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Radiation: How much is harmful?

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How dangerous to human health is radiation from the crippled nuclear power plant in Japan? How does it compare with other nuclear accidents and our everyday radiation exposure?


April 07, 2011

Not all radiation is the same, so scientists use the ‘sievert’ to measure the health risks of radiation. A one-sievert dose of radiation would cause immediate radiation sickness. But most radiation doses are much smaller, so you’ll see them measured in millisieverts or even smaller microsieverts.

1 sievert = 1000 millisieverts
1 millisievert = 1000 microsieverts


Radiation Dose: 0.09 microsieverts

Exposure: One year living within 50 miles of a nuclear power plant (1/20)

It should be safe to live beside a normally functioning nuclear power station. The radiation it sends into the surrounding environment is 100 times less than our daily dose of background radiation from natural and man-made sources.

In fact, you’d receive more radiation exposure—0.3 microsieverts a year—living next to a coal-fired power plant, because coal smoke contains radioactive substances.

(Source: Reuters)


Radiation Dose: 0.1 microsieverts

Exposure: Eating one banana (2/20)

Bananas are radioactive. It’s true! They contain tiny quantities of naturally occurring Potassium-40, as do Brazil nuts, which also contain Radium-226. A bag of Brazil nuts has a dose of about 10 microsieverts. Fortunately, our body’s metabolism easily deals with these radioactive materials.

In fact, most foods are ever-so-slightly radioactive. Over 10 percent of the average person’s total annual dose of background radiation comes from food: that’s about 400 microsieverts, 40 bags of Brazil nuts, or 4000 bananas!

(Source: Reuters)


Radiation Dose: 3.5 microsieverts

Exposure: Lowest daily dose within 30-60km of the Fukushima nuclear plant (3/20)

A Geiger counter for measuring radiation sits on a map of the Fuksuhima area. Radiation levels in the areas surrounding the crippled reactor vary wildly from day to day and place to place. On March 18, Japan’s science ministry measured radiation at 18 locations in areas 30 to 60 kilometers from the power plant.

The lowest levels were about 0.5 microsieverts per hour. Assuming somebody was continuously exposed (i.e. outdoors) for 7 hours, they received a dose of about 3.5 microsieverts, just over one third of their normal daily dose of background radiation. A dental X-ray gives a dose of 5 microsieverts.

(Source: Reuters)


Radiation Dose: 50-100 microsieverts

Exposure: One chest X-ray (4/20)

Radioactive X-rays are passed through the patient to produce an image that is used for diagnosis. For some diseases, gamma rays emitted by radioactive materials are introduced into the patient by injection, or by swallowing or by inhalation.

Cancers may be treated through radiotherapy, in which X-rays or gamma rays from cobalt-60 or similar sources are aimed at diseased cells. Medical radiation makes up the vast majority of man-made radiation doses, about 14 percent of the average person’s annual dose of background radiation.

(Source: Reuters)


Radiation Dose: 200 microsieverts

Exposure: Round trip flight from New York to Tokyo (5/20)

The Earth is continuously bombarded by cosmic radiation from the sun and other energy sources in space. The atmosphere acts as a shield, but the higher you go, the less the protection you get.

An aircraft fuselage isn’t much protection either. Someone flying from New York to Tokyo on March 18, 2011 would have received a dose of 10 or 20 times more radiation than some people living within 60km of the Fukushima nuclear power plant.

(Source: Reuters)


Radiation Dose: 1 millisievert (1000 microsieverts)

Exposure: Maximum dose within 10 miles of the Three Mile Island accident (6/20)

In 1979, the biggest nuclear accident in America’s history caused a partial meltdown of a nuclear reactor core, allowing radioactive coolant to escape.

The doses of radiation received by the general public living near the plant were about a third of the average background radiation for U.S. residents, and medical studies have concluded that the accident had no substantial health effects.

(Source: Reuters)


Radiation Dose: 1.2 millisieverts

Exposure: Highest daily dose within 30-60km of the Fukushima nuclear plant (7/20)

On March 18, the Japanese science ministry measured radiation at 18 locations in areas 30 to 60 kilometers from the crippled nuclear power plant. The measurements varied hugely.

The highest levels were about 170 microsieverts per hour. Assuming somebody was continuously exposed (i.e. outdoors) for 7 hours, they received a dose of about 1.2 millisieverts. That’s an extra third of their normal annual dose of background radiation in just one day, and well above Japanese safety limits.

(Source: Reuters)


Radiation Dose: 3.65 millisieverts

Exposure: Average annual dose of background radiation from natural and man-made sources (8/20)

Cosmic rays, the Earth’s crust and soils, the buildings we live in, and the food we eat account for about 85 percent of our annual dose of natural radiation. Almost all the rest comes from medical scans.

Radioactive uranium, thorium, and potassium-40 exist in rocks and soil and so end up in building materials. Radon is a radioactive gas that comes from uranium and seeps out of rocks and soil. It accumulates in buildings and, when inhaled, can lodge in the lungs.

The average background dose is the equivalent of about 0.35 microsieverts an hour. By comparison, Tokyo reached a high of 0.109 microsieverts per hour on March 31, 2011.

(Source: Reuters)


Radiation Dose: 4.4 millisieverts

Exposure: One year working in an Australian uranium mine (9/20)

The uranium used in nuclear reactors comes from uranium ore dug out of the ground. Miners working in underground and open cast pits inhale radioactive uranium ore dust and radon particles.

Doses range from 3 to 20 millisieverts a year (the industry limit) with the average annual dose 4.4 millisieverts. Exposure to the maximum dose over 40 years increases fatal cancer risk by about 3 percent.

(Source: Reuters)


Radiation Dose: 5-6 millisieverts

Exposure: One chest CT scan (10/20)

Computed tomography (CT) scanning uses much higher doses of radioactive X-rays than normal scans in order to create more detailed images of both soft and hard tissues. There are concerns that CT scans raise the risk of cancer, especially in women and children.

According to estimates reported by the New Scientist, a full-body CT scan dose of about 10 millisieverts is about the same one-off dose as that received by a person about 2-3km from the atomic bomb blasts at Hiroshima and Nagasaki.

(Source: Reuters)


Radiation Dose: 6 millisieverts

Exposure: One hour on the grounds of Chernobyl in 2010 (11/20)

A visitor holds a radiation warning device as he stands in front of the damaged fourth reactor at the Chernobyl nuclear power plant.

Twenty-five years after the world’s worst nuclear accident in Chernobyl, Ukraine, the area around the power plant is still irradiated by radioactive material and unsafe for humans to stay for any length of time. A 30km exclusion zone remains in place today.

(Source: Reuters)


Radiation Dose: 13 millisieverts

Exposure: Smoking 1.5 packs of cigarettes a day for one year (12/20)

Naturally-occurring radium accumulates on tobacco leaves as the plant grows. Added to this are more radium, lead-210, and polonium-210 contained within phosphate fertilizers.

The radium emits alpha and gamma radiation. The lead-210 and polonium-210 particles lodge in the smoker’s lungs, where they accumulate for decades. The tar from tobacco traps even more of these particles and so, over time, these particles can cause lung cancer.

(Source: Reuters)


Dose: 30-40 millisieverts

Exposure: Highest annual dose of natural background radiation (13/20)

Background radiation exposure varies hugely, from an annual average of 2.2 millisieverts in the UK to above 30 millisieverts in the Indian state of Kerala (pictured) and the city of Chennai, and as high as 40 millisieverts in parts of Brazil and Sudan, according to the World Nuclear Association.

It all depends on local geology. India, for example, has large amounts of radioactive thorium in its soil. The province of Ramsar in Iran has hot springs full of dissolved radium-226, making it the most naturally radioactive place in the world, with doses up to 250 millisieverts a year, five times the permitted dose for American radiation workers.

(Source: Reuters)


Radiation Dose: 50 millisieverts

Exposure: Maximum yearly dose for nuclear industry workers (14/20)

This is an internationally recognized limit and in Japan is also the limit for emergency services involved in disaster prevention. If workers are involved in lifesaving emergency operations, the dose limit rises to 250 millisieverts.

Over 5 years, however, the recommended limit for radiation workers is not 250 millisieverts but just 100 millisieverts, reflecting the fact that high doses of radiation over a short period of time are more harmful than the same doses spread over a longer period.

(Source: Reuters)


Radiation Dose: 100+ millisieverts

Exposure: 12 days within the evacuation zone around the Fukushima plant from March 11 (15/20)

Japan's Nuclear Safety Technology Center estimated the cumulative exposure in the 20km no-go zone and calculated that the maximum safe dose for infants was regularly exceeded. Worryingly, it also found that areas as far as 40km away also went above the 100 millisieverts limit, which is internationally recognized as the point at which cancer risk increases.

However, the simulation is based on the scenario of a person staying outdoors the entire time. Once people go indoors, radiation levels typically drop to a quarter to one-tenth of outdoor levels, the Center said.

(Source: Reuters)


Radiation Dose: 200 millisieverts

Exposure: Average dose of survivors within 2.6km of the Hiroshima and Nagasaki atom bombs (16/20)

The Radiation Effects Research Institute (RERF) studied over about 50,000 atomic-bomb survivors who were within 2.4km of the Hiroshima blast and 2.6km of the Nagasaki explosion and therefore received significant doses of radiation, anything from 5 millisieverts upwards.

The RERF found that about 850 survivors died from leukemia or other cancers directly attributable to radiation between 1950 and 1990. This was out of a total of 4800 cancer deaths.

(Source: Reuters)


Radiation Dose: 350 millisieverts

Exposure: Chernobyl residents evacuated after the disaster in 1986 (17/20)

After the catastrophic reactor meltdown and explosion spewed a cloud of radioactive smoke into the air, over 300,000 people were evacuated from the area around the plant.

A United Nations study found that about 6000 thyroid cancers in children from Belarus, the Russian Federation and Ukraine could be attributed to the fallout. The main cause was drinking contaminated milk. The study found no other substantial health effects on the population caused by radiation, but local activists, Greenpeace, and others dispute the findings.

(Source: Reuters)


Radiation Dose: 1 sievert (1000 millisieverts)

Exposure: One hour reading from water leaking from Fukushima No. 2 reactor (18/20)

This was then the highest level of radiation recorded at the plant, found in water that flooded a turbine hall. Direct exposure at that level causes radiation sickness such as nausea and decreased white blood cell count, but not death. However, for every sievert a person absorbs, the risk of dying from cancer in the future increases by about five percentage points.

Fortunately, nobody was directly exposed to this water but on March 24, three workers were hospitalized after receiving doses of about 170 millisieverts from contaminated water they were standing in. As for radiation in the air, the highest levels have been 400-500 millisieverts an hour, forcing evacuation of the plant.

(Source: Reuters)


Radiation Dose: 2-5 sieverts

Exposure: Severe radiation poisoning, often fatal (19/20)

An employee of the Chernobyl nuclear power plant undergoes medical inspection at a medical centre after the explosion of the fourth reactor at the plant in 1986.

Obviously the bigger the dose the less the likelihood of surviving, but anything above 4 sieverts would require very prompt treatment to avoid death. Possible causes of death would be gastrointestinal injury, bone marrow failure, central nervous system or cardiovascular collapse.

(Source: Reuters)


Radiation Dose: 6 sievert

Exposure: Typical for Chernobyl workers who died within a month (20/20)

Anyone receiving a dose of above 5 sieverts has only a 50-50 chance of survival. At Chernobyl, the nuclear workers and firefighters had no protective clothing. Those working near the destroyed reactor core would have received fatal doses of 50 sieverts within ten minutes.

In the aftermath of the accident, 237 people suffered from acute radiation sickness, including radiation burns, of whom 31 died within the first three months.

(Source: Reuters)

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