X-ray equipment is available for the detection of weapons within baggage or other carried items. For single-energy units appropriate for school applications, a vacuum tube emits x-rays on and through these items. These x-rays come from inside the top of the unit and scan downward as baggage is automatically moved through the equipment. Sensors collect the magnitude of the signals that make it through scanned items, with low Z-number material allowing more energy through and material with high Z-numbers allowing less energy through. (A "Z number" is the atomic number of a particular element; a low Z in x-ray terms is any atomic number less than 26. A high Z in x-ray terms is any atomic number equal to or greater than 26.) The resulting images are transferred to a TV monitor, where an operator must carefully examine each image for evidence of firearms or knives.
The safety aspect of x-ray equipment for baggage inspection has improved greatly over the past two decades. This application of x-rays previously used a large cone of energy in order to make an image of an entire piece of baggage at one time. Today's x-ray machines for baggage use a much lower energy pencil-thin beam of radiation that generally scans back-and-forth across a piece of baggage as the baggage moves beneath it. More sensitive sensors can now adequately capture an image with these lower dosage x-rays. Infrared (IR) beams installed within the equipment can accurately start and stop the x-ray beam source so that the x-rays are not operational when there is not a piece of baggage located in imaging position. Add to these improvements the excellent shielding built into x-ray detectors, and it is easy to understand what has made modern baggage detectors quite safe and of negligible health risk to either the operator of the equipment or to the general public. Indeed, the radiation exposure to operators from baggage scanners has been shown to be only a few microrems per hour, which is equivalent to standing in the sunlight for a few minutes. Even smoking a cigarette gives a person a larger dose of radiation. About the only potential health risk from an x-ray baggage machine would be to someone attempting to ride the conveyor belt through the equipment, which would still result in substantially less radiation exposure than would be gained from a medical x-ray.
There have been concerns raised about the safety of exposing food to baggage x-ray machines. The U.S. Food and Drug Administration (FDA) has approved much higher doses of radiation for normal food preservation methods than any food items would receive going through x-ray baggage equipment. Most scientists feel that the FDA is quite conservative in the limits it has established.
Over the past 10-15 years, x-ray detectors have become quite safe for camera film because of lower dose x rays. This would include the x-ray equipment most schools would normally consider purchasing today but not, perhaps, an older piece of equipment that has been donated for the school's use. One modern exception to this is the much more sophisticated $1million x-ray machines that are used on some airline flights to examine checked baggage. This equipment is used to search checked baggage for explosives, and it may well damage camera film.
Research Report: The Appropriate and Effective Use of Security Technologies in U.S. Schools