists who believe that the clouds have grown, even if there's no global consensus on the issue.
Recently, an unusually large cloud dusted the Eastern Caribbean, generating hazy skies and vivid sunsets before drifting over Mexico's Yucatan Peninsula and moving on to be detected as far away as Wyoming in the United States.
Satellite images from NASA show these huge, smoky clouds wafting westward from Africa and blanketing hundreds of square miles.
Although the microscopic dust particles sent aloft by African sandstorms have hitherto been accorded little more than moderate interest, experts are now saying that the particulate matter may be cause for health concerns and merit more study to understand their potential impact.
According to Braulio Jimenez-Velez, a specialist in molecular and environmental toxicology at the University of Puerto Rico at Mayaguez, "It is a matter of great magnitude, interest and importance for health."
So far this year, Sahara dust has prompted two health alerts in Puerto Rico for asthma sufferers and people with allergies. The Dominican Republic also issued a warning.
Many Caribbean territories, including Puerto Rico, have high asthma rates, but no direct link has been established between African dust and higher rates of asthma or lung cancer.
Over time, human activity has changed the composition of the clouds, with scientists saying that they now contain trace amounts of metals, microorganisms, bacteria, spores, pesticides and faecal matter, although no evidence exists that the quantities are sufficient to pose a threat.
African dust sampled in Barbados also had elevated levels of arsenic and cadmium, according to Joseph M. Prospero, professor emeritus of marine and atmospheric chemistry at the University of Miami.
"The specific impact on health is not known here or anywhere else. It has been extremely difficult to link specific particle composition to health effects," said Prospero, who is lead author of a paper on the dust to be published in September by the bulletin of the American Meteorological Society.
"So it cannot be said what effect all this dust has, but there is reason for some concern," the expert added.
Eugenio Mojena of Cuba's Institute of Meteorology said the particles are believed to originate in the semi-arid Sahel region south of the Sahara Desert, where farmers raise livestock and employ chemical fertilizers and pesticides.
Experts also worry that chemicals in the clouds may pose a threat to coral, although the theory is still a subject of debate.
The dust clouds can also complicate air traffic by reducing visibility to less than 3 miles, said Jason Dunion, a researcher at the National Oceanic and Atmospheric Administration (NOAA).
On a more positive note, the dust-laden clouds may inhibit the formation of hurricanes and other tropical weather systems in the Caribbean.
According to Prospero, lower rainfall in West Africa presumably causes more dust, which reduces sunlight, lowers water temperatures and cuts evaporation, all factors in cyclonic formation