In experiments, the cream-based ultra-violet (UV) filters -- used to protect skin from the harmful effects of sun exposure -- caused bleaching of coral reefs even in small quantities, the study found.

Coral reefs are among the most biologically productive and diverse of ecosystems, and directly sustain half a billion people. But some 60 percent of these reef systems are threatened by a deadly combination of climate change, industrial pollution and excess UV radiation.

The new study, published in US journal Environmental Health Perspectives, has now added sun screens to the list of damaging agents, and estimates that up to 10 per cent of the world's reefs are at risk of sunscreen-induced coral bleaching.

Chemical compounds in sunscreen and other personal skin care products have been detected near both sea and freshwater tourist areas. Previous research has shown that these chemicals can accumulate in aquatic animals, and biodegrade into toxic by-products.

Researchers led by Roberto Danovaro at the University of Pisa in Italy added controlled amounts of three brands of sunscreen to seawater surrounding coral reefs in Mexico, Indonesia, Thailand and Egypt.

Even small doses provoked large discharges of coral mucous -- a clear sign of environmental stress -- within 18 to 48 hours. Within 96 hours complete bleaching of corals had occurred.

Virus levels in seawater surrounding coral branches increased to 15 times the level found in control samples, suggesting that sunscreens might stimulate latent viral infections, the study found.

Pesticides, hydrocarbons and other contaminants have also been found to induce algae or coral to release viruses, hastening the bleaching process.

According to the World Trade Organisation, around 10 per cent of tourism takes place in tropical areas, with 78 million tourists visiting coral reefs each year.

An estimated 4,000 to 6,000 tonnes of sunscreen are released annually in reef areas, with 25 per cent of the sunscreen ingredients on skin released into water over the course of a 20 minute submersion.

Sunscreens are made of around 20 compounds acting as UV filters and preservatives. Seven were tested for the study, including parabens, cinnamates, benzophenones and camphor derivatives.

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