Answer to Question #5554 Submitted to "Ask the Experts"

Do external dose rates in an area (or ambient gamma dose rates in an area) fluctuate with time? If so, what can be the possible causes and what are safe limits for these dose rates? Where can I find data over a period of time showing these dose rates?

External radiation dose rates will always vary, even on different time scales. Let me give you some examples:

Radiation is emitted by natural uranium, thorium, and potassium in the rocks and soils. This radiation is emitted randomly, so at any one time you might get more or less emitted (higher or lower dose). If you take a radiation meter into the field, you can see these fluctuations from second to second, although they will average out to something fairly steady over longer time spans. Also, every time there is a volcanic eruption, a flood, or any other event that changes the mixture of rocks and soils in a particular place, radiation dose from the new rocks and soils will change, even if only very slightly. Over longer spans of time (millions to billions of years), the dose from geologic emitters has changed as well because the earth's crust has changed chemically with time—there is now much more uranium, thorium, and potassium in the crust than there was when the earth was first formed, while radioactive decay has reduced their overall radioactivity. This also affects the amount of radon in the air because radon comes from the radioactive decay of uranium in the soil—higher levels of uranium produce higher levels of radon.
 
That's the big picture. This so-called terrestrial radiation is not constant over the surface of the earth, but is related to how much natural radioactivity there is in the soil. Thus, in the Rocky Mountain plateau, levels will be greater than in Mississippi or over water, which contains relatively little natural radioactivity. Radon concentrations in air are determined not only by the amount of radium in the soil, but also by the type of soil, atmospheric conditions, and several other factors, and may vary by as much as 100-fold.
 
Radiation is also emitted by the sun, and cosmic radiation from outside the solar system strikes our planet all the time. This varies by the hour, throughout the year, and through the solar cycle. When the sun is down, for example, we lose that input to radiation dose. When the earth is farther from the sun (in the summer, believe it or not), solar dose is also somewhat lower. And, during the solar minimum, solar radiation is lower (although galactic cosmic rays increase). Over the history of the earth, the sun has become brighter and hotter, but less active, while our atmosphere has become richer in oxygen and ozone, but somewhat thinner in total pressure. As a result, ultraviolet radiation is lower now than in the distant past, as is cosmic radiation exposure. Cosmic-ray radiation levels are relatively constant over the surface of the earth, varying a few percent with latitude, and considerably more so with altitude, roughly doubling every 5,000 feet.
 
The other place that natural radiation comes from is internal radioactivity (primarily potassium) in our own bodies. This remains largely constant from day to day and from year to year, and even over millions of years. However, going back a few billion years, potassium activity in the earth has changed (dropped) by a factor of nearly 10.
 
So, there is nothing constant about our exposure to radiation on either the very long or very short time spans. However, even with this degree of change, we can still talk accurately about average (or typical) background radiation levels. In general, the background radiation levels are going to remain largely steady because the fluctuations I mentioned above are either too short-lived or happen over too grand a timescale to really affect us or they are too small-scale to really worry about. Which means that, the first few paragraphs not withstanding, we can treat background radiation levels as though they are constant because the fluctuations we see really do not have any impact on our health.
 
P. Andrew Karam, PhD, CHP