Microsievert to Millisievert

A New York-to-London flight delivers roughly 50–80 µSv of cosmic radiation, depending on solar activity and the specific flight path over the pole. That is equivalent to about 3–4 chest X-rays. Pilots and cabin crew who fly long-haul routes accumulate 2–5 mSv per year — enough that airlines in the EU are legally required to monitor their doses. Passengers on a once-a-year vacation flight have nothing to worry about; frequent business travellers crossing the Atlantic weekly might accumulate a few extra millisieverts annually, still well within safe limits.

A dental bitewing exposes a few square centimeters of jaw to a brief, low-energy X-ray pulse — about 5 µSv. A chest CT scans the entire thorax in a spiral, delivering radiation from every angle to build a 3D image — roughly 7,000 µSv. The dose difference (about 1,400×) comes from three factors: the area exposed, the beam energy, and the duration. Dental X-rays use narrow, collimated beams at 60–70 kVp for milliseconds; CT scanners use wide fans at 120 kVp for several seconds of continuous rotation.

No. Radiation is completely imperceptible to human senses at any dose below the threshold for acute radiation syndrome (roughly 250,000 µSv as a sudden whole-body exposure). You cannot feel a chest X-ray, a CT scan, or even the elevated cosmic radiation at cruising altitude. The only "sensation" from radiation occurs at extremely high doses — a metallic taste reported by some Chernobyl liquidators, which was likely caused by ozone and nitrogen oxides generated by intense gamma fields ionising the air, not by direct neural stimulation.

A dosimeter records the cumulative equivalent dose to the wearer, typically in µSv or mSv. Film badges (now largely replaced), thermoluminescent dosimeters (TLDs), and optically stimulated luminescence (OSL) badges are worn monthly then read by a lab. Electronic personal dosimeters (EPDs) give real-time µSv/hr readings with audible alarms. Nuclear workers, radiologists, interventional cardiologists, industrial radiographers, and airline crew in some countries are all required to wear them. The legal dose limit for most workers is 20 mSv/year.

No — phones and Wi-Fi emit non-ionising radio-frequency radiation, which does not cause the kind of DNA damage that ionising radiation (X-rays, gamma rays, alpha particles) causes. Microsieverts apply exclusively to ionising radiation. Radio waves are measured in watts per kilogram (specific absorption rate, or SAR) for phones, and microwatts per square centimeter for environmental RF. Comparing a phone signal to a chest X-ray in microsieverts is like comparing the temperature of a warm bath to the speed of a car — they are fundamentally different physical quantities.

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.