Radiation comes from everywhere. Most of it is natural, produced by radioactive elements in the ground, cosmic rays from space, and even isotopes inside your own body. The average person in the United States absorbs about 6.2 millisieverts (mSv) of radiation per year, split roughly in half between natural background sources and man-made sources like medical imaging.
Radioactive Elements in the Ground
The Earth’s crust contains uranium, thorium, and potassium-40, all of which are naturally radioactive. These elements have been present since the planet formed, and they continuously release energy as they decay. The gamma radiation they emit varies dramatically by location, from nearly zero in some areas to over 100 nanograys per hour in regions with uranium-rich or thorium-rich geology. Granite, for instance, tends to be more radioactive than sandstone or limestone.
The most significant ground-based source for most people is radon, a colorless, odorless gas produced when uranium and thorium in soil and rock break down. Radon seeps upward through the ground and enters buildings through cracks in foundations, floors, and walls. Once inside, it gets trapped and accumulates, especially in basements and crawl spaces. Radon is the second leading cause of lung cancer after smoking, which is why home testing kits exist and why some regions require radon mitigation systems in new construction.
Cosmic Radiation From Space
High-energy particles constantly stream toward Earth from the sun and from distant sources across the galaxy. The atmosphere and Earth’s magnetic field block most of this cosmic radiation before it reaches ground level, but some gets through. The higher you go in altitude, the less atmosphere sits above you, and the more cosmic radiation you absorb.
This matters most for airline crews and frequent flyers. Flights at high altitudes, high latitudes, and polar routes expose passengers to noticeably more cosmic radiation than flights closer to the equator at lower altitudes. A NIOSH study found that flight attendants exposed to 0.36 mSv or more of cosmic radiation during the first trimester of pregnancy faced an increased risk of miscarriage. The FAA maintains an online tool (the CARI program) that lets you estimate your cosmic radiation dose for any specific flight route.
People living at high elevations, like Denver or Mexico City, also receive a slightly higher annual dose than people at sea level, simply because there’s less air shielding them from cosmic rays.
Radiation Inside Your Own Body
Your body is mildly radioactive right now. Every time you eat or drink, you take in trace amounts of naturally radioactive isotopes that behave chemically like the nutrients your body needs. Potassium-40 is the biggest contributor. Your body absorbs it at the same rate as regular potassium, around 80 to 100 percent efficiency, because it can’t tell the difference. Bananas contain about 130 becquerels per kilogram of potassium-40, nuts around 207, and carrots about 126. Carbon-14 and tritium (a form of hydrogen) also enter your body through food, water, and air, becoming part of your organic tissues and body water.
This internal radiation is a small but constant part of your annual dose. Your body is always replacing these isotopes as old ones decay and new ones arrive through your diet, so the level stays roughly stable.
Medical Imaging
Medical procedures now account for nearly half of the average American’s annual radiation exposure, at about 3 mSv per year. The doses vary enormously depending on the type of scan. A standard chest X-ray delivers just 0.02 mSv, which is negligible. A two-view chest X-ray bumps that to 0.1 mSv. But CT scans are in a different category entirely: a head CT delivers about 2 mSv, a chest CT about 6.1 mSv, and an abdominal/pelvic CT around 7.7 mSv. A PET scan without an accompanying CT delivers roughly 7 mSv.
To put that in perspective, a single abdominal CT scan gives you more radiation than an entire year of natural background exposure. This doesn’t mean CT scans are dangerous when medically necessary, but it’s why doctors weigh the diagnostic benefit against the dose, particularly for children or patients who need repeated imaging.
Consumer Products
Several everyday items contain small amounts of radioactive material. Most household smoke detectors use americium-241 to detect smoke particles. Some illuminated EXIT signs contain tritium gas, which glows without electricity or batteries. Certain luminous watches and clocks use tritium or promethium-147 on their dials, and older models made before 1970 may contain radium-226 paint.
Antique and vintage items can be surprisingly radioactive. Camera lenses from the 1950s through 1970s sometimes incorporated thorium into the glass for better optical properties. Pre-1972 Fiesta dinnerware used uranium-based glazes that can exceed normal background radiation levels. Yellow or greenish antique glassware, sometimes called vaseline glass, often contains detectable quantities of uranium. Even commercial fertilizers can be measurably radioactive because they contain potassium and phosphorous derived from uranium-bearing phosphate ore.
The radiation from these products contributes only about 0.1 mSv to the average person’s annual dose, a tiny fraction of the total.
Nuclear and Coal Power Plants
Nuclear power plants release far less radiation to the surrounding public than most people assume. During normal operation, a nuclear plant exposes nearby residents to roughly 0.0003 mSv per year from airborne emissions. That’s thousands of times below the regulatory limit and far below what you’d absorb from a single chest X-ray.
Coal-fired power plants actually expose surrounding communities to more radiation than nuclear plants do. Burning coal concentrates naturally occurring uranium and thorium from the coal into fly ash, which gets released into the air. Research comparing the two found that the effective external radiation dose from a coal plant was about 40 times higher than from a nuclear plant. Both remain well within regulatory safety limits, but the comparison surprises people who associate radiation exclusively with nuclear energy.
Non-Ionizing vs. Ionizing Radiation
Everything discussed so far involves ionizing radiation, the kind with enough energy to strip electrons from atoms and potentially damage DNA. But there’s a whole other category called non-ionizing radiation that comes from sources you interact with constantly: cell phones, Wi-Fi routers, microwave ovens, power lines, and visible light. These all sit on the lower-energy end of the electromagnetic spectrum.
The dividing line between the two falls in the ultraviolet range. Everything at ultraviolet energy and below (radio waves, microwaves, infrared, visible light) is non-ionizing. Everything above (X-rays, gamma rays) is ionizing. Non-ionizing radiation doesn’t have enough energy to damage molecules the way ionizing radiation does, though intense, direct exposure to radiofrequency or microwave radiation can cause tissue heating. Ultraviolet radiation from sunlight and tanning beds sits right at the boundary and can damage skin cells, which is why it causes sunburn and increases skin cancer risk.
How Annual Exposure Adds Up
For the average American, the 6.2 mSv annual total breaks down to about 3.1 mSv from natural background sources (radon, cosmic rays, terrestrial radiation, and internal isotopes) and roughly 3 mSv from medical exposures. Consumer products contribute about 0.1 mSv. Your individual number could be significantly higher or lower depending on where you live, how often you fly, and whether you’ve had any imaging done recently.
For context, the international guideline for occupational exposure (people who work with radiation) caps the dose at 20 mSv per year averaged over five years, with no single year exceeding 50 mSv. The average person’s exposure sits well below those thresholds, though anyone receiving multiple CT scans in a year can approach or exceed the occupational average.