By Dr. P. Andrew Karam
People are bad at evaluating risks—especially from artificial, exotic, or invisible sources. When it comes to radiation, most people are not only bad at evaluating risks, but can be irrational to the point of being phobic about them. Interestingly, many of those who are frightened of radiation do understand that it’s part of the environment—they’re just scared of artificial radiation, mistakenly thinking that it’s somehow different from natural radiation. One of my colleagues at the Ohio Department of Health took a phone call from a worried citizen in the early 1990s and tried to explain that the radiation exposure they were concerned about (from Ohio’s uranium enrichment plant) was actually less than the radiation from a cross-country flight. “Yeah, but that’s that good cosmetic radiation” was the caller’s answer, to which my colleague simply replied with a sigh.
The fact is, we cannot escape radiation and radioactivity. Every organism that has ever lived on Earth (and probably the rest of the universe) has been exposed to radiation, and for most of geologic history the radiation levels have been higher than they are today. The obvious ones are the natural sources, although these aren’t quite as well understood, even by radiation and nuclear professionals, as one might hope (more on this shortly). But we’re also exposed to a fair amount of radiation from artificial sources—obvious ones such as medical x-rays, but also a number of sources that we might not have considered. So let’s go through some of these, and then perhaps a snapshot of my typical day with radiation.
We can start with the cosmic radiation that our caller didn’t mind receiving. We can’t get away from this—even at night—because the major component is galactic cosmic rays—highly energetic particles accelerated to high energies by supernovae as well as by forces we still don’t quite understand. The sun makes a bit of a contribution as well, but not much until we get to a reasonably high altitude. While solar cosmic ray exposure waxes and wanes with the solar cycle, overall cosmic radiation exposure remains fairly constant from year-to-year because, when solar activity is high, the solar wind helps sweep galactic cosmic rays from the inner solar system. In addition to increasing at higher altitudes (or higher elevations for those living in the mountains), they also increase with rising geomagnetic latitude—those living in the far north or south will receive about 15 percent more cosmic radiation exposure than those living near the equator. Incidentally, it’s thought that Earth is approaching a magnetic field reversal, which will include some time with a near-zero magnetic field strength. But lest you worry that this might prove fatal to life on Earth, never fear—life has survived dozens of reversals and will most likely survive the impending one. Cosmic radiation exposes us to between 0.2-0.3 mSv (20-30 mrem) annually.
We’re also exposed to radiation from geologic materials—almost entirely potassium (K-40) as well as uranium, thorium, and their decay series nuclides. Some clays—illite is a prime example—contain a fairly high concentration of potassium (several percent by weight) and anything made from these clays (bricks and concrete for example) will have elevated radiation levels. Some rocks contain not only potassium, but higher concentrations of uranium and thorium (and decay series nuclides) too. As a general rule of thumb, dark sedimentary rocks (coal, black shales, etc.) tend to be richer in uranium than lighter-colored rocks—this is because uranium is insoluble in the oxygen-deficient conditions that lead to the formation of these rocks. Igneous rocks, on the other hand, tend to be richer in radioactivity when they’re lighter-colored (red, pink, gray granites for example). This is because uranium, thorium, and potassium are large ions that are preferentially partitioned into the magma that eventually forms granites and other light rocks. As an aside—black “granite” is not what a geologist would call a granite and it tends to be bereft in radioactivity. All in all, radiation from geologic sources (rocks, soil, and things made of rocks and soils) exposes us to about the same amount of radiation as do cosmic rays. They’re the gift that keeps on giving, since radon (specifically Rn-222) is one of the U-238 decay progeny. Higher levels of uranium in the bedrock and soils leads to increased levels of radon emanation as well. On average, we are exposed to about 2 mSv (200 mrem) from radon in an average year.
The last source of natural radiation comes from radionuclides in our own bodies—mostly potassium, but also traces of tritium and carbon-14 (both formed by cosmic ray interactions in the atmosphere) as well as uranium and thorium (from dust we inhale or ingest). This comes out to about 0.4 mSv (40 mrem) annually. Put this together with the other sources of natural radiation and we pick up about 3 mSv (300 mrem) annually from natural sources.
In addition to natural radiation there are artificial sources of radiation which are primarily medical in nature, contributing about 3 mSv (300 mrem) each year. But this number is an average that is not really descriptive of anyone. People who are ill and possibly receiving multiple CT scans, nuclear medicine, x-rays, fluoroscopy, and/or radiation therapy are likely to be receiving a lot more than 3 mSv a year. A single whole-body CT scan can give you 10 mSv (1 rem) or more. People who are healthy and have none of the aforementioned have no exposure at all. So the average number, again, doesn’t really describe any real person.
After medical radiation, the rest of our annual background exposure is pretty small. Some industrial materials and consumer products contain radioactivity—smoke detectors, static eliminators, glossy magazines, cathode ray-type TV and computer monitors, some type of high-quality optical glass, jet turbine blades, some dentures, and much more. But these are a minor source of radiation (less than 0.1 mSv or 10 mrem annually). And with regards to smoke detectors, PLEASE don’t get rid of yours, because the radiation dose is trivial. Other sources of radiation (nuclear energy, fallout from nuclear weapons testing, etc.) are actually fairly trivial—they’re there, but account for only a few percent of our annual exposure.
Okay, so with this as a backdrop, let’s go through one of my days to see where I encounter radiation. I should say that my experience isn’t typical—or, rather, my exposure is fairly typical (except for my calibration laboratory), but my instruments are exceptional so I’m more aware of radiation in daily life than most. In case you’re wondering, I work for a major police department as a rad/nuke scientist in Counterterrorism, so I have some nice detectors! And I should say that everything noted here actually happened to me—although not necessarily during the same flight.
- 0600 Skids up for a radiological survey flight in our helicopter prior to a major event. The instruments use the natural K-40 gamma for energy calibration—we lift off and move the helicopter to a natural hot spot on the West Pad to help the instruments stabilize more quickly (about 1-2 minutes compared to 10-30 in the air).
- 0630 Readings spike up briefly as we fly over a cemetery (granite headstones).
- 0640 Readings take a major dip as we fly over the harbor (water is a pretty good shield).
- 0655 Spike in rad levels—nuclide ID is Tc-99m—we probably flew over a nuclear medicine patient. Looking at the map, we’re close to a hospital. At this altitude (about 300 feet) the signal strength is consistent with a diagnostic dose.
- 0710 Flew a couple of passes at low altitude along the route we’re concerned about. No spikes, but a few noise complaints. On the other hand, a terrorist attack will be a lot more disruptive, so my sympathy is limited. Besides, increasing our altitude from 300 feet to 1500 feet will reduce our sensitivity by a factor of 25.
- 0725 Heading back to the airfield and got a huge spike (Ir-192). Took a look at the map and had the pilot circle back to the correct intersection. Looked out the window and saw a construction site below—best guess is industrial radiography. But to be safe, called Counterterrorism and they sent a car out to confirm.
- 0740 Running low on fuel and time to head back. Shut down the instruments and reported back to Counterterrorism. No more flights for today, time to head back and link up with a ground team. Background has varied noticeably depending on what we’re flying over, and to a lesser degree based on altitude. At these elevations, dose rate drops off with altitude (due to increasing distance from the ground)—I’ve noticed when I fly commercial planes, dose rate continues to drop until about 10,000–12,000 feet, when the atmosphere is thin enough that cosmic radiation starts to kick in.
- 0910 Made it back to Counterterrorism and grabbed a ride to the event, linked up with a ground team. Got a hit right away, but it seemed odd—Co-60. Looking at the spectrum, there was only one peak—finally realized that the instrument in the vehicle had self-calibrated (on K-40) when it was cold and, as the vehicle warmed up, the detectors’ energy ID drifted—the K-40 peak (1.46 MeV) had drifted down until it looked like the high-energy Co-60 peak. Turned the instrument off and back on again, re-calibrated the energy setting, and everything went back to normal.
- 1220 We get a low-energy “hit” outside a medical clinic—after some investigation we determine it’s leakage from a poorly shielded medical x-ray machine.
- Got a few more nuclear medicine hits—all confirmed as patients.
- Background goes up and down depending on the local architecture—primarily whether or not the buildings are fronted with granite. The hottest spot in the downtown area is in front of a memorial wall built of granite (about double normal levels).
- 1445 Deployment ends and back to base—no hits along the way.
- The rest of the day is pretty normal—but a quick inventory of radioactive objects at work and home is interesting:
- Various sources for instrument checks and calibrations.
- Smoke detectors at work and home.
- Explosive trace detectors (use a Ni-63 source for ion mobility spectrum analysis).
- A variety of “hot” rocks I’ve collected over the years.
- A stainless steel soap dispenser contaminated with Co-60 from an accident in India in 2010.
- Some “Vaseline glass” colored with uranium, a Fiestaware plate colored with uranium.
- One small piece of trinitite (desert sand fused to glass by the first atomic bomb explosion)—emits low levels of alpha and gamma radiation.
- Some thoriated welding electrodes and old gas lantern mantles I keep as a check source for my GM and gamma spec units.
- 200 grams of salt substitute I keep in a plastic container—we Velcro this to the side of the Aviation detectors to help speed up crystal stabilization if there’s no time to warm up the instruments.
I could go on—but you get the idea!
Two last points….
- Variations in natural background radiation levels don’t seem to affect the health of people living in those areas, whether it’s in the United States or overseas. Cancer incidence, rates of birth defects, life spans—all are similar in areas of elevated background radiation levels and in areas where exposure is lower.
- Increasing use of nuclear energy, medical radiation, smoke detectors (and, for that matter, cell phones and high-tension power lines) doesn’t seem to affect cancer rates when we look at society as a whole—age-adjusted cancer incidence rates have dropped steadily over the last half-century, in spite of increases in the use of most of these.
The bottom line is that we can’t escape radiation exposure from any number of sources—we run into it every day from both natural and artificial sources. Radiation is part of our environment.
- Ghiassi-nejad M et al. Very high background radiation areas of Ramsar, Iran: Preliminary biological studies. Health Physics 82(1):87-93. 2002
- Howlader N, Noone AM, Krapcho M, Miller D, Bishop K, Kosary CL, Yu M, Ruhl J, Tatalovich Z, Mariotto A, Lewis DR, Chen HS, Feuer EJ, Cronin KA (eds). SEER Cancer Statistics Review, 1975-2014, National Cancer Institute. Bethesda, MD, https://seer.cancer.gov/csr/1975_2014/, based on November 2016 SEER data submission, posted to the SEER web site, April 2017
- National Council on Radiation Protection and Measurements. Report No. 160: Ionizing Radiation Exposure of the Population of the United States. NCRP, Bethesda MD, 2009
- United Nations Science Committee on the Effects of Atomic Radiation, Report to the General Assembly (Sources and Effects). United Nations, New York City, various publications dates.
- United States Food and Drug Administration. National Evaluation of X-ray Trends. 2014 (last accessed 8/15/2017)
Andrew Karam, Ph.D., CHP, has more than 35 years of experience in radiation safety. He is a member of the Health Physics Society and the German-Swiss Radiation Federation, and is an author of many articles on various aspects of science and radiation safety. He currently works as a rad/nuke counterterrorism scientist in New York City.