Answer to Question #10330 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

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


I purchased a Radiation Alert Inspector Geiger Counter and have been checking my background levels in my home. I just tested a bag of Maine coast sea vegetables—kelp, wild atlantic kombu—and the background was 40 counts per minute (CPM) or 0.11 µSv h-1 while a 10-minute reading over the sea vegetables was around 78 CPM or 0.19 µSv h-1. I could not find anything that says what a level of alarm or concern is. Could someone either guide me to the Idiot's Guide to Geiger Counter Reading or let me know what I am dealing with?


Geiger Mueller (GM) detectors are commonly used for routine measurements in a variety of settings. Using the instrument for assessing typical background levels and for looking for deviations from the normal background is a relatively common endeavor and has increased in incidence among members of the public concerned about the potential impact of radioactivity from the Fukushima accident area being transported to the shores of the United States. Usual radiation background readings in the United States vary from about 0.05 Sv h-1 to 0.2 µSv h-1 depending on where the surveyor is located and what materials are in the vicinity. In some other parts of the world where high levels of naturally occurring radioactive materials are close to the surface of the earth normal background radiation levels are much higher than they are in the United States. Individuals living in those areas have suffered no demonstrable adverse health effects associated with the elevated background radiation.

According to information on the website of Maine Coast Sea Vegetables, the company has contracted with the University of Maine to perform radioactivity assessments of its products to ensure that no abnormal radioactivity of concern is in their products from the sea. This caution has grown out of general concern among some people about the possible influence of radioactivity released from the Fukushima site. The University of Maine has reported that only naturally occurring radioactivity has been detected in the sea vegetables.

Seaweeds are among the highest known edible sources of potassium. As you may know, about 0.01% of all naturally occurring potassium is a long-lived radioisotope called potassium-40. It decays with the emission of rather high-energy beta radiation and relatively high-energy gamma radiation. Thus, materials, including foods that contain significant potassium, will exhibit signs of radioactivity. Indeed, depending on the amount of seaweed present, whether it was dry or raw, and how the GM probe was situated with respect to the seaweed, some elevation in the reading above normal background may well have been observed, the reading being attributable to the beta and gamma radiation from potassium-40.

The reading you cite, about two times the normal background level is not of any concern from a health standpoint. If it is associated with the potassium-40, as I expect it is, there should be no concern on your part about ingesting the additional radioactive potassium. Potassium is an essential mineral and maintaining an appropriate systemic level is important to good health. Keep in mind that if your body already has a sufficient level of potassium, when you take in additional potassium, your body will attempt to maintain a suitable equilibrium associated with what is referred to as a state of homeostasis and will excrete any excess potassium not required by the body.

A point to keep in mind when using the GM detector is that most usually such instruments are calibrated with gamma radiation, most often from a defined cesium-137 source, and are adjusted to yield the correct exposure rate (mR h-1). If you use the detector to attempt to measure radiation that might be coming from radionuclides incorporated into foodstuffs or other materials, the radiations emitted and the geometry of the probe relative to the sample will likely not simulate the calibration field conditions, and quantitative results may not be possible. More often the measurements are intended simply to indicate whether significant radioactivity might be present. In order to quantify the amount of specific radionuclides present, more sophisticated laboratory measurements are generally required.

If you want a bit more insight on radiation levels and expected GM responses you might review Question 10281 and its answer on the HPS Ask the Experts website.

George Chabot, 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.
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