Dr Krishnan explains how the PVC treatment seals in the phthalates

The chemicals known as phthalates, which are incorporated into PVC to make it softer and more pliable, have been accused of causing cancer by environmental campaigners.

Groups such as Greenpeace have been trying to get them banned.

The new Indian process, reported in the science journal Nature, is targeted at medical PVC equipment to prevent phthalates entering the bloodstream of patients undergoing dialysis or blood transfusions.

"I have been working in this area for seven years and seen no other work which produces total prevention of phthalate leaching," said Dr Jaya Krishnan, at the Sree Chitra Tirunal Institute for Medical Sciences in Trivandrum, India.

It might also be useful to treat PVC cling-film sometimes used to wrap food and possibly children's PVC toys, which have been the focus of green concerns.

Body fluids

Plasticised poly-vinyl chloride (PVC) is used widely in medical equipment such as blood bags and tubing. It usually contains up to 40% di-(2-ethylhexyl)phthalate (DEHP).

However, DEHP can leach out of PVC and into body fluids and has been found in stored blood products as well as the body fluids of patients. In rats, and possibly in humans, it is a fat-removing carcinogen in the liver. Phthalates are also suspected of attacking the human reproductive system, particularly the testes.

The new approach holds promise for manufacturing use because of its simplicity, said Dr Krishnan. "It is a very straightforward process and uses very inexpensive materials."

His team reacted the PVC with sodium sulphide in water with a catalyst. They found that this replaced the chloride ions on the surface of the PVC with sulphide ions, causing the polymer chains to cross-link. This locks in the DEHP.

The researchers placed treated and untreated PVC samples in petroleum ether, which extracts DEHP. The unmodified PVC lost almost all its DEHP in a day but the surface-treated PVC lost virtually none in six months.

New PVC treatment only "sticking plaster solution to toy danger"

Gwynne Lyons, pollution consultant for the Worldwide Fund for Nature, responded cautiously to the research. "For medical applications, there may be a benefit but for baby toys it would only be a sticking-plaster solution. There are many other safe teething toys for children," she told BBC News Online.