Historically, concern about the state of the oceans has mainly been generated by pollution. Over the last decades, increasing understanding of the seriousness of other threats - such as overfishing and the  destruction of habitats – and of the damage they cause, has tended to overshadow it. But it has enormous effects on health and the environment.

 Many studies show that respiratory and intestinal diseases and infections among bathers rise steadily in step with the amount of sewage pollution in the water. They demonstrate, too, that bathers are at risk even in lightly contaminated waters that meet the pollution standards laid  down by the European Union and the US Environmental Protection Agency. A recent WHO report has estimated that one in every 20 bathers in “acceptable waters”, will become ill after venturing just once into the sea.

The GESAMP/WHO study - based on global estimates of the number of tourists who bathe, and WHO estimates of the relative risks at various levels of contamination - estimates that bathing in polluted seas causes some 250  million cases of gastroenteritis and upper respiratory disease every year. Some of these people will be disabled  over the longer term. The global impact can be measured by adding up the total years of healthy life that are lost through disease, disability and death using a new measurement - the Disability Adjusted Life Year (DALY) – developed by WHO and the World Bank. When this is done, the world-wide burden of disease incurred by bathing in the sea, adds up to some 400,000 DALYs, comparable to the global impacts of diphtheria and leprosy. It is estimate to cost society, worldwide, about US $1.6 billion a year1  The toll from consuming contaminated shellfish is even greater. One study suggests that seafood is involved in 11 per cent of all the outbreaks of disease carried in food in  the United States, 20 per cent of them in Australia, and over 70 per cent in Japan, which has a particularly strong tradition of eating raw fish and shellfish.  Pathogenic bacteria can survive in the sea for days and weeks; viruses can survive in the water - or in fish and shellfish - for months. The particularly virulent infectious hepatitis virus - which has caused many outbreaks of the disease associated with eating shellfish - can remain viable in the sea for over a year. Shellfish, like oysters, mussels, clams and cockles, feed by filtering huge amounts of seawater - and can concentrate viruses and bacteria a hundredfold from the water in which they live.

A series of studies has found viruses in about a fifth of the shellfish taken from waters that meet US bacteriological standards for growing and harvesting them. There is strong evidence that fresh shellfish - on sale for food -frequently contain enough viruses to make many of those who eat them ill. They are often eaten raw, or after only a light steaming which is not enough to kill most of the viruses or bacteria.

One US study suggested that one in every hundred people eating relatively lightly contaminated raw shellfish will be infected with a moderately serious intestinal virus disease; the risk rises to up to 50 in a 100 if the virus is highly infectious. Other studies in both the United States and the United Kingdom suggest that a quarter of those who are taken to hospital suffering from infectious hepatitis - a disease that can confine sufferers to bed for two to three months - have caught it from eating raw or lightly steamed shellfish.

 Some eight billion meals of shellfish are thought to be eaten worldwide each year. The GESAMP/WHO study estimates that eating sewage-contaminated shellfish raw causes some 2.5 million cases of infectious hepatitis each year. Some 25,000 of the victims die and another 25,000 suffer long-term disability from liver damage. The global burden on human health equals some 3.2 million DALYs a year - comparable to the worldwide impact of all upper  respiratory infections and intestinal worm diseases – and costs world society some US$ 10 billion annually.

 This new evidence of the dangers of sewage pollution is just one example of a general reappraisal of the relative importance of different pollutants of the sea. Some of those  once thought to be the most damaging worldwide are now believed to be much less important, either because more is known about them or because they have been brought under control.  

• The supposed effects of man-made radionuclides discharged into the sea still loom large in the minds of the general public and politicians. Although threats from accidental releases cannot be ruled out, radionuclides now probably worry scientists less than any other category of marine pollutants. 

• Similarly, highly publicised and exaggerated concerns about the extent of contamination of the seas and their life by heavy metals cannot be justified; it is probably far less serious than pollution by nutrients and some persistent organic chemicals. 

• The effects of even the most dramatic oil spills are generally localised; gross pollution from them disappears relatively rapidly, though some subtle effects may last for decades, with enormous economic costs.

• Some eight billion meals of shellfish are thought to be eaten worldwide each year. The GESAMP/WHO study estimates that eating sewage-contaminated shellfish raw causes some 2.5 million cases of infectious hepatitis each year. Some 25,000 of the victims die, and another 25,000 suffer long-term disability from liver damage. The global burden on human health equals some 3.2 million DALYs a year - comparable to the worldwide impact of all upper respiratory infections and intestinal worm diseases – and costs world society some US$ 10 billion annually.

 Until recently, most attention concentrated on pollutants which directly or indirectly poisoned sea life and those consuming it - or were suspected of doing so. Less attention was paid to the potential effects of the persistent organic chemicals, some of which may have much more subtle, but possibly even more damaging effects. These include changes in the structure and function of communities of marine life, through disrupting reproduction and altering behaviour, and effects at the molecular level, such as causing cancer or mutations or disrupting endocrine systems. Evidence that concentrations of these substances now in the marine environment are causing such effects is mostly inconclusive. Risks to human health usually only occur where concentrations are high, or where people are exposed to them in unusual ways, such as in the Arctic where fish and seafood form an extremely high percentage of the diet.

It is now well-established that some chemicals can harm the endocrine systems of a wide range of wildlife species, both on land and at sea, and may give rise to strange ‘genderbending’ effects. Tributyl tin, for example - which has been widely used in anti-fouling coatings on ships and in fish farming - appears to have made female sea snails grow false penises, and to have severely affected oyster fisheries in some areas. Its use has now been restricted in most developed countries, but it is still being traded in some markets. It is possible that other environmental contaminants could ‘sneak up on us’, causing further unexpected effects.

A group of researchers has found a connection between ocean temperatures and cholera outbreaks, and plans to test the model worldwide for predicting the incidence of the deadly water-borne disease.

Using data from three ocean-monitoring satellites, the scientists found that increased sea-surface temperatures correlated with cholera outbreaks in Calcutta. Higher temperatures make the coastal waters "bloom" with plankton that carries the deadly cholera bacteria.

Now the plan is to merge ocean temperature forecasts with proven public health measures for reducing the incidence of cholera -- such as instructing residents who drink hazardous water to filter it first through layers of cloth.

Cholera is a bacterial infection that affects mainly the small intestine and can be fatal. In 1993, a total of 296,206 new cases were reported in South America. Most cholera outbreaks start in coastal areas.

Huq worked with Rita Colwell, director of the National Science Foundation, and others on the research, recently published as a paper in PNAS (Proceedings of the National Academy of Sciences).

Huq, Colwell and their colleagues studied satellite data and found that rising sea temperatures and ocean height near the coast of Bangladesh in the Bay of Bengal from 1992 to 1995 often preceded sudden growth of plankton and the cholera outbreaks.

The group correlated years of hospital cholera records from Bangladesh with sea temperature and ocean height data that came from a variety of satellites and sea-surface observations.

The satellites, including the U.S.- French TOPEX/Poseidon oceanography satellite, measured water temperature and ocean height, as well as colors that indicated plankton growth.

Scientists believe that sea height also affects cholera outbreaks because tides reach further inland to affect more people who drink or bathe in brackish water carrying cholera.

"Bangladesh is very low and flat," said co-author Louisa Beck of California State University at Monterey Bay and a resident scientist at Ames, "and tidal effects are felt almost halfway up into the country."

Scientists have used similar approaches with satellite data to study malaria, Lyme disease and Rift Valley fever.

If the model that worked in Bangladesh could be extended globally, it could serve as an early warning system, said Brad Lobitz, a scientist at the NASA Ames Research Center, and the lead researcher on the project.

Once alerted to rising temperatures in coastal waters, public-health officials could issue warnings to prevent or respond to the spread of cholera, such as instructing and reminding people about cloth filters, Huq said.

"When four folds of a sari are used as a filter, 99 percent of the attached bacteria onto plankton and particles are removed," Huq said. Huq has demonstrated the method with women in the area where data were collected for the study.