Millirem to Microsievert

ALARA stands for "As Low As Reasonably Achievable" — the idea that radiation doses should be minimized beyond what regulations require, using a cost-benefit analysis. In practice, a hospital might install additional lead shielding in a catheterisation lab wall (reducing staff dose from 300 mrem/year to 50 mrem/year) because the shielding cost is modest compared to the dose reduction. But spending $1 million to reduce a dose from 5 mrem to 4 mrem would not be "reasonable." ALARA is a philosophy, not a number — it forces every radiation facility to continuously ask "can we do better without being absurd?"

Almost everything. A nuclear power plant delivers roughly 0.1–1 mrem/year to its nearest neighbors. Eating one banana: 0.01 mrem. Sleeping next to another person for a year (their K-40): about 0.5 mrem. A cross-country flight: 2–5 mrem. Moving from a wood-frame house to a brick one: ~10 mrem/year from terrestrial gamma. A single chest X-ray: 2 mrem. Living in Denver instead of Miami adds ~50 mrem/year from cosmic rays. Even the potassium in your own body irradiates you at ~17 mrem/year. The nuclear plant next door is the least significant radiation source in most people's lives.

About 620 mrem (6.2 mSv). The breakdown is roughly: radon inhalation 200 mrem, medical imaging 300 mrem (CT scans are the big driver), cosmic radiation 33 mrem, terrestrial gamma 21 mrem, internal radionuclides 29 mrem, and consumer products (smoke detectors, certain ceramics) about 10 mrem. The medical imaging component has nearly doubled since the 1980s due to the explosion of CT and nuclear medicine scans. A single abdominal CT at 1,000–2,000 mrem can exceed a year's worth of natural background in one sitting.

Before the 1920s, radiologists routinely tested X-ray machines by placing their own hands in the beam to check image quality. Cumulative doses to their fingers reached tens of sieverts over years — enough to cause chronic radiation dermatitis, ulceration, and eventually squamous cell carcinoma. Dozens of pioneering radiologists had fingers amputated; some died of metastatic cancer. The "Martyrs of Radiology" memorial in Hamburg lists over 350 names. Their suffering directly led to the first dose limits (the 1928 ICRP recommendations) and the fundamental principle that no one should use their own body as a radiation detection instrument.

A quarterly dosimeter report lists: deep dose equivalent (whole-body penetrating radiation, in mrem), lens of eye dose, shallow dose (skin dose from beta or low-energy photons), and sometimes extremity dose (from ring dosimeters worn in labs). Most workers see "M" for minimal — below the reporting threshold of 10 mrem. A nuclear medicine technologist might report 100–300 mrem/quarter; an interventional cardiologist might see 500+. If any reading exceeds an administrative action level (often 500 mrem/quarter), the radiation safety officer investigates whether something went wrong or if the work simply required it.

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.