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March 22, 2011

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Sources of radiation in daily life

THE air we breathe, the food we eat, the cigarettes we smoke, the building materials in our homes and all our electronic gadgets are sources of radiation. It's all around us - and most is not harmful. Yao Minji reports.

Fears about Japan's crippled nuclear power station have turned the word "radiation" into a very scary term, making it far scarier than it is.

"Radiation is natural and all around us, from cosmic, earth, stone, food, among many other sources," says Professor Zhu Guoying, a researcher at the Radiomedicine Institute of Fudan University.

"There is no need to get panicked about radiation. It all depends on the amount and the time of exposure."

For example, the radiation level recorded yesterday morning at 500 meters northwest of Fukushima nuclear plant's Unit 3 is 2.1 mSv per hour.

The effects of exposure to that level of radiation for an hour is about the same as flying 10 times (round-trip) between New York and Tokyo.

Radiation is part of everyday life for everyone. It comes from the air, the ground and even within the body, in the form of ionizing and non-ionizing radiation. It's contained in food, such as long-life irradiated milk, potatos and onions.

Ionizing radiation is more energetic and can damage living tissue, possibly causing cancer in human if the accumulated amount is excessive.

Experts have been long arguing about the definition of "low exposure" or "excessive exposure" of ionizing radiation. Many countries set different health standards concerning ionizing radiation.

The effective dose is measured in a unit called the sievert (Sv), named after Swedish medical physicist Rolf Sievert. This unit takes into consideration the different biological effects from different radiation sources. For example, alpha particles and neutrons cause more damage than gamma or beta radiation per unit.

In other words, one sievert of radiation produces the same biological effect whether it comes from the crippled nuclear power station in Fukushima or from the air.

According to the website of World Nuclear Association, a lethal exposure is 5,000 millisievert (1 Sv = 1,000 mSv) at one time, which will kill about half those receiving it within a month. And the lowest level at which any increase in cancer is clearly evident is 100 mSv per year.

Without exposure to anything else, it is common for a person to receive about 1.5 to 3.5 mSv background radiation per year from the earth, the rocks, the air and cosmic rays, among other sources. The precise amount depends on the geological features where radiation is measured.

One of the major sources of background radiation is radon, a naturally occurring odorless, tasteless and colorless radioactive gas produced by the natural breakdown of uranium and radium in soil, rock and water. It exists in many buildings and interior decoration materials, and has become the second leading cause of lung cancer in the United States, second only to smoking.

Radon may be found in tiles, stone, granite, gypsum (common in wallboard/sheetrock/drywall), concrete, cement and other materials.

Experts strongly recommend testing for radon before moving into a new apartment. In the US and Europe, pre-purchase home inspections almost always check for radon.

In addition to the natural background radiation, exposure to artificial radiation depends on lifestyle and habits.

For example, a chest CT scan produces about six to 18 mSv at one time, so people are encouraged to check with doctors and dentists about the necessity of x-rays.

Smoking 30 cigarettes a day can also produce around 13 mSv for a year. Living with the person who smokes 30 cigarettes a day poses danger for exposure as well, from second-hand smoke.

The easiest way to avoid more exposure is to quit smoking, and stay away from smokers.

Electromagnetic radiation is the most common form of non-ionizing radiation, rarely a serious problem but also unavoidable since it mainly comes from electric necessities in daily life including computers, televisions, microwave ovens, mobile phones, refrigerators, among many more.

Electric power lines also create electromagnetic radiation around them.

Such man-made radiation, whether ionizing or non-ionizing, is normally not too harmful due to the small amount and can be greatly reduced by simple changes of habits.

See more on B2, B3

What is ionizing radiation?

High-energy radiation that can add or detach an electron from an atom or molecule, giving it an electric charge. It produces free radicals that can damage tissue and DNA. There are four types.

Alpha radiation can be stopped by the skin. Particles entering the body through food or lungs are dangerous.

Beta radiation consists of electrons. Less dangerous than alpha particles.

Gamma radiation consists of electromagnetic waves similar to X-rays.

Neutrons themselves don't ionize matter, but they interact with other atoms to generate other types of radiation.

Most radiation exposure is from natural sources. These include: radioactivity in rocks and soil of the earth's crust; radon, a radioactive gas given out by many volcanic rocks and uranium ore; and cosmic radiation. The human environment has always been radioactive and accounts for up to 85% of the annual human radiation dose.


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