How Radiation Threatens Health

The developing crisis at the Fukushima Daiichi nuclear power plant in the wake of the March 11 earthquake and tsunami has raised concerns over the health effects of radiation exposure: What is a“dangerous” level of radiation? How does radiation damage health? What are the consequences of acute and long-term low-dose radiation?Though radioactive steam has been released to reduce pressure within the plant’s reactors and there has been additional radiation leakage from the three explosions at the plants， the resulting spikes in radiation levels have not been sustained. Radiation levels in Tokyo， about 220 kilometers to the southwest， have been reported to be only slightly above normal.According to Abel Gonzalez， vice-chairman of the International Commission on Radiological Protection who studied the 1986 Chernobyl disaster， current information coming from Japan about levels of radiation leakage are incomplete at best and speculations about “worst-case scenarios” are as-of-yet irrelevant. The health effects of radiation depend on its level， type and the exposure duration.Level of radiationThe average person is exposed to 0.2 to 0.3 millisieverts of background radiation per year， a combination of cosmic radiation and emissions from building materials and natural radioactive substances in the environment.Radiation sickness (or acute radiation syndrome) usually sets in after a whole-body dose of 3 sieverts， that is， 3，000 times the recommended public dose limit per year， says Langhorst. The first symptoms of radiation sickness， nausea， vomiting， and diarrhea， can take mere minutes or up to days to manifest， according to the Centers for Disease Control. A period of serious illness， including loss or appetite， fatigue， fever， gastrointestinal problems， and possible seizures or coma， may follow and last from hours to months.Type of radiationThe type of radiation of concern in the current situation is ionizing radiation， which is produced by spontaneously decaying heavy atoms， such as iodine-131 and cesium-137. Ionizing radiation is so-called because it has sufficient energy to ionize atoms (change the charge on them， usually by knocking out electrons)， giving them the potential to tamper with the atoms and molecules within living tissues.Ionizing radiation takes different forms. In gamma and X-ray radiation， atoms release energetic light particles that are powerful enough to penetrate throughout the body. Alpha and beta radiation are of lower energy and can often be blocked by just a sheet of paper. However， if radioactive material is ingested or inhaled into the body， it is actually the lower energy alpha and beta radiation that becomes the more dangerous. That’s because a large portion of gamma and Xray radiation will pass directly through the body without interacting with the tissue， while alpha and beta radiation， unable to penetrate tissue， will lose all their energy by colliding with the atoms in the body and likely cause more damage.In the Fukushima situation， the radioactive materials detected， iodine-131 and cesium-137， emit both gamma and beta radiation. These radioactive materials are products of the nuclear fission reactions that generate power in the nuclear power plants.The Japanese government has evacuated 180，000 people from within a 20-kilometer radius of the Fukushima Daiichi plant. They are urging people within 30 kilometers of the plant to remain indoors， close all windows， and to change clothes and wash exposed skin after coming in from the outside. These measures are mainly aimed at reducing the potential for inhaling or ingesting beta-emitting radioactive material.Duration of exposureA very high single dose of radiation can be more harmful than the same dose accumulated over time. According to the World Nuclear Association， a single dose of 1 sievert is likely to cause temporary radiation sickness and lower white blood cell count， but does not cause death. A single dose of 5 sieverts would likely kill half of those exposed within a month. At 10 sieverts， death occurs within a few weeks.The effects of long-term， low-dose radiation are much more difficult to gauge. DNA damage from radiation can cause mutations that lead to cancer， especially in tissues with high rates of cell division， such as the gastrointestinal tract， reproductive cells and bone marrow. But the increase in cancer risk is so small as to be difficult to determine without studying a very large population of people exposed to radiation. As an example， according to Langhorst， 10，000 people exposed to a 0.01 seivert whole-body dose of radiation would potentially increase the total number of cancers in that population by eight. (scientificamerican.com)