All living things are exposed to natural ionising radiation from the environment. We normally do not think about this exposure to radiation because it comes from things we accept in our everyday lives. Although there is worldwide variation in levels, we have always been, and will always be, exposed to background radiation from natural sources. In Slovenia, each of us receives an average dose of 5 mSv per year from a typical Slovenian lifestyle. Globally, this ranges from 1 to 13 mSv per year.
What are the sources?
Rocks and soil
Most of us have heard of uranium, but little about thorium. These two elements have occurred naturally on our planet at varying concentrations since the formation of the Earth. We are familiar with uranium mines where concentrations occur naturally at higher levels; however, small amounts of uranium and thorium are present everywhere. These elements undergo radioactive decay, producing more radioactive elements until the chain of decays leads to a final element that is not radioactive. Another major contribution is from radioactive potassium-40. Potassium is an essential life-sustaining element, but a small fraction of potassium is naturally radioactive. It has also existed since the formation of the Earth. Rocks containing these elements lead to an exposure, just by being near them. Further, rocks and stones are also building materials. Rocks exposed to weather and water erode over time. This causes these elements to dissolve, become part of the soil, and be transferred into water in lakes, rivers, and oceans.
Food and water
Food grown in soil, animals that live in water and animals that feed on plants and other animals all take in these elements as part of living in their environment. It follows naturally that all human food sources, including drinking water, result in us ingesting these elements when we eat.
Radon
Radon is a naturally occurring radioactive gas, which has no smell, colour or taste. It comes from the radioactive decay of uranium and thorium, which are present in small amounts in rocks and soil. Radon is chemically inert, so it can easily escape from the ground into the air, where it can be inhaled. Every breath we take exposes us to a small amount of radon gas.
Space
The sun, like all stars, is essentially a gigantic nuclear reactor, changing one element into another as it burns its fuel to supply the heat and light needed to sustain life. This process produces a large amount of ionising radiation, known as cosmic radiation. The Earth’s atmosphere shields us from most of this cosmic radiation. Exposure to the small amount that reaches the surface varies primarily with altitude and latitude. In other words, if you live high in the mountains and closer to the north or south poles, your exposure will be higher than at sea level near the equator. The highest exposure to cosmic radiation occurs during flights.
Humans’ exposure to ionising radiation
| Source of exposure | Exposure per year (mSv) |
|---|---|
| One CT (computed tomography) scan to the chest | 5 |
| Radon exposure | 5 |
| Total natural radiation in Slovenia | 1.7 |
| Slovenian cave guide | 1 |
| One return flight from Ljubljana to New York a year (passenger) | 0.11 |
| One chest X-ray (two views) a year | 0.06 |
| Nuclear fallout (from atmospheric tests in the 1950s and 1960s) | 0.0 |
World average of natural background ionising radiation
| Source of exposure | Exposure per year (mSv) |
|---|---|
| Inhalation (radon gas) | 0.2-10 |
| External terrestrial | 0.3-1 |
| Ingestion | 0.2-1 |
| Cosmic radiation | 0.3-1 |
| Total natural | 1-13 |