In 1969, the late Pete Conrad was the third human to set foot on the Moon as Commander of Apollo 12. Part of his crew's mission was to retrieve parts of the spacecraft Surveyor 3 - which had been sitting on the surface of the Moon for two-and-a-half years - and take them back to Earth so that scientists could figure out what had happened to them during their stay in space.

To NASA's amazement, viable cells of Streptococcus mitis were found on the parts. They were natives of the Earth that had done what any scientist would have told you was impossible - survived the journey to the Moon, and a very long stretch in the Moon's cold, oxygen-less, irradiated atmosphere.

"I always thought that the most significant thing that we ever found on the Moon was the little bacteria who came back and lived and nobody ever said anything about it," Conrad said.

It may not have been good news for NASA - it seems to mean that in future they cannot rely solely on the process of space travel to ensure Earth-bound bacteria don't contaminate other life-supporting planets.

But it also underscores some bad news that scientists are beginning to ferret out about the ease with which pollutants are transmitted around the Earth. Oceans, seas, deserts and mountains are not the barriers they might seem to be.

In this month's issue of the American Scientist, the magazine of Sigma XI, the Scientific Research Society, four scientists present evidence demonstrating that a veritable river of dust, microorganisms and toxic chemicals flows constantly through the Earth's atmosphere.

It is that river, they say, that may well be to blame for the decline of coral reefs in the Caribbean.

The authors are Dale W Griffin, a microbiologist who is a postdoctoral scientist at the US Geological Survey, Center for Coastal and Regional Marine Sciences in Florida; Christina A Kellogg, a marine biologist and a Mendenhall Fellow at the USGS; Virginia H Garrison, a coral-reef ecologist at the USGS; and Eugene A Shinn, a USGS senior geologist.

"By some estimates as much as two billion metric tons of dust are lifted into the Earth's atmosphere every year," they say.

"Most of this dust is stirred up by storms, the more dramatic of which are aptly named dust storms. But more than mere dust is carried aloft.

"Drifting with the suspended dust particles are soil pollutants such as herbicides and pesticides and a significant number of microorganisms - bacteria, viruses and fungi. We can gain some appreciation of how much microbial life is actually floating in our atmosphere by performing a quick calculation.

"There are typically about one million bacteria per gram of soil, but let's be conservative and suppose there are only 10,000 bacteria per gram of airborne sediment. Assuming a modest one billion tons of sediment in the atmosphere, these numbers quickly translate into a quintillion (ten to the power of 18) sediment-borne bacteria moving around the planet each year - enough to form a microbial bridge between Earth and Jupiter.

Those who watch the movement of tropical systems in the summer will be aware of the winds that push these systems from their spawning grounds off the coast of Africa across the Atlantic to the Caribbean, often making a beeline for the islands just to the north of the coast of South America - like Barbados and the Windward Islands.

And anyone who has been to the Caribbean knows of days when the air seems laden with fine particles, drawing the limit of vision in as close as it might be in light rain. The cause is dust from the African continent, transported across the Atlantic by the same prevailing winds. Research seems to indicate that during the hurricane season, most of the African dust blowing westwards lands in the Caribbean and in the US. From November through May, it takes a path with a little more south in it, and falls on South America.

Bermuda is not normally in the direct line of fire of these winds, and may have escaped reef damage as serious as that experienced further south for that reason.

The article in American Scientist claims: "Today, scientists are just beginning to grasp the impact and full extent of African dust fallout. It's been estimated that 13 million tons of African sediment fall on the North Amazon Basin of South America every year. A single, large dust storm may deliver more than 200 metric tons.

"And the Americas are not the only dustbins for African sediments. Dust storms originating in North Africa routinely affect the air quality in Europe and the Middle East. Reports of a fine red layer of African dust on automobiles and snow are not uncommon in Western Europe."

North Africa is not the only source. Arid regions in Russia, China, the United States, Australia and South America also contribute.

But the Sahara and Sahel regions of North Africa, both of which have been in drought since the late 1960s, are thought to be the most significant contributors. Cultural practices in those regions affect what is carried north and west. Mali, for example, whose northern half is covered by the Sahara, is economically poor, lacking basic sanitary facilities. The Niger River, which flows through thousands of miles of the country's arid lands, is the repository for all kinds of waste, including animal feces and excreted pharmaceuticals that have been used against a wide variety of human diseases.

Once a year, the Niger deposits a load of fine sediment on the floodplain, along with whatever else it carries. People plant crops on the newly deposited soil, add pesticides and burn garbage to fertilise the soil. Recently, the nature of the burned garbage has changed, to include plastic products and rubber tires. When they are burned, all kinds of unpleasant material is released, including dioxin and concentrated heavy metals.

According to American Scientist, a scientist at the University of Miami has discovered a direct correlation between the worsening African drought and increases in dust landing in the Caribbean. This scientist's research group has also cultured numerous colonies of bacteria and fungi from the atmosphere over the Island of Barbados, noting that their concentrations increase sharply with increasing concentrations of African dust in the region.

Among the crop pathogens that make their way across the Atlantic are thought to be those that cause sugar cane rust, coffee rust and banana leaf spot. The Caribbean-wide sea fan disease agent Aspergillus sydowii is also found in the Caribbean atmosphere during African dust events. Scientists expect to be able to show that many other coral diseases are spread by dust from both Africa and Asia.

The dust also has a direct effect on human health. Quite apart from the outbreaks of meningitis that dust events cause on the continent of Africa, investigators are studying a connection between African dust and high rates of asthma in the Caribbean. For example, there has been a 17-fold increase in the incidence of asthma on Barbados since 1973, which corresponds to the period when quantities of dust in the region began to increase. Some of the airborne particles are so small that once in the lungs, they cannot be exhaled. The effects are not clear, but some of the contaminants are endocrine disrupters (pesticides and polyaromatic hydrocarbons), some are carcinogens (dioxin and radioactive isotopes) and others are simply toxic to cells (like heavy metals).

Pathogens notwithstanding, the iron-rich dust has complex effects on plant life in the Western hemisphere. It serves as a source of nutrients for plants in the upper canopy of the South American rain forest. But it is also thought to cause red tide-type algae blooms in Florida's coastal waters.

"At this point," the authors say, "we are just stepping through a door into a wide-open area of research.

"It is important to recognise that the knowledge in this field is based on occasional point samples and not on continuous surveys. The results are affected by many factors, including location, altitude, season, weather, wind and the methods used to collect the samples. Understanding the potential ramifications of atmospheric transport of hazardous materials and organisms may have fundamental importance for the health of a society.

"There is an air bridge that spans the great oceans, and we haven't been paying much attention to it."

I should record my gratitude to American Scientist for allowing me to quote at such length from the article in their May-June edition.

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