QUESTION

How much radiation is needed to kill a person?

ANSWER

The short answer: A single dose of 5,000 mSv will kill half the people in 30 days. Half will live and half will die. Gives a bizarre and macabre twist to the concept of “half-life”!

The universally accepted unit of radiation today is the sievert, named after Rolf Maximilian Sievert (1896-1966), a Swedish medical physicist who made major contributions in the study of the biological effects of radiation.

The older unit was the REM (Roentgen Equivalent Man), which is still used, and other units include the gray, curie, and becquerel. It can really get confusing.  A sievert (Sv) is 100 REM.

Because of the recent earthquake off Japan and the troubles at the Fukushima nuclear reactors, much interest and concern has been focused on radiation exposure. The millisievert (mSv/year) is the unit we will see most often in the coming weeks and months.  The prefix milli is one-thousandth or .001.

Everything in our environment has some radioactivity; air, water, soil, rocks, trees, our bodies, simply everything.  Yes, our bodies are radioactive! We all have small amounts of Potassium-40, Carbon-14, and Radium-226 in our tissue, blood, and bones.  It’s called background radiation. The natural background radiation varies from place to place but is typically about 2.5 mSv per year.

Health physicists generally agree that a person’s exposure beyond that background radiation of 2.5 mSv should be no more than 1 mSv. Most of the radiation that you and I will be exposed to will come from dental and medical X-rays, CT scans, and flying at high altitude.

Living near a coal power plant will expose a person to more radiation than living near a nuclear power plant. Unless, or course, the nuclear power plant is in Japan. Different story! Coal comes out of the ground, and stuff out of the ground is high in radioactivity.

A dental X-ray is about .01 mSv.  A full body CAT scan is worth about 10 m/Sv. A gastrointestinal series X-ray will rack up about 14 mSv. The medical profession is very careful about limiting radiation exposure from both diagnostic tests and using radiation for cancer treatment. Much progress has been made in the last 100 years.

About 1,000 mSv causes radiation sickness, which means nausea, lack of energy, and some hair loss. Recovery is close to 100 per cent. Same symptoms a person experiences when undergoing treatment for cancer. Keep in mind that 1,000 mSv is 400 times average background radiation.

Certain human activities will increase the dosage. Live in the basement? Radon gas settles in the lowest possible places.  The average in U.S. homes is 2 mSv/year. Smoke 1 pack per day? That’s 9 mSv/year. Flight crew from New York to Tokyo? Add 9 mSv/year.  Remember that the Feds require radiation exposure from licensed activities to be below 1 mSv/year above background radiation. This limit does not apply to radiation received from medical procedures.

The only ones at risk from dying from radiation exposure, even in Japan, are those people working at the reactor, responding to this catastrophic event.