Shields to cut mobile phone radiation can work but some of the devices whose benefits are often described in quasi-scientific language do not, experts say.

Shields to cut mobile phone radiation can work but some of the devices whose benefits are often described in quasi-scientific language do not, experts say.

Research by a team of independent scientists for the Government concluded that while many of the shielding devices on sale can reduce the amount of radiation that reaches the head, they can also reduce the effectiveness of the phone by blocking its signal, and make it use up its battery.

But the researchers were dismissive of the small button-like devices which are sold as clip-on products and often contain fluids or materials that are claimed to "attract" or absorb the majority of potentially harmful radiation. Tests on three of the varieties on the market found that "none of the devices are very effective in reducing" the radiation that reaches the head, also known as the "specific absorption rate" (SAR).

The report found that hands-free kits "greatly reduced" SAR to the head – but it warned: "The phone should not be placed in contact with other parts of the body. A shielded case would be equally applicable for a phone clipped to a belt, for example, and could still be used with a hands-free kit, in which case, both head and body SAR would be reduced."

Mobile phones work by encoding conversations into radio-frequency waves in the microwave band. They try to connect to any base station for their network that is in range; if none is found or the signal is weak, the phone will increase its power output to try to establish the link.

That has thrown up questions on whether the microwave radiation could have some biological effect, including causing tumours. In the US a number of lawsuits has been filed making such claims. None has yet been concluded.

The latest report was commissioned in May 2000 by the Independent Expert Group on Mobile Phones, chaired by Sir William Stewart.

Current figures suggest that nearly 75 per cent of the adult population in Britain owns a mobile, and more than half of the entire population. That has led to a number of health scares, although no adverse effects have been proved through long-term use of a mobile phone.

A number of studies have shown some warming effects in the head from the microwave radiation emitted from the phone – but no adverse effects have been established. Studies have not shown any increase in the incidence of tumours.

But Sir William recommended that children should not use mobiles extensively, and earlier this year oversaw a raft of new research into phones and their effects.

The newly published work investigated the radiation received in a dummy head when a mobile phone was held near it with and without various shielding devices. The ideal would be that the device would cut the radiation received in the dummy head without affecting its usefulness. But that is not possible, the report noted: "Generally, the SAR reduction is due to the device limiting the useful, radiated power from the phone, which has the disadvantage of reducing the performance of the phone in weak signal areas or inside buildings," it said.

Shielding the phone could also make it try to connect with its base station by upping its power output and that could make the battery run out more quickly.