Answer to Question #13272 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Shielding

The following question was answered by an expert in the appropriate field:

Q

My daughter and I were at an urgent care facility recently for a physical, and when we were done, we had to walk by the x-ray room to leave. There were glass windows in this room and the door was opened while the person was inside having an x ray. Were we exposed to radiation from this?

A

The dose from an inadvertent exposure such as you describe, is a small fraction of the daily dose received naturally by everyone from background radiation.

X-ray rooms usually contain material in walls, doors, and windows designed to absorb radiation. This is typically lead or leaded glass but may also be concrete or steel. The purpose of this shielding is to reduce radiation dose to people walking by the room to acceptable levels (see Structural Shielding Design for Medical X-ray Imaging Facilities, NCRP Report 147). The dose allowed is 1 milliSievert (mSv) annually. An urgent care facility may take hundreds, if not thousands, of x rays per year. These procedures typically involve imaging the upper body (chest x ray) or the extremities (hands, feet, legs, and arms). The total radiation dose produced in the hallway outside of the x-ray examination room from all these x rays combined should not exceed 1 mSv per year.

Most of the radiation produced by the x-ray machine is absorbed by the patient or the imaging equipment. A small amount is scattered away from the patient in all directions. For a chest x ray, the patient typically receives a dose of 0.1 mSv from the x-ray machine. At 1 meter from the patient, the scattered radiation dose is 1/1000th of this dose or 0.0001 mSv. The intensity of this scatted radiation falls off as the inverse square of the distance. For example, at two meters the dose drops by a factor of four to 0.000025 mSv, at three meters the dose is 1/9th or 0.000011 mSv. This radiation and a small amount from the x-ray tube itself are what the shielding is designed to absorb to keep the dose below the limit of 1 mSv per year. 

To put this into perspective, natural background sources, such as cosmic, terrestrial, and natural internal sources of radiation cause a dose of approximately 0.01 millisieverts per day. If someone stood in an open doorway two meters away from the patient during a chest x ray, the dose they would receive (0.000025 mSv) would be 1/400th of the dose received from background radiation sources per day.

The dose from an inadvertent exposure such as you describe, is a small fraction of the daily dose received naturally by everyone.

Thomas L. Morgan, PhD, CHP

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 14 January 2020. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.