Millisievert to Sievert

The UNSCEAR figure of 2.4 mSv/year is a population-weighted average across all countries. But the real range is enormous: 1–1.5 mSv in flat coastal cities with low radon, up to 50+ mSv in places like Ramsar, Iran, where naturally occurring radium hot springs push radon levels through the roof. The 2.4 figure is useful as a benchmark — when a doctor says "this CT scan is equivalent to 3 years of background," they mean 3 × 2.4 = 7.2 mSv — but it should not be mistaken for what any specific individual actually receives.

A head CT delivers about 2 mSv; a chest CT about 7 mSv; an abdomen/pelvis CT about 10–20 mSv. For context, the increased cancer risk from a 10 mSv CT is estimated at roughly 1 in 2,000 — compared to the baseline lifetime cancer risk of about 1 in 3. If the scan detects a tumor, blood clot, or appendicitis, the diagnostic benefit massively outweighs that tiny added risk. The concern is not one scan but cumulative dose from repeated scans, particularly in children, who are more radiosensitive and have more years ahead for a potential cancer to develop.

The ICRP recommends 20 mSv/year averaged over 5 years, with no single year exceeding 50 mSv. This limit was derived from epidemiological data on atomic bomb survivors, radium dial painters, and early radiologists — groups whose cancer rates could be correlated with estimated doses. The 20 mSv figure is set so that a worker exposed at the limit for an entire 40-year career (total: ~800 mSv) faces an additional cancer risk of about 3–4% — roughly the same as working in a slightly more hazardous industry. Most workers actually receive well under 5 mSv/year.

Radon-222, a decay product of uranium in soil, seeps into buildings and is inhaled continuously. Its short-lived decay products (Po-218 and Po-214) lodge in lung tissue and blast it with alpha particles. Alpha radiation deposits 20 times more biological damage per unit of energy than gamma rays, which is why the sievert weighting factor for alpha is 20. The global average radon contribution is about 1.26 mSv/year — more than cosmic radiation, terrestrial gamma, and internal radionuclides combined. In areas with granite bedrock or uranium-rich soils, radon can dominate the dose budget even further.

Studies on airline crew consistently show a small but statistically detectable increase in certain cancers (melanoma, breast cancer in female crew), though it is difficult to separate radiation effects from other occupational factors like jet lag, irregular sleep, and UV exposure during layovers. A long-haul pilot accumulates about 2–5 mSv/year from cosmic radiation — comparable to a couple of CT scans. EU regulations classify aircrew as radiation workers if they exceed 1 mSv/year, requiring dose monitoring and schedule management to keep exposure ALARA. The US has no equivalent requirement.

At 1 Sv: nausea, vomiting, and fatigue within hours; blood cell counts drop but most people survive with supportive care. At 3 Sv: severe bone marrow damage, bleeding, infection risk, hair loss — survival is possible (~50% with intensive treatment) but uncertain. At 6 Sv: near-total bone marrow destruction plus gastrointestinal damage; even with aggressive treatment including bone marrow transplant, survival is unlikely. Above 10 Sv the gut lining disintegrates and death occurs within days regardless of treatment. These thresholds apply to whole-body, acute exposure — the same total dose spread over months causes far less harm.

Alpha particles are massive (two protons + two neutrons) and highly charged, so they dump all their energy in a tiny volume of tissue — a few cell diameters. That concentrated damage creates complex, hard-to-repair DNA double-strand breaks. Gamma photons spread their energy thinly across a larger volume, causing simpler damage that cells repair more easily. The radiation weighting factor (w_R = 20 for alphas, 1 for gammas) converts absorbed dose in gray to equivalent dose in sieverts, capturing this biological difference. It is a simplification — the real damage depends on many factors — but it works well enough for radiation protection.

ISS crew receive about 0.5–1 mSv per day — roughly 150–200 mSv during a six-month mission — primarily from galactic cosmic rays and occasional solar particle events. NASA limits career dose based on age and sex (historically 1–4 Sv lifetime, recently standardized to 600 mSv), monitored via personal dosimeters and area monitors. During solar storms, crew shelter in the most heavily shielded module (usually near the water tanks). A Mars mission would deliver roughly 1 Sv total, pushing against career limits and making radiation the single biggest health obstacle to deep space travel.

This is one of the most debated questions in radiation science. The "linear no-threshold" (LNT) model, used by most regulators, assumes every additional dose carries some cancer risk proportional to the dose — so there is no perfectly safe level. Critics point out that below about 100 mSv, the added risk is too small to detect statistically against the high natural cancer rate. Some researchers argue low doses may even stimulate repair mechanisms (hormesis). Current regulatory policy uses LNT as a conservative precaution, not because there is proof of harm at very low doses.

Rolf Sievert spent decades in the early-to-mid 1900s building dosimetry instruments and gathering data on how radiation damages living tissue. His work showed that the physical dose (energy deposited) and the biological effect (tissue damage) are not the same thing — you need weighting factors for radiation type and tissue sensitivity. The ICRP formalised this into the concepts of equivalent dose and effective dose, and the 1979 CGPM named the unit after Sievert. One sievert of any radiation type, to any tissue, is supposed to carry the same overall cancer risk — a bold simplification that works reasonably well in practice.