Pilots' unions for US Airways and American Airlines are urging their members to avoid full-body scanning at airport security checkpoints, citing health risks and concerns about intrusiveness and security officer behavior.

"Pilots should NOT submit to AIT (Advanced Imaging Technology) screening," wrote Capt. Mike Cleary, president of the U.S. Airline Pilots Association, in a letter to members this week. USAPA represents more than 5,000 US Airways pilots.

"Based on currently available medical information, USAPA has determined that frequent exposure to TSA-operated scanner devices may subject pilots to significant health risks," Cleary wrote.

American Airlines pilots have also received guidance from their union, the Allied Pilots Association, to decline full-body scanning. APA represents 11,000 pilots.

"It's safe to say that most of the APA leadership shares my view that no pilot at American Airlines should subject themselves to the needless privacy invasion and potential health risks caused by the AIT body scanners," APA president David Bates said in a letter to members.

Both unions are concerned about the effects of repeated exposure to small doses of radiation emitted by the backscatter technology used in some of the Transportation Security Administration's full-body scanners.

In the course of their daily duties, pilots are routinely exposed to elevated levels of naturally occurring atmospheric radiation, which increases at higher altitudes.

The unions urge members to choose security lines that use standard metal detectors whenever possible. When faced with AIT screening, pilots should opt for enhanced pat-downs, although this security procedure also concerns the unions.

Unions are encouraging pilots to request private pat-downs. USAPA urges members to make sure a witness is present during the procedure.

USAPA refers to incidents where Transportation Security Administration officers may have implemented the screening technique inappropriately.

One pilot described his experience as "sexual molestation," according to Cleary's letter. Bates wrote, "There is absolutely no denying that the enhanced pat-down is a demeaning experience."

Both unions are looking for long-term solutions to airline crew screening.

"Pilots really should never have been subjected to this type of screening, ever. Because when we walk through these machines, within a few hundred yards we get into what potentially could be the biggest weapon on the airport, and that's the airplane," said James Ray, a USAirways captain and spokesman for USAPA.

Pilots are well screened with security background checks and regular medical and mental health checks, he said. The union suggests implementing alternate identity verification technology that would allow pilots to bypass regular passenger screening.

The TSA said it welcomes further discussion with pilots and emphasized the agency's role in addressing security threats.

"We are frequently reminded that our enemy is creative and willing to go to great lengths to evade detection. TSA utilizes the latest intelligence to inform the deployment of new technology and procedures in order to stay ahead of evolving threats," the TSA said in a statement.