Citizens testing radiation levels is both good and bad. It is good because more people will become familiar with the normal radiation everyone in the world lives with daily. Bad because some will jump to conclusions that may frighten others.
Setting basic standards for testing will help increase the good and decrease the bad. The various detectors that are purchased by private citizens vary greatly. The main differences between detectors will be the biological impact readings. These are generally given in MicroSieverts or Roetgens Equivalent Man (REM). Biological impact depends on the type of radio isotope, whether its radiation is internal or external, how easily the isotope is inhaled or ingest. For example, Uranium 238 is pretty common, is an alpha particle emitter and is not very harmful unless ingested or inhaled. Alpha particles have high energy, but they can be stopped by a sheet of paper, the human skin, even air restricts the distance it can travel significantly. A dose meter would assign a fairly high microSievert reading to a sample of Uranium, but really it would have virtually no biological impact.
Iodine 131 is a beta/gamma emitter with a short half life that can be very harmful, so a microsievert setting for Uranium would underestimate the biological impact. Since radio Iodine is so harmful, dose meters may be calibrated for that harm, so they would over estimate the harm of uranium. Most dose meters are calibrated for Cesium 137 as a compromise, but may have settings for other isotopes like Iodine 131. Without knowing the calibration and calculations used, the microsievert reading is nearly useless. If calibration and calculation are known, it can be invaluable for determining the potential harm of known isotopes.
For the amateur, The counts per minutes is much more useful. To get the most use, you should have a standard method for testing and recording your data. For instance a common natural radiation in the background is Radon gas. Radon has a half life of four days. It can react in rain to form compounds that are solids, so it can be rinsed out of the air and collect in drainage ditch, culverts, and soil. With a half life of only four days, a Citizen Radiation Patrol (CRP) participant can measure a level after a rain at actually determine if the radiation is due to Radon by measuring the same spot over a number of days the same way. This would be a reasonably scientific method of testing.
To make it better, record the type of meter, background level of the area, time of day, weather conditions and an average of more than one test per day of the site in question. Three five minute tests should be enough to determine a reasonable average in counts per minute or second. Recording the microsievert reading as well could provide more information on how conservative you detector is. It should be conservative, read higher, because it is a safety device. Repeating the tests in the same manner over a number of days may give you an indication of the types of radioisotopes present. Radon222 and Iodine131 should be the easiest to isolate since they have half lives of 4 and 8 days respectively. Other isotopes with longer half lives would require more complex test equipment, but the CRP can gather pretty good information if they use a standardized test method.
Food testing can also be done with reasonable accuracy, provided natural levels in food are considered. I found a couple of good references for natural radiation in foods, but to simplify, 125 Becquerel per kilogram is good average to expect. Since the average radiation detector cannot test the whole one kilogram mass, a small amount, approximate 1 gram can be tested which should produce 125/1000 (0.125)becquerel or counts per second. Since there is normal background radiation in the air, your meter may measure virtually nothing in the food sample. Then again it may show a few counts above background for perfectly normal food. A much higher reading is what to expect if the food is contaminated. Since 10 to 15 counts per minute is a typical background level, food with more than 30 counts per minute (0.5 counts per second)may be suspect, but over 60 counts per minute ( 1 counts per second)would indicate significant contamination. That does not mean the food would exceed safe limits, only that it has more than just natural radiation. It takes a very strict method to produce repeatable results. Considering the numbers and limitations, four times normal background is a good indication of other than natural radiation, anything less is a maybe.
With high quality equipment and proper test procedure, one gram of a food item with the 500 Becquerel per kilogram upper limit would measure 0.5 counts per second or 30 counts per minute. Natural radiation levels would produce about 7.5 counts per minute. With normal background which should be subtracted, 17.5 to 20 counts per minute may be perfectly normal.
With background measurement the same should apply. Twice normal background is not unusual, four time normal background is an indication of significant contamination.
Even with readings that indicate significant contamination, that does not mean unsafe conditions. Japan like most countries has areas with higher natural radiation. There are natural springs high in radiation. So there may be other areas with higher than normal background levels. This leads to a good deal of confusion. Man made radioisotopes are assumed to be more dangerous. Some are and some are not. Radon, which is a decay product of radium is the prime example.
"222Rn belongs to the radium and uranium-238 decay chain, and has a half-life of 3.8235 days. Its four first products (excluding marginal decay schemes) are very short-lived, meaning that the corresponding disintegrations are indicative of the initial radon distribution. Its decay goes through the following sequence:[20]
218Po, 3.10 minutes, alpha decaying to...
214Pb, 26.8 minutes, beta decaying to...
214Bi, 19.9 minutes, beta decaying to...
214Po, 0.1643 ms, alpha decaying to...
At the next step, 214Po decays to 210Pb, which has a much longer half-life of 22.3 years. Its progenies are:
210Bi, 5.013 days, beta decaying to...
210Po, 138.376 days, alpha decaying to...
206Pb, stable."
From Wikipedia, Radon has a 50% chance of decaying to 210Pb (unstable lead) in the sequence above in 4 days. Then alpha decays, where the atomic weight drops by 4, have an average energy of 5,000 KeV (thousand electron Volts) and the beta decays (elemental change with the same weight) release an average energy of roughly 1,000 KeV. So the total energy from 222Radon is about 8,000 KeV. From Radium the total energy would be about 13,000 KeV, (I had a typo in the Plutonium post). With a possibility of another 11,000 KeV to stable 206Pb from 214Pb with a 22.3 year half life.
Plutonium 239 for example, considered the most dangerous man made isotope, has about a 24,000 year have life and alpha decays to Uranium 235 which has a half life of 700 million years. Energy wise, radon 222 is just as harmful if not more so.
Do double check my numbers, but I think you will find radiation deserves respect but not fear. Everything I have read supports the limits imposed by different countries for safety with a few overly conservation limits that could be relaxed.
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