Answer to Question #10177 Submitted to "Ask the Experts"

I am concerned about radiation exposure in everyday life and purchased a radiation detector. However, I have no idea when to consider a meter reading as hazardous. The meter beeps and blinks briefly when held close to many objects (like electronics and even paper), but the readings usually don't exceed 20–40 cpm or 0.02 mR h^-1. What do these readings mean? At what meter reading should I be concerned about the hazard or consider an object to be radioactive? For how long should I take a reading?

I am pleased to see that you are interested in the radiation environment that you occupy. Before saying anything about radiation levels, I should first state that if you are using any radiation-measuring instrument for quantitative measurements, as you apparently are, you should be sure that the instrument has been designed for the kinds of measurements that you are intending to do and that the instrument has been properly calibrated.

Generally, when an instrument is purchased new from a manufacturer, it is calibrated by the manufacturer. The kinds of measurements that the instrument is suited for and for which it has been calibrated may vary. For example, a thin-window Geiger-Mueller detector may be calibrated to read exposure rate (e.g., mR h^-1) or dose rate (mSv h^-1) for purposes of assessing external gamma exposure rates or dose rates. If the instrument is going to be used for assessments of the levels of beta-emitting contamination on surfaces (e.g., disintegrations per minute per square centimeter), different calibrations are required. In general, an instrument should be calibrated at least annually. If it receives considerable heavy use/abuse it may require more frequent calibration.

Typical detector readings of ambient radiation exposure rates range from about 0.005 mR h^-1 to 0.02 mR h^-1. At these low dose rates, the instrument may change rapidly and noticeably because of normal statistical fluctuations. The detector value that you cite (around 0.02 mR h^-1 on the usual upper end) is not out of line with expectations. If you move into an area where a significant amount of granite is present, as in many older buildings, you may see the background dose rate change markedly, possibly by a factor of two or more. The fact is that virtually all materials that use any resource from the Earth in their fabrication will contain natural radioactive materials. For most cases, the radiation levels are quite low and not discernible from ambient background levels, but there are exceptions, as in the case of granite. The effect is even measurable in granite that has become popular for surfaces of kitchen counters.

There are several areas throughout the world where the external radiation background levels are much higher than they are in the United States, a number being about 10 times higher and at least one where residents receive annual doses more than 200 times that typical of US residents. These higher background dose rates are associated with unusually high concentrations of natural radioactive materials, especially ⁴⁰K, uranium, and thorium, and their radioactive progeny, that are close to the land surface where people live. Several epidemiological studies have been conducted for some of these high background areas, and none have shown any elevated increase in the incidences of radiation-induced disease.

The point I am trying to make here is that it is extremely unlikely that you will ever encounter a natural radiation level that will present a health hazard to you or others in the area. This is not to say, however, that if you do encounter significantly deviant levels you should dismiss them out of hand. An unexpected reading elevated noticeably beyond your “normal” background level may be cause for some investigation to ensure that an event has not occurred that may have the potential for more serious implications. Because of the natural statistical variations in background, it is generally advisable to select an "alert" value sufficiently high so as to avoid a lot of false-positive conclusions.

For your purposes, I would recommend a sustained reading of about three times your usual background exposure rate as a level that warrants additional investigation. Such would be a level that is not directly hazardous, but one that is far enough away from "normal" to raise suspicions about the source. Note that I have specified a "sustained" reading, and by "sustained" I mean a reading that persists for more than 10 seconds or so. This is important because there can be natural sources that might produce short-term elevations in background readings, depending on where you are. Such can occur at times of unusual solar activity (especially solar minima) that affects the amount of galactic cosmic radiation reaching the Earth.

You should also be aware that many radiation-measuring instruments are sensitive to radiofrequency (RF) radiations as are produced by various electronic sources. The RF radiation is not ionizing radiation (which is what the detector is designed to measure), but it can be picked up by various components used in the instrument and induce currents that produce false readings on the instrument.

You can find considerable information about background radiation on the HPS Ask the Experts website. You can review the topics and questions to see if any appear of interest to you. I hope you find your measurements informative. Good luck.

George Chabot, PhD, CHP