A passenger is checked inside a body scanner at Schiphol airport, Netherlands. (Cynthia Boll/Associated Press)

On Jan. 5, 2010, the federal government announced that full-body scanners would be installed during the year at several Canadian airports. Ottawa ordered 44 of the machines, which could be used on passengers who were selected for further screening.

The move followed a tightening of security at American airports after a passenger on a flight from Amsterdam to Detroit tried to set off a bomb over the busy Christmas travel season.

People who refuse to go through the scanner — whether over concerns of being subjected to radiation or because the scanners produce a three-dimensional outline of a person's naked body — would have the option of being subjected to a pat-down search instead.

Currently, all luggage — checked or carried on — goes through X-ray machines that reveal suspicious objects. Your carry-on bag will be hit with low-level radiation — far less than what you would be subject to for a chest X-ray. You'll be hit with a bit, too. But you'd have to put your bag through the scanner about 5,000 times to equal the radiation you are exposed to in one chest X-ray. The machines used to X-ray luggage differ from medical X-ray devices in two important ways:

• There is more shielding on airport X-ray machines — the entire belt is shielded; in medicine, the patient table is not shielded.
• Baggage scanners use less radiation because the image does not need to be as clear as it does when X-raying a patient.

While airport workers might be exposed to some radiation, airport security devices must meet federal standards. Health Canada has set recommended levels for radiation exposure.

The scientific unit of measurement for radiation exposure is the millisievert (mSv). Radiation exposure is also measured by the millirem (mrem). One mSv is equivalent to 100 mrem. The average person is exposed to 3.6 mSv of radiation a year naturally.

That number can vary substantially from location to location depending on elevation, soil and latitude. The higher your home's altitude, the more background radiation you will be exposed to.

Most of that background radiation exposure — about 2.6 mSv — comes from radon gas.

Get a chest X-ray, and you'll add 0.1 mSv — or about 10 days' worth of naturally occurring radiation — to your annual exposure total. Go for a CT scan of your abdomen and pelvis, and your total is boosted by 10 mSv — or about three years' worth of naturally occurring radiation.

A Transportation Security Administration officer looks over the image taken from an X-ray security scanner in a demonstration at Sky Harbor International Airport, in Phoenix, Ariz. (Elaine Thompson/Associated Press)

At the airport, your exposure to radiation is barely boosted. All passengers must pass through a metal detector, which uses pulse induction technology to scan you for items you shouldn't be taking onto an airplane.

The machine sends short powerful bursts of current through coils of wire. Each pulse generates a magnetic field. When the pulse ends, the magnetic field collapses, leading to a sharp electrical spike, which causes another current to run through the coil. The detector creates anywhere from 25 to 1,000 pulses per second.

If a metal object passes through the detector, the pulse creates an opposite magnetic field in the object. That makes it take longer for a pulse to disappear — creating an echo, similar to the way sound hits a hard surface leading to a noticeable echo.

If the machine beeps, a security guard will use a hand-held scanner to try to pinpoint the location of the item that caught the machine's attention. Radiation exposure here is zero.

However, if you make the metal detector beep and you're given the option of a pat-down search or a trip through one of those new scanners that are coming to airports in Vancouver, Calgary, Edmonton, Winnipeg, Toronto, Ottawa, Montreal and Halifax this year, you will be exposed to a little radiation.

Advanced scanning technology

There are two types of enhanced body scanners. One relies on millimetre wave technology, which uses extremely high frequency radio waves to produce images. There is no ionizing radiation — the kind of radiation associated with medical X-rays — but the scanners can "see" through your clothes, revealing not just concealed weapons but breast implants, colostomy bags and any body piercings you might not want people to know about.

The image quality resembles a fuzzy photo negative. The software can be enhanced to obscure the face. The U.S. Transportation Security Administration (TSA) says the scanners it has tested produce 10,000 times less energy than a cellphone transmission.

The other enhanced body scanner uses a technology known as "backscatter." Low-level X-rays produce the same kind of "see-through" images that millimetre wave technology produces. Backscatter images resemble a chalk etching. Again, the technology can be adapted to obscure faces.

Unlike medical X-rays, the X-rays used in backscatter technology bounce off the skin, revealing what's under your clothes, but not what's under your skin. Medical X-rays go through the body, revealing bones and internal organs.

The TSA says you will be exposed to as much radiation undergoing a backscatter body scan as you would during two minutes of your flight. The high altitude of a cross-Canada flight exposes you to about 0.03 mSv of radiation. You'd need to fly across the country 100 times to equal the amount of radiation you are naturally exposed to in a year.

Manufacturers of both types of machines insist that images cannot be stored, broadcast or re-transmitted. After the person passes through the machine and the person has been cleared, the image is gone forever.

Last October, Canada's privacy commissioner said her office is satisfied that the scanners are not a threat to people's privacy.

While Health Canada says the scanners are safe, it also recommends caution when it comes to any kind of radiation exposure. In a section of its website dealing with environmental and workplace health, the agency notes:

"It is emphasized that any irradiation may involve some degree of risk … All doses should be kept as low as reasonably achievable, and any unnecessary radiation exposure must be avoided."