Answer to Question #127 Submitted to "Ask the Experts"

Thank you for your questions. Let me first address your statement regarding the x-ray/CT room and door shielding. All facilities in which radiation-producing equipment is used are required to meet radiation dose limiting regulations. If a person were to stand outside the room for a year, the room must be shielded so dose limits could not be exceeded. Regulations also state that any “penetration” of a shielded wall must be shielded so it offers the same protective capability of the shielded wall. A penetration might be an electrical outlet, window, door, piping, etc. So if there is a door or window to the room, the institution must make sure that it offers the same level of protection as the wall.

The annual regulatory radiation exposure limit for a member of the general public is 100 mrem. Doors should not be open while the machine is producing x rays (the machine will produce noise even when it is not producing x rays). However, doors can be open when the machine is not producing x rays. Whether there would be exposure in the hallway if the door was open while the machine was operating would depend on how far the machine is from the doorway and what type of procedure was being performed. In general, most x-ray exposures are a fraction of a second long so the potential for exposure is small even for someone who would walk by the open doorway at the exact time the machine is generating x rays. If a person stood in the open doorway for a longer period of time while the machine was producing x rays, he/she could receive some exposure—how large or small would depend on time spent in the doorway and x-ray machine settings.

Is there any risk of radiation exposure when passing an open-doored x-ray room? While there is a small chance of exposure to someone passing by an open door, the level of risk [of biological effects] is negligible. A general rule of thumb is that while standing three feet away from the point of entry of the x rays into the patient, you could receive about 0.01percent of the exposure received by the patient. In addition, any radiation that would exit the door would be scatter radiation from the patient–radiation that has less energy and is less penetrating–so it is even more unlikely that the fetus would be exposed since the mother’s abdominal tissue acts somewhat as a shield and stops this radiation.

How much radiation does a regular diagnostic x ray contain? For the patient, it will vary due to body shapes and sizes. In general, though, let me give some skin exposure numbers for common diagnostic x-ray procedures (this is the radiation exposure of the skin where the x rays are entering; don’t worry about the units mR, mrad or mrem–for your purposes just consider them the same). These are for just one x ray.

A radiation worker is allowed to receive up to 5000 mrem (5 rem) each year as a result of exposure received on the job. In most states, the radiation dose to a fetus of a declared pregnant worker (a radiation worker who has told the Radiation Safety Officer of her pregnancy) must be kept below 500 mrem (0.5 rem) during gestation. It is also important to note that low levels of ionizing radiation are not foreign to our bodies. Each year the average U.S. citizen receives an exposure of approximately 300 mrem (0.3 rem) from natural radiation sources. If you have concerns regarding the situation in your specific facility, you might try to contact your facility’s Radiation Safety Officer.

Kelly Classic
Certified Medical Health Physicist