Radiation has disastrous effects on the human body and health. The dose of radiation a person is exposed to is measured in grays, named after English physicist Louis Harold Gray. 1 gray (Gy) is equal to 1 joule of radiation energy absorbed by 2 lb (1 kg) of matter.
But for the sake of practicality, milligrays (mGy) are used because the standard unit is way too large. To provide some perspective, an X-ray photo exposes a person to 1.4 mGy.
Radiation exposure in quantities greater than 1 gray is considered moderate, as well as dangerous, because 1 gray is enough to cause symptoms of radiation sickness to appear. A dose of 4 Gy is deadly for nearly half of those exposed.
Doses of radiation exposure can also be measured in sieverts (Sv) or millisieverts (mSv), with 1 sievert equal to 1 gray and therefore 1 millisievert equaling 1 milligray.
According to experts, the maximum annual dose of radiation for a healthy person is 50 mSv. If a person is exposed to over 100 mSv per year, they need to seek treatment for radiation sickness. Such a dose causes the development of cancer in every 5 out of 100 people.
A dose of 1000 millisieverts induces radiation sickness, which is characterized by diarrhea, headache, high temperature. These symptoms then disappear briefly, only to come back in a much more severe form.
A curious fact, cancer patients undergoing therapy are treated with doses up to 7 millisieverts, although exposure is done only to localized parts of the body and different tissues react differently to radiation therapy.