Answer to Question #12361 Submitted to "Ask the Experts"
Category: Security Screening — Airport Screening
The following question was answered by an expert in the appropriate field:
I have heard that an x-ray baggage scanner emits less than 1 millirad. I want to know exactly how it is measured because in the hospital x rays are taken within a fraction of seconds, but an x-ray baggage scanner takes some seconds (like a fluoroscopy) and the bag also receives radiation in the form of the primary beam as well as scattered beam. How much will it emit per inspection when it is operated at 160 kilovolt (kV) and 1 milliamp (mA)? How much does the scatter beam radiation decrease in comparison to the primary beam?
For clarity, there are two types of screening systems used in airports. First is the "checked baggage" screening system, often referred to as an explosive detection system (similar to a computed tomography [CT] scan). The other screening is for "carry-on baggage," with which most people are familiar when they go through airport security. Carry-on baggage screening is sometimes referred to as a transmission x ray, similar to a posterior-to-anterior (PA) x ray. Newer carry-on baggage screening technologies are being installed that include the same CT scanning capabilities as the explosive detection systems. Both screening technologies generate (or emit) radiation based on the kiloVolt potential (kVp) and milliamp-second (mAs) settings, as mentioned in your question. Specific details cannot be provided for security reasons.
The amount of radiation absorbed by the materials varies greatly depending on the materials in the baggage, time in the primary beam field, and surrounding materials. All of these factors affect the amount of radiation that is also scattered from the primary beam which can vary to extremes.
The operational settings for checked baggage systems differ from the traditional carry-on baggage systems. Checked baggage and the newer carry-on screening systems produce slightly higher energy x rays, which also results in higher radiation levels within the unit. Some of these systems are activated (e.g., x ray on) throughout the entire screening process to produce a detailed image of the entire baggage. Others are activated randomly throughout the screening process to produce multiple images (e.g., 2 mm–5 mm slices of the baggage) during the scanning process. In other words, while checked baggage and the newer carry-on screening systems may emit the same amount of radiation intensity during the screening, the dose delivered to the materials will vary greatly because the unit isn't always active. Traditional carry-on baggage screening systems emit lower intensity x rays and these systems also vary with the amount of time the unit is active.
The National Institute for Occupational Safety and Health conducted a study to measure the amount of radiation potentially received by materials imaged from both screening technologies. Sixty-seven dosimeters were intentionally irradiated in groups of three, from one to 10 times, in "checked baggage" and traditional "carry-on baggage" machines to characterize the response of the dosimeters when directly exposed to the beam. Data from dosimeter badges passed through a traditional carry-on baggage machine showed from none to very small amounts of measurable radiation. On page 24, the study notes that the highest dose measured on a dosimeter that was passed 36 times through the machine was 4 mrem or 0.04 millisievert (mSv).
Dosimeters that were passed through the "checked baggage" system that randomly activates the x ray had highly variable doses. If the dosimeters were near the area randomly selected by the software to activate the x-ray source, a higher dose would be measured. The average dose, after 10 passes through this type of system was about 28 mrem per scan (0.28 mSv per scan).
Dosimeters that were passed through the type of "checked baggage" system that stayed active for the entire screening process had an average dose of 156 mrem per scan (1.56 mSv) per scan. The newer carry-on screening systems are expected to deliver similar doses.
Please note that these results reflect the potential dose to items passing through these screening systems as measured by a dosimeter calibrated for human measurements. Actual doses will vary depending on the materials in the checked- or carry-on baggage. Please see the NIOSH study for an assessment of potential radiation exposure to the workers who operate these systems.
John Cardarelli II, PhD, CHP, CIH, PE, RSO