A common form of skin cancer could be diagnosed by the distinctive chemical "scent" it gives off, say US experts.

Philadelphia's Monell Center sampled the air directly above basal cell carcinomas and found it was different to similar samples from healthy skin.

They told a conference it offered the chance of cheap and painless testing.

Other scientists are trying to spot the "smell" of cancer, with a UK team using dogs to sniff out bladder tumours from urine samples.

All human skin releases chemicals called "volatile organic compounds", many of which do have a scent.

The researchers from the Monell Center used a technology called gas chromatography-mass spectrometry to identify their precise chemical composition.

A total of 22 patients, 11 with and 11 without basal cell carcinomas, were tested. All the air samples contained the same ingredients, but the equipment revealed that the patients with cancer had markedly different concentrations of certain chemicals.

Dr Michelle Gallagher, presenting the results of the project at the American Chemical Society's annual conference, said that a "profile" of the cancer could be built up.

"Our findings may someday allow doctors to screen for and diagnose skin cancers at very early stages," she said.

She now plans to try to construct profiles of other types of skin cancer, including the much more dangerous malignant melanoma.

Dog detector

Dr Carolyn Willis, a dermatology researcher from Amersham Hospital in Buckinghamshire, is trying to develop a cancer test using the same principles - but substituting a living sensor.

Her team has trained dogs to detect subtle changes in the odour of urine which could indicate bladder cancer, and is hoping to detect prostate and skin cancers the same way.

The dog's nose, she said, was one of the most sensitive instruments available, and had the advantage of being attached to a brain already programmed to identify different patterns in the scents it received.

She said: "This has great potential as a screening tool. The detection of these volatile organic compounds could make a major contribution to diagnosis.

"It's a non-invasive and simple way of detecting disease."

Other projects worldwide have included checking the composition of exhaled breath for distinctive chemicals given out by lung tumours.