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A quantity that determines how much energy has been absorbed by matter from radiation. If ionizing radiation delivered 1 Joule of energy to 1 kilogram of matter, the matter received a dose of 1 gray. For example, in a chest CT scan, your thyroid receives about 10 mGy. Often the dose received by a person, such as a patient or a plant employee, is measured. Instruments called dosimeters are used to measure personal dose. Because different types of radiation have different effects, the dose is recalculated according to what type of radiation the mass or individual was exposed to. In the case of human dose, we speak of equivalent dose and measure it in units called sievert. For a dose, it is also important to know over how long a period of time it was received (in the same way that it makes a difference whether you drink a bottle of whisky in a minute or a year). For example, the equivalent dose from natural background in the USA is 3 mSv/year.

Equivalent dose of common activities

Activity Dose (mSv)
Sleeping next to another person for one night 0.00005
Eating a banana 0.0001
Watching TV with an old CRT monitor, one hour 0.002
Arm X-ray 0.01
One flight from New York to Tokyo 0.25
Living in Australia for a year 1.5
Living in the USA for a year 3
Spending a day near the Chernobyl power plant in 2010 6
Flying as a pilot on the New York — Tokyo route for one year 9
Smoking daily one and a half packs of cigarettes for one year 36
Living one year in Guarapari, Brazil 175
Undergoing radiotherapy for prostate cancer 800
Spending 10 minutes in the reactor core of the Chernobyl power station after fuel meltdown 50,000