So-called "naked" body-­scanning machines at airports, the latest defence against would-be plane bombers, have already raised concern for breaching flyers' privacy and, potentially, feeding the voyeurism of security officials. But could being screened also pose a health risk?

The question arises because one of the two types of new ­scanner – those that deploy "back-scatter" x-ray technology – uses ionising radiation to generate the images that indicate if someone is concealing something dangerous. The Department for Transport, which ordered the introduction of whole-body scanners at all UK airports after the plot to blow up an aeroplane over Detroit on Christmas Day, says that they are completely safe.

"The level of radiation that you usually receive from a back-scatter machine is equivalent to what you would naturally receive [from the sun] from two minutes of flying at about 35,000ft," says a DfT spokesman. He points to a report this week by the Health Protection Agency, which says that standing to be examined by back-scatter technology involves ­receiving a dose of just 0.02 micro­sieverts or less of radiation, a tiny fraction of the 2,700 microsieverts that a typical Briton is ­exposed to naturally every year from sources such as radon gas, cosmic rays and building materials.

"That's a very small dose of radiation," says Professor Richard Wakefield, a radiation expert at Manchester University's Dalton Nuclear Institute. "I can't say that these scanners pose no risk, but at the doses you are talking about it's verging on the ridiculous to be worried about them." Many of us may well prefer the notional risk from those minute doses to the risk of being on a plane that is blown up, anyway.

But Douglas Boreham, professor in medical physics and applied radiation sciences at ­McMaster University in Ontario, Canada, cautions that there is a small possibility of harm for frequent flyers or those who are sensitive to the effects of radiation. Radiation from x-ray scanners could be more highly concentrated than radiation encountered naturally at high altitudes, he says. He wants the possible impact to be monitored. "We don't have enough information to make a decision on whether there's going to be a ­biological effect or not," he says.