Researchers in the United States say dust clouds from dry African deserts may be a threat to the environment and human health. They say the dust may contain many small organisms(有机物) that could be dangerous to some people.

Each year, huge dust storms form in the Sahara and Sahel deserts of northern Africa. Winds carry the dust across the Atlantic Ocean. The movement of dust across the Atlantic Ocean has been increasing in recent years because of longer periods without rain in Africa.

From February to April, the dust settles in South America. From June to October, the winds change and transport the dust to North America, Central America and the Caribbean(加勒比海地区). The dust clouds travel several thousand meters above sea level. It takes five to seven days for the dust to cross the Atlantic Ocean.

Researchers have long known that the dust clouds could travel long distances. But they thought few microorganisms could survive the trip because of damaging ultraviolet radiation(紫外线) from the sun. Researchers now believe that the dust clouds block enough of the light to protect viruses, bacteria, and fungi in the dust.

Researchers say these microorganisms may be a health risk to some people. About half the bacteria and fungi that survive the trip from Africa are known to cause disease in people or plants.

More than half of the dust that reaches the United States settles in the state of Florida. For many years, it has caused the skies there to turn red. Now researchers say there may be a link between the dust storms and increased health risks in Florida. They believe the dust causes higher rates of asthma, allergies, and other breathing problems in people there. The dust also has been linked to a large increase in lung problems in at least one Caribbean nation.

58. Which statement about African dust storms is TRUE?

A. They form in the north of Africa and move eastwards.

B. They form in the north of Africa and move westwards.

C. They settle in South Africa from February to April.

D. It takes over a week for them to cross the Atlantic Ocean.

59. The worst thing about African dust storm is ______.

A. the heat they give off                              B. the distance they cover

C. the microorganisms they contain             D. the time they last

60. Which is NOT the effect of the dust in Florida?

A. For many years , the sky there turns red.

B. Health risks increase there.

C.The rates of breathing problems is higher there.

D.Half the bacteria and fungi survived there.