Gray (Gy) measures the physical energy absorbed by matter. Sievert (Sv) measures the potential biological damage to humans. For X-rays and Gamma rays, 1 Gy = 1 Sv. But for Alpha particles, 1 Gy can equal 20 Sv because they are more damaging.

There is no 'zero risk' level, but the average person receives about 2.4 to 3.0 mSv (millisieverts) per year from natural sources. Occupational limits (e.g., nuclear workers) are usually 20-50 mSv per year.

A dental X-ray delivers about 5 µSv (microsieverts) of dose. This is extremely low—equivalent to eating 50 bananas or taking a short flight. It is considered negligible risk.

A standard chest X-ray is ~100 µSv. A 'Banana Equivalent Dose' is ~0.1 µSv. So, one chest X-ray is roughly equivalent to eating 1,000 bananas.

The LD50 (lethal dose for 50% of people) without medical treatment is roughly 4 to 5 Sieverts (Sv) of acute whole-body exposure. With intense medical care, survival is possible up to 6-8 Sv.

Rad and Rem are older US units. 100 Rad = 1 Gray and 100 Rem = 1 Sievert. Most of the world uses the metric Gray/Sievert, but Rad/Rem are still used in the USA.

Yes. At high altitudes (30,000+ ft), there is less atmosphere to shield you from cosmic rays. A flight from New York to London exposes you to about 40 µSv, which is safe but measurable.

A Geiger counter detects ionizing radiation (alpha, beta, gamma). It clicks when a particle hits the detector. It measures 'counts per minute' which can be converted to µSv/hr.

Divide by 1,000. 1,000 µSv = 1 mSv. For example, 5000 µSv = 5 mSv.

Alpha particles are heavy and charged. If they hit you from outside, skin stops them. But if inhaled or swallowed (e.g., Radon gas), they deposit massive energy into sensitive lung cells, causing 20x the damage of X-rays.