Cell phone use raises the risk of mouth cancer, a new study has found. Five years of heavy use increased the chances of developing a tumor by around 50 percent compared with people who had never used a mobile phone, scientists found. The study is the latest to link cell phone use to a serious disease, and raises concerns that mobile phones could be interfering with the body in ways that scientists simply do not yet understand.

Previous studies into the links between phones and cancer have generated conflicting results with the vast majority claiming to have found no evidence of serious health risks. A number of studies have found links between cell phone usage and cancers of the head and ear. The latest research, carried out in Israel, was published in the respected American Journal of Epidemiology.

The lifestyles of 402 people with benign mouth tumors and 56 with malignant tumors were compared to a control group of 1,266 people. Those who used mobile phones were more likely than normal to develop parotid gland tumors. The parotid is the largest of the salivary glands and sits at the back of the mouth not far from the ear. Long-term users of mobile phones tended to develop tumors on the same side of the head as the phone was normally held. People who used mobile phones in rural areas, where the phone has to work harder to make contact with the nearest base station, were found to be at greater risk. The cause of the heightened risk was not established.

Most studies have looked at the way the electromagnetic fields created by phones warm tissue; however, the levels of the fields are thought too small to have a heating effect. Instead, some researchers believe the fields have the power to disrupt chemical bonds within cells or damage DNA. The lead researcher - Dr. Siegal Sadetzki, from the Chaim Sheba Medical Centre in Tel Hashomer, Israel - urges caution regarding drawing conclusions. A similar study published a year ago in the same journal and based on mouth cancer sufferers in Denmark and Sweden found no increased risk. But campaigners against mobile use leapt on the findings.

Graham Philips of PowerWatch - a nonprofit, independent advocacy group working in the United Kingdom's electromagnetic field and microwave radiation health debate - states that this is "more evidence - and strong evidence too - that mobile phones have a biological effect. It brings into question once again the official guidelines on safe exposure which are based on the heating effects, not the non-thermal effects that they seem to have shown here."

Other scientists urged caution as the cancers studied are extremely rare. Only 400 parotid tumor cases are diagnosed each year in the United Kingdom and, of these, only 60 are found to be malignant. The advice from the Department of Health is that there is still no evidence that mobile phones pose a health risk; however, a Government funded six-year study in September found a hint of a higher cancer risk and concluded that a danger cannot be ruled out.