Escaping radiation entirely is impossible — because the Earth itself is slightly radioactive. The soil under your feet, the air you breathe, the food you eat and the rays from space all give you a small dose every day. This is natural background radiation. The good news: this level is considered harmless for healthy people; but its size surprises most people — it is on a level comparable to medical imaging.
What is background radiation?
Background radiation is the ionizing radiation we are exposed to naturally, without any medical or artificial source. The average natural background in the US is about 3.1 mSv/yr;1 the worldwide average is about 2.4 mSv/yr.2 This varies markedly with region, altitude and lifestyle — for example, in a high-altitude city the cosmic share rises.
Where does it come from?
Natural background comes from several sources, and the biggest share usually comes from an unexpected place: radon gas.
- Radon: a radioactive gas from the decay of uranium in the soil, which can accumulate in basements and lower floors of homes. It is the single largest component of natural background (more than half in many countries).2
- Terrestrial: natural radioactive elements in soil, rocks and building materials (uranium, thorium, potassium-40).
- Cosmic: high-energy rays from space; they increase with altitude (on a mountain, on a plane).
- Internal (food/body): natural radioactive atoms in what we eat and drink — for example potassium-40 in a banana. Our own body is slightly radioactive too.
Compared with imaging
The best way to grasp the size of background radiation is to compare it with familiar exams. A chest radiograph (~0.1 mSv) equals about 12 days of natural background; an abdomen CT (~8 mSv) equals about 2.5 years of background.1 The striking part: the average per-capita medical dose in the US (~3 mSv/yr) is now almost equal to the average natural background.1 That is why justifying and optimizing medical exposure (ALARA) matters — detail in Why Does Dose Matter?.
References
- NCRP Report No. 160. Ionizing Radiation Exposure of the Population of the United States, 2009 — ABD'de ortalama doğal fon radyasyonu ~3,1 mSv/yıl; en büyük pay radon; kişi başı tıbbi doz ~3,0 mSv/yıl (Bushberg s.399 üzerinden). Değerler ortalama/temsilîdir, bölgeye ve yaşam tarzına göre değişir.
- UNSCEAR 2008 Report — dünya genelinde ortalama doğal fon radyasyonu ~2,4 mSv/yıl (radon ~1,3; karasal ~0,5; kozmik ~0,4; içsel ~0,3 mSv). unscear.org
- İlişkili: Radyasyon Nedir? · Radyasyon Birimleri · Doz Neden Önemli?