Answer to Question #10383 Submitted to "Ask the Experts"

My husband and I are US citizens currently residing in Bangalore, India. Because of my concerns regarding local radiation levels (the presence of rocky soil, quarries, and radon-contaminated borewells; the use of concrete, granite, and marble building materials; and the overall elevation of 900 m above sea level), I recently purchased an Inspector+ Geiger counter to monitor our house. I am hoping to minimize our risks at home, given that we are also exposed to radiation from our frequent international travels (we travel at least 175,000 km per year).

In attempting to familiarize myself with the various measurements, I have read several question-and-answer texts, including ATE Q/A 8910 on this site, and have done research via other online resources as well. However, I am still a bit confused regarding levels of concern for normal background radiation and regarding the methods for computing measurements on my new detector.

Specifically, I've conducted several 10-minute, count-per-minute (cpm) tests indoors that have generally ranged from 50 to 67 cpm (with the exception of our bathrooms and kitchen where rates were much higher), and I have also held the detector near several items in our house, with readings varying from as low as 0.014 mR h^-1 (near furniture) to greater than 0.040 mR h^-1 (near our ceramic sinks, ceramic tiles, and granite countertops). Midrange readings of the air and along our concrete walls showed 0.015 to 0.030 mR h^-1, averaging around 0.02 mR h^-1. The count rates in our bathrooms averaged in the 80–90 cpm range, with some over 100 cpm (and individual spikes more than 120 cpm), and our kitchen’s count rates averaged in the 90s and 100s of counts, at times averaging even 100–120 cpm, with individual spikes more than 130 cpm. Outdoors, along our street, the count rates were generally in the high 50s and low 60s but with individual spikes as high as 100 cpm.

The manual for my detector notes that it has a sensitivity of 3,500 cpm per mR h^-1, referenced to ¹³⁷Cs. It measures 0.001 to 100 mR h^-1, and it measures 0 to 350,000 cpm. I would thus like to confirm how the sensitivity value links the cpm and mR h^-1 readings of my detector. It would seem that if 3,500 cpm = 1 mR h^-1 on my detector, then 65 cpm would equate to around 0.0185 mR h^-1, or 18.5 µR h^-1.

However, while certain references note that count rate is dependent on the detector's calibration, several others seem to indicate that readings over 100 cpm are a safety concern, without regard to exposure rate in mR h^-1 or other measurements. For instance, Radiatio Network seems to indicate that 5 to 60 cpm is normal but that anything exceeding 100 cpm is an “alert level.”

My questions are as follows.

1. Are my computations accurate? I would like to make certain I'm converting the cpm to mR h^-1 properly.
2. Is the count rate totally detector-dependent, or is it accurate to apply a blanket statement that count rates of greater than 100 cpm indicate a radiation exposure risk?
3. If count rate is detector-dependent, then is it accurate to say that my high count rates are due to the sensitivity of the Geiger-Mueller pancake tube in my Inspector+ detector? (For instance, a detector with a less sensitive tube might only be calibrated to 1,000 cpm per mR h^-1, thus resulting in a substantially higher measurement in mR h^-1.)
4. Since my Inspector+ measures alpha, beta, gamma, and x rays in cpm but the measurement in mR is calibrated to the gamma emission from ¹³⁷Cs, is it accurate to translate the result in cpm to mR? (For instance, is it accurate to say that 100 cpm is approximately equal to 0.0285 mR, given that this detector is calibrated to a sensitivity of 3,500 cpm per mR h^-1 for ¹³⁷Cs?)
5. Would the spikes in outdoor count rates generally be due to air particulates and solar/atmospheric changes? Could it also be from living near a contaminated lake or a treatment plant that cleans borewell water and thus might release radon into the air?
6. Are the count rates in our bathrooms and kitchen, which varied between 75 cpm and 120 cpm (approximately 0.021 to 0.034 mR h^-1) and averaged in the 80s to 90s, of concern even though the rest of the house seemed to average around 60 to 67 cpm? Factoring in showering, cooking, and general usage, we probably spend no more than two hours a day in the bathrooms and kitchen, but the rates still seem quite high.
7. Would those bathroom readings likely be due to the ceramic materials used on the walls, flooring, and sink? Might it also be radon released via drains? I could not seem to find any particular item that caused spikes (the readings seemed equally high near the sinks, toilets, wall tile, and so forth), and measuring with the bath fan running reduced the overall count rate only slightly (for example, from 88 cpm to 83 cpm).
8. Assuming a rough average of 57–65 cpm in the main parts of our home, 85–95 cpm in our bathrooms, and 90–110 cpm in our kitchen, are those levels of any health concern by your group’s guidelines? I know it’s relative to many other variables that factor into annual dose, but would you personally consider a house with those levels a healthful environment?
9. How do those averages compute to annual dose (in sieverts per year [Sv y^-1], millisieverts per year [mSv y^-1], or microsieverts per year [µSv y^-1]), in terms of what is generally deemed safe long-term exposure? I saw that the Radiation FAQs portion of your site lists "normal" background radiation as generally being under 0.1 µSv/hr, and ours is considerably higher than that (assuming 0.04 mR h^-1 equates to 0.4 µSv h^-1), so I am wondering whether we fall, statistically, into normal rates for our yearly dose, given the fact that the 1 to 1.5 mSv y^-1 dosage is apparently in addition to normal background radiation levels. The Nuclear Regulatory Commission (NRC) guidelines seem to vary, saying that the average US dose, from all sources, can be up to 6.2 mSv y^-1 but then also noting, on the personal dosage calculator, that the average dose from all sources is 3.5 mSv y^-1. I read an Idaho State University (ISU) text that better explained it, describing how modern-day medical procedures have upped the amount to around 6.0 mSv y^-1, and I saw an HPS statement saying that anything below a lifetime maximum of 0.1 Sv over background rates is not generally considered a health hazard, but what background rate is that based on? In that regard, what is the annual "safe" dose for an average person (not working in hazardous/radiation-related fields)? The NRC calculator page, above, notes that 2.0 mSv generally come from air, 0.4 mSv come from food, etc. The NRC calculator page also says: "The annual average dose per person from all natural and man-made sources is about 3.5 mSv, but it is not uncommon for any of us to receive more than that in a given year (largely due to medical procedures). Consequently, to protect health and safety, the NRC has established standards that allow exposures of up to 50 mSv per year for those who work with and around radioactive material, and 1 mSv per year for members of the public (in addition to the radiation we receive from natural background sources)." So does that mean 3.5 mSv + 1 mSv, not inclusive of the public limit of 1 mSv? And how do I ascertain how much of that 3.5 mSv is assigned to the rate applied for a household? For instance, say my house averages 0.02 mR h^-1; does that include the terrestrial and cosmic radiation noted on the ISU list? I'm basically trying to understand how my house's rate is affecting my annual yearly.
10. Since I know that my bathrooms and kitchen have the highest ratings in our home, should I consider storing our foodstuffs, medicines, and other comestibles outside of those rooms or hanging our towels, robes, and clothing elsewhere? The rates inside my bathroom storage cabinets seemed to average lower count rates than the open parts of the bathroom.
11. Is there anything I can coat our tiles with (such as heavy lead-based paint), to lower their emissions?
12. Is there a reliable data source indicating what the average background radiation rate is in India (or more specifically, Bangalore), and given the above, how can I best estimate my annual average total dose including all natural and man-made sources? I've read that parts of Kerala, India (the state below ours, which is Karnataka), have extremely high terrestrial rates, and I assume parts of Karnataka may have issues too.

We have lived in this house for four years, and we just found out that we will likely spend another two years in Bangalore. We have the ability to move to another house, if required, so I want to assess the need to move by seeking your input. Please provide your honest take on whether you'd seek another home, if it were your family involved.

I have read your introductory statement and your questions, and it is clear that you have a number of concerns and have been diligent in doing some homework in attempting to answer your questions. I should start out by making a few observations that may provide some useful perspective for you.

First, the area of India that you are in has been generally surveyed by various individuals. One of these researchers is Shiva Prasad who, along with professional colleagues, made a variety of measurements of naturally occurring radioactive ²²⁶Ra, ²³²Th, and ⁴⁰K in soils of the Bangalore region and measurements of external gamma radiation dose rates. Their results showed generally that average levels in this region were pretty much the same as levels in other parts of “normal-background” India as well as the rest of the world. The average gamma dose rate was 117 nGy h^-1, which is equivalent to 0.0117 mR h^-1. Their work was published in the journal Health Physics (Shiva Prasad NG, Nagaiah N, Ashok GV, Karunakara N. Concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in the soils of Bangalore Region, India. Health Physics 94(3):264-271; 2008).

Second, there have also been studies of radon and thoron in air in Bangalore. One such study was by Satish et al. in which they measured radon and thoron levels in 10 different sections of Bangalore (about 15 houses in each area). Their findings showed results substantially the same as the worldwide averages, with no excess hazard to the populace. Naturally, individual radon and thoron levels can vary markedly with specific location and home construction details, but the results tend to confirm that the area does not have any special problems.

Third, as you have noted, there are areas of India, particularly the state of Kerala, where radioactivity in the earth leads to much elevated radiation levels compared to the norm. The average external radiation levels in Kerala are five to 10 times higher than is typical for the rest of India and the world. The important point to note is that, despite these elevated levels in Kerala and other anomalous locations in the world, no excess of health defects have been observed.

I point out these facts only to reinforce the notion that, based on the information you have provided, I do not believe you should have any concerns about possible negative health influences associated with your present living environment. I shall now attempt to address each of the specific questions you have asked.

1. Assuming that the calibration of the detector is appropriate for the measurements being made, your computations are indeed accurate. As long as the dose rates are not excessively high, the count rate varies linearly with the dose rate, as you have properly assumed.

2 and 3. Your inference is correct that the relationship between count rate and radiation field intensity does depend strongly on the specific detector characteristics. Unless we know the characteristics of the particular detector, it is not appropriate to state that a count rate in excess of 100 cpm is a concern. The relationship between count rate and dose rate then varies with detector characteristics and another detector may show a considerably reduced response compared to yours.

4. If you are using the detector to measure gamma radiation dose rates (or count rates) from natural sources, then the energy response characteristics of the detector are such that you would expect to measure approximately the correct values. You do need to exercise some caution if you are measuring radiation levels with the thin window of the detector in proximity to some surfaces that contain certain radioactive materials. In the home this might be a concern if you are using the instrument to measure levels at the surface of a granite countertop or at the surface of some ceramic tiles. In such instances it is possible for radiations other than gamma radiation to affect the response of the detector; naturally occurring radioactive species may emit alpha and beta radiations and, at the surface of the material, such radiation could enter through the thin window of the detector and produce a reading in excess of what the gamma radiation contributes. If the detector has been calibrated against gamma radiation from ¹³⁷Cs, such excess readings from these additional radiations cannot be interpreted using the same calibration factor that is used for the gamma radiation. For generalized area measurements, not close to a possibly radioactive surface, your conversion of 100 cpm to 0.0285 mR h^-1 is appropriate.

5. It is unlikely that any spikes that you are observing are due to radioactive particulates in the air. Any such particle that was close enough to the detector to produce an elevated reading would very likely attach to the detector face and yield a consistently higher reading. Also, I would be hard-pressed to identify any source of such hot particles in your area. It is possible that occasional short-term spikes could be associated with cosmic ray bursts. It is also possible that nonionizing radiation sources that emit radiofrequency (RF) radiation (some appliances, motors, switches, etc.) may induce false readings on the detector. Any radon releases from a nearby treatment plant would not be expected to produce spikes in readings because any releases would get dispersed in both space and time so that if such releases occurred they would tend to produce readings that might increase somewhat for some interval of time and then decline but would not likely appear as a real spike over a short interval.

6. The readings in your bathroom and kitchen may be a bit higher than average outdoor readings, but they are certainly not high enough to warrant any health concerns or any remedial action. The rates are somewhat higher than those in other areas of the house but not any higher than those often observed in many dwellings and other buildings in the United States. They are appreciably lower than natural levels in many other parts of the world, especially given the low occupancy time for the areas.

7. The somewhat elevated readings could indeed be associated with the building materials. Ceramic tiles have often been identified with higher concentrations of naturally occurring materials compared to materials that contain no minerals. It is unlikely that radon would produce the elevated readings, especially since the elevated readings seem confined to one or two areas, but if you haven't made radon measurements in your home, I would recommend doing them. The cost is quite small and, in my opinion, it is one of the possible radioprotective measures that is worthwhile for the general public. You should be able to find service providers on the Internet.

8 and 9. The levels that you cite are not a health problem and should not engender concern on your part. Keep in mind that in the United States for example, the average annual dose from natural background sources, including radon, is about 3.0 mSv (equivalent to 300 mR if all were gamma radiation induced). On average, an individual gets about an equal amount of radiation from medical procedures using ionizing radiation (mostly diagnostic) so that the average individual total annual dose is about 6.0 mSv. If such a dose were all delivered externally by gamma radiation it would represent an exposure rate of about 0.069 mR h^-1, and this has not been associated with any negative health effects. I would have no personal qualms about living in your house. The values in your house, assuming they represent gamma exposure rates, are on the higher side of the usual range of values that run from perhaps 0.005 to 0.02 mR h^-1 that we might expect, but they do not represent a problem. Your readings do include contributions from both terrestrial radiation and cosmic radiation.

You are correct in assuming that 1 mSv is the value recommended as an allowed added annual dose to nonoccupationally exposed individuals, beyond the normal background. Note further that this 1 mSv is assumed to be incidental dose resulting from operations involving licensed radioactive materials, and it does not include any dose to an individual from prescribed medical diagnostic or therapeutic procedures that might be received during the year. The 6.2 mSv y^-1 that you refer to is not an NRC guideline; it was simply meant as an observation that the average individual in the United States receives about that amount of exposure with about half from typical background and half from medical procedures, neither of which are dose-controlled by the US NRC. There are small additional doses from consumer products and other sources.

The second NRC value of 3.5 mSv y^-1 that you refer to is, I believe, based on older data that has not been updated on the NRC page. It assumes a lower contribution from medical procedures that applied prior to a review and publication by the National Council on Radiation Protection and Measurements (NCRP) in its 2009 report, Ionizing Radiation Exposure of the Population of the United States (Report 160).

Your observation regarding the HPS recommendations is correct. The HPS issued a position statement, Radiation Risk in Perspective, that recommends that we should not attempt to assign any risk to annual doses of less than 0.05 Sv or to lifetime doses less than 0.1 Sv. These values make no specific assumption about dose rate. It makes no difference whether the annual 0.05 Sv is accrued in a single second or was spread evenly over the entire year. A similar rationale applies to the 0.1 Sv lifetime doses. These recommendations grew out of observations that no link to deleterious effects had been observed in individuals exposed to short-term acute doses of these magnitudes. Studies of occupationally exposed individuals have not revealed any deleterious effects associated with occupational exposures within recommended limits.

As an aside, there is a large volume of laboratory data that supports the observation that moderate short-term doses of radiation (doses from tens of mSv up to perhaps 0.5 Sv) produce a radioprotective effect that reduces the biological impact of subsequent larger doses (referred to as radiation hormesis). The radiation protection community is aware of such information but primarily because of a lack of human data, has so far chosen not to use it in implementing exposure recommendations for occupational workers or for other members of the public.

All of these considerations lead to confusion among members of the public, as well as among some radiation protection professionals, as to what represents a level of safe radiation exposure. I do believe that most protection specialists, including myself, would agree that chronic radiation annual doses of less than 50 mSv y^-1 pose no significant elevated risk to individuals and are safe. The fact that many thousands of people are exposed to natural levels of this magnitude every year provides added support for this belief. The regulating agencies have taken no action in attempting to limit natural background exposure other than to suggest that we reduce radon exposure in our homes (as recommended by the US Environmental Protection Agency), an exposure that is controllable.

If your readings in your home are representative of actual gamma dose rates, the approximate factor of two by which your levels exceed the average in the United States should not be a health concern, and I would have no reservations about continuing to live in your home.

10. You should not worry about storing your edible goods and clothing in other locations. The dose rates are not high enough to represent any concern from any standpoint.

11. There is no practical coating that you can apply to significantly impact the gamma dose rate. If some of the detector response near the surface of the tiles is associated with particulate radiation (alpha and beta), then covering with paint or other materials could reduce the readings somewhat. Based on the values you have provided, I would not recommend taking any such action.

12. As discussed above in my introductory response, measurements have been made in the Bangalore area (Shiva Prasad et al.), and external radiation levels and radon levels have been found to be more-or-less typical of world-wide average values. Your annual average total dose may be estimated using the readings you have obtained and some of the typical values that have been measured, as cited above. For example, if we assume that you spend 16 hours a day in your home, where the assumed external exposure rate is 0.020 mR h^-1, and 8 hours a day in other locations where the assumed rate is 0.0117 mR h^-1, your annual external dose would be

(0.020 mR h^-1)(16 hr/day) + (0.0117 mR h^-1)(8 hr/day)(365 days y^-1) = 151 mR y^-1

which equates to an external gamma dose of 1.51 mSv y^-1.

If we use the concentrations of Satish et al. (cited above) for radon and thoron, but apply the dose conversion factors from recommendations of the United Nations Scientific Committee on the Effects of Atomic Radiation (2.5 mSv per Bq m^-3 for radon and 2.2 mSv per Bq m^-3 for thoron; these conversion factors yield higher doses than the conversions used by Satish), we would obtain an annual expected dose of 1.31 mSv from radon plus thoron. Using this estimate, we would evaluate your total annual dose from natural sources to be about 2.82 mSv (1.51 mSv + 1.31 mSv). This number is not significantly different from the typical average natural doses received by individuals throughout the world. We cannot estimate your dose from manmade sources without knowledge of your exposure history, most especially any medical tests involving x-rays and/or radioactive materials. Doses from other sources, such as consumer products, amount to only about 0.13 mSv y^-1 in the United States and are probably no higher in India. If your medical exposure is similar to the average in the United States, you could add about 3 mSv y^-1, giving a total annual dose from all sources, natural and manmade, of about 5.95 mSv, again consistent with current averages.

I hope this discussion is helpful in addressing some of your concerns and relieving any anxiety you may be experiencing. I hope you enjoy the remainder of your stay in India.

George Chabot, PhD, CHP