The levels of background radiation we quote come from a variety of sources, as described in more detail on our radiation 101 page, or from external references (Background Radiation Wiki). Note that the numbers we quote for regions people inhabit refer to natural sources and do not include exposure resulting from travel, medical exposure, or occupation. These numbers are typically quoted in mSv/yr rather than µSv/hr. On the rates shown on the DoseNet main page, we have made the conversion to µSv/hr to demonstrate the corresponding average hourly exposure for more direct comparisons with the data shown across our network. However, it is important to understand that hourly levels can vary dramatically while remaining well within these yearly averages, and risk from exposure at these levels is only meaningful on the year(s) time scale. This is why the occupational limits we include are set as yearly rates.
Regionally, levels can vary dramatically as well. For example, the average exposure level across the United States is ~3mSv/yr, which corresponds to roughly 0.34 µSv/hr, and is more than twice the levels generally seen in the Bay Area. (Check out our measurements!) While in other areas, like the Colorado plateau, radiation levels are closer to twice the national average.
For reference, we've also included the average exposure rate for flight crews, as an example of the increased exposure that can result from spending extended periods of time at high altitudes, where there is less atmosphere to shield cosmic radiation. It is also important to note that that level still falls well below the upper limit for occupational exposure set by the International Commission on Radiological Protection, discussed in much more detail here. Though we have stated these numbers in µSv/hr, these limits are set for yearly exposure. Hourly rates that go above these limits temporarily for hours or even days typically do not represent a health risk.
High background regions
We've also included several examples of areas around the world that are inhabited where background radiation levels are significantly higher than the world average. Areas of Iran, India and Europe, for example, have uncommonly high levels of naturally occuring terrestrial radiation, ranging from 30-40 mSv/yr to over 100 mSv/yr.
Beaches and hot springs
It is not uncommon for beaches to have higher than average levels of radioactivity, depending on the types of sand present. For example, there is a coastal town in Brazil, Guarapari, with beaches that have extremely high levels of natural radiation. Exposure in some areas can be as high as 175 mSv per year (20μSv/h), though the average exposure for residents of the town are of course much lower. In fact, the highest level of natural background radiation on record (800mSv/yr) is on another Brazilian beach.
Though the localized radiation levels in some of the beaches and hot springs already mentioned are among the highest natural levels measured, those are not places people spend extended periods of time, so the average radiation level is much lower than the level at the beach or hot spring. However, for residents in Ramsar, in northern Iran, the level is much higher. One reason for this is the use of the naturally radioactive limestone found in that area as a building material. There are on-going investigations of whether long-term exposure to background levels this high have adverse health effects, such as increased cancer rates amoung residents of this area, but thus far no measurable effects have been found.
These are just a few examples of the range of radiation levels people around the world are exposed to, without measurable adverse effects. You can find a variety of other examples of various levels of exposure
here. More information about the range of natural and occupational exposure levels, as well as the various limits set, can be found here.