Stealthy, silent destroyer of DNA

Adam Cresswell, Health editor
From: The Australian March 16, 2011

IT can't be seen, heard or felt. Even the handheld Geiger counters being wielded by Japanese nuclear safety officials are useless at detecting the most dangerous forms of it once inside the body.

But radiation is nonetheless a stealthy, silent killer, and is already intense enough to threaten workers still battling to bring Japan's tsunami-devastated Fukushima nuclear reactors under control.

Some reports have stated that radiation levels at the reactors have reached 400 millisieverts an hour - enough for experts to urge a rapid rotation of emergency crews, to limit their exposure to its DNA-destroying energy.

Tilman Ruff, a public health physician at the University of Melbourne's Nossal Institute for Global Health, said this intensity of radiation was far above the maximum safe level for the general public, of just 1 millisievert per annum, and much higher even than the limit for workers in occupational settings, of 20 millisieverts annually.

At these levels, the 50 workers still on site ran the real risk of developing acute radiation sickness if not given proper protection, said Associate Professor Ruff, while the general population faced separate but no less real risks from chronic exposure to any lower-level contaminants released into the environment.

Symptoms of acute radiation poisoning start at doses above half to one sievert (500 to 1000 millisieverts), with symptoms such as nausea, headache and fatigue.

Doses of between 2 and 4 sieverts can cause bleeding, nausea and ulceration of the intestinal tract, and can cause a horrible death, either within a few days of exposure, or after a more lingering illness such as anaemia, infection and blood loss take their toll.

Higher doses can damage the brain and central nervous system directly, causing coma and death within days or hours.

However, the bigger potential threat facing Japan lies in a repeat of the Chernobyl accident, in which radioactive contamination was blown over the surrounding area and into the atmosphere.

While this has not yet occurred in Japan, the health effects in this situation would be caused by a more insidious and chronic exposure to radiation of a different type.

Intense bursts of radiation that cause rapidly fatal cases of acute poisoning are often caused by gamma rays, a type of ionising radiation that, like X-rays, damages cells and DNA by stripping atoms of some of their charged particles.

However, radioactive isotopes often emitted in nuclear accidents in many cases emit a different type of radiation called alpha particles.

Outside the body, these are not usually a problem, because they do not travel long distances and cannot penetrate the skin or even a sheet of paper.

But swallowed or inhaled, they can do great damage.

"The lethal dose for a human can be no more, in energy terms, than the heat in a cup of coffee," Professor Ruff said. "It's not that it's a particularly large amount of energy - it's just packaged in such a way that's particularly damaging to the large molecules that are key to how we work."

The isotope iodine-131 - which accounts for about 3 per cent of the products of uranium fission - is easily absorbed by the thyroid gland, which produces hormones essential for cell metabolism. Once there, there is little to protect surrounding DNA from the particle bombardment that alpha radiation involves.

While doctors could give people non-radioactive iodine as a preventative measure to saturate the thyroid and prevent it taking up any radioactive molecules, this had to be taken within 36 hours of exposure.

Professor Ruff said reactors contained a "complex soup" of radioactive isotopes, and caesium-137 was a particular threat as it behaved in chemical terms like potassium and was readily taken up by cells throughout the body.

Likewise, strontium-90 behaved like calcium, leading to its accumulation in bones and teeth.

Iodine-131 has a half-life of just eight days, which means that after this time half the substance has decayed, in this case into the inert gas xenon. But many other radioactive compounds persist for much longer.

Strontium-90 and caesium-137 both have half lives of about 30 years, while plutonium-239 - another product of the Japanese reactors, which Professor Ruff describes as "probably the most hazardous substance in existence" - has a half-life of 24,200 years.

No treatments exist that can reverse the damage involved in radiation poisoning, although doctors can try to treat symptoms, for instance by giving antibiotics to prop up a failing immune system.

Professor Ruff said any large radiation leak could have serious consequences over a wide area, as the Chernobyl blast had resulted in radiation hotspots in areas as far-flung as northern Sweden, southern Greece and other parts of Europe.

It was unlikely to affect Australia, as the separation of climate systems in the northern and southern hemispheres would make it difficult for airborne contamination to cross the equator.

But for those in Japan, increases in cancer rates could be expected to follow a significant radiation leak - as had already been seen in the 23 years since the Ukraine accident. "At Chernobyl, 42 workers died from acute radiation sickness, all within the first month," Professor Ruff said.

"But it's estimated that somewhere between 10,000 and 60,000 people in total will die from fall-out, and from cancer, over a period of decades."

Rates of thyroid cancer around Chernobyl rose within five years of the accident, Professor Ruff said, while rates of childhood leukemia rose within 10 years.

"Then the various solid tumours - lung, stomach, ovary, breast, colon - (have started to rise after) 10 years and onwards," he said.

"The incidence of cancer in Hiroshima and Nagasaki survivors is still going up.

"You can't put a name on it and say, 'That cancer is due to that radiation exposure', because cancer caused by radiation doesn't look any different to cancer caused by smoking, or anything else. But overall, it could be a very significant increase in rates."

Other experts yesterday sought to tone down the health concerns.

Peter Burns, former chief executive of the Australian Radiation Protection and Nuclear Safety Agency, said radiation releases so far in Japan were "I suspect . . . a million times, probably several million times, less" than in Chernobyl, where basic prevention measures would have greatly lessened the health impacts.

"The problem at Chernobyl (was that) people still drank the milk and ate the vegetables," he said.

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