Answer to Question #10886 Submitted to "Ask the Experts"

I live in Tijuana, Mexico, and cross the border once or twice every day. There are several screening devices that I have been concerned about, including:

1. LEXRIS (low emission x-ray inspection systems). Our cars go through these machines when sent to secondary inspection (which happens about once a week when randomly assigned by a computer or more often depending on the odds). I always opt for exiting the vehicle with my children and carpooled children and an officer drives my car through the machine. We (the passengers) are held inside a gated area that is next to the machine while we wait for the inspection to occur. I am concerned about the scattered radiation we are receiving while waiting.

2. Stationary radiation detectors. We drive past these detectors to arrive at the port where the officer is (many times, we and our cars are inside the stall within the detectors because of waiting times). Same question as above.

Thank you for your questions. The dose to officers or to individuals who might choose to stay in their vehicles while being scanned is very small.

The average radiation dose per scan to a person sitting in a vehicle that passes through one of the vehicle scanners would be, maximally, about 0.05 µSv per screening and probably closer to about 0.02 µSv because of the fast speed at which a vehicle is screened (µSv, or microsievert, is a unit of effective radiation dose). This is far below the level of radiation exposure that is known to cause cancer or any other health effect.

Ordinary background radiation received each year from cosmic radiation and radioactivity in soils and building materials is about 3,000 µSv. Another comparison is a chest x ray, where a person would receive about 100 µSv.

The American National Standards Institute has studied the issue of vehicle scanners (ANSI N43-17 published in 2009) and has recommended that vehicle screening expose an individual inside the vehicle to no more than 0.25 µSv per screening, which would allow someone to pass through a screening unit 1,000 times before reaching regulatory limits on exposure for the public. As I mentioned, though, the actual doses received are much smaller than this.

The radiation dose limit in areas where people who operate the machines might be located or where people might stand while their cars are being driven through are even lower.

According to AS&E, a maker of a vehicle screening unit, if a car goes below a specified speed, the x-ray unit is shut off to minimize the possibility that a person could receive extra radiation dose. Most screening units are set for a car to go about three miles per hour.

As for the justification for this use, the Health Physics Society has a position statement on the topic of using ionizing radiation on humans for security screening and believes that it is a justified practice if certain criteria are met.

Kelly Classic
Certified Medical Health Physicist