African dust clouds that cross the Atlantic into south Florida during the dry season every year, producing beautiful sunsets, may harbor dangerous bacteria and fungi that are dangerous to inhale.

The dust originates in the Sahara Desert and contains fine particles of dry topsoil. It is transported by winds and can be carried more than 10,000 feet in the air. The clouds of dust reach the southern U.S. and the Caribbean in about 5 to 7 days. Florida receives more than 50 percent of the microbe-carrying African dust.

?These dust events are cyclical,? says Dale Griffin of the U.S. Geological Survey (USGS). ?Studies by other researchers have shown that from February to April, winds bring an estimated 13 million tons to the Northeastern Amazon Basin. From June to October the winds shift and typically bring dust to North and Central America and the Caribbean.?

During the peak of the dust season in July of 2000, Virginia Garrison of the USGS collected daily samples of airborne pollutants and dust in the air over the island of St. John in the Virgin Islands. She sent them to the USGS laboratory in St. Petersburg, Florida, where they were analyzed by Griffin. They found that microbe levels in the air were high when dust storms were sweeping in from Africa.

?In the week it takes for North African dust to cross the Atlantic some of the microbes die because of exposure to ultraviolet rays of the Sun,? says Griffith. ?However, microbes in the cracks and crevasses of dust particles may be shielded from UV. We also believe that the upper altitudes of the dust clouds deflect harmful UV rays, shielding microbes at lower altitudes. Additionally, when dust clouds move over open water in lower latitudes, the moderate temperatures and high humidity enhance microbial survival.?

The U.S. Environmental Protection Agency says these tiny dust particles can penetrate deep into airways and react with lung tissue. ?The National Institute of Health?s National Institute of Allergy and Infectious Diseases identifies airborne dust as the primary source of allergic stress worldwide,? says Eugene Shinn of USGS. A study by M.E. Howitt of the Queen Elizabeth Hospital in Barbados between 1973 and 1996 found 17 times as many allergy attacks during the period of African dust clouds.

Clouds of dust regularly reach us from China as well. On April 20, we reported that a massive cloud of dust that originated in the Chinese desert, containing industrial pollution, bits of dinosaur fossils and even mummy particles, was floating over the U.S.

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