It's been almost a year since the disaster at Japan's Fukushima Dai-ichi nuclear facility. And still the question remains: is it safe?
Many residents of the surrounding region have moved back to their homes and farms, but the Japanese government has still kept a large part of the city of Fukushima closed -- the so-called "no-go" zone that extends 12 miles around the facility.
While none of the nuclear plant workers died of radiation exposure last March, many got doses beyond safe levels as they fought to keep the plant from melting down in the days after the killer earthquake and tsunami knocked out power to the reactors.
Experts in the U.S. say the amount of radiation that spewed from the crippled reactors was about 10 percent of what was released at the Chernobyl disaster in 1986. Still, they caution that the long-term health effects on both residents and workers are still mostly uncertain.
"People are scared to death," Wolfgang Weiss, chairman of the U.N. Scientific Committee on the Effects of Atomic Radiation, which is studying Fukushima, told the Associated Press this week. "They are thinking, 'Tell me. Is it good or bad?' We can't tell them. ... Life is risky."
In Fukushima and nearby areas, outside the 20-kilometer evacuation zone, the annual exposure is 20 millisieverts in some places and as high as 50 in others, according to the AP. Fifty millisieverts (or 5 REMs) is equivalent to the yearly occupational exposure limit for nuclear workers. "It's a low dose," said Lewis Pepper, an occupational health physician at the Queens College of New York.
Below 100, experts can't say for sure whether it's safe, just that a link to cancer can't be proven. Pepper says the risk to plant workers is greater than those living nearby.
"There is no safe level," he said. Pepper has spent the past 25 years studying nuclear plant workers in the United States.
"The model which addresses radiation risk and cancer suggests there is no threshold beneath which there is no risk."
In Japan, radioactive iodine-131 released from the plant dissipated fairly quickly since it has a half-life of only eight days. But cesium-137 -- another particle emitted during the nuclear fuel cycle -- has a 30-year half-life and is likely still in the environment, Pepper said.
For people living nearby, "there remains a chronic risk from food exposure, from the vegetables or animals which eat that radiation and which people consume," he said. Cesium collects in the bones and can lead to various forms of leukemia or bone cancers.
These cancers won't be known for years to come, according to Pepper. But perhaps worse is the rise in stress-related deaths that experts are noticing in survivors of the disaster, even those who left to start new lives beyond the danger zone.
"They are worried that they were poisoned and didn’t know what would happen to them," Pepper said.
Japanese health officials this week estimated 1,331 people -- mostly elderly -- have died so far from stress-related causes, such as heart attacks or pneumonia contracted from living in evacuation shelters, according to the Kyodo News Service.
While the worst of it was felt around Fukushima, wind-borne particles of cesium and iodine did travel across the Pacific. They were barely detected by radiation monitors in the United States, according to EPA officials in Washington.
"All of the levels detected by EPA were very low, and were always well below any level of public health concern," an EPA spokesperson said in a statement provided to Discovery News. "At such extremely low levels, the risk of cancer is so small that it cannot be distinguished from exposure to natural background radiation, which is around us all of the time."
Nuclear experts say the Fukushima disaster also left a legacy of engineering mistakes.
"There were some things in the design that were obvious failures," said Ed Morse, professor of nuclear engineering at the University of California, Berkeley.
Morse said that of the four reactors at Fukushima shared two emergency exhaust stacks, a flaw that led to the destruction of reactor number four, even though it had no nuclear fuel inside. In addition, the plant's power supply was knocked out and emergency generators were located below sea level.
Members of the U.S. Nuclear Regulatory Commission voted this month to require all 104 U.S. reactors have back-up diesel generators and pumps, as well as new equipment to check water levels in spent fuel pools.
Older U.S. reactors similar to the one at Fukushima will also have to build new vent stacks to prevent the kind of explosion that occurred in Japan, according to an agency spokesman. A back-up generator at the North Anna nuclear plant in Virginia failed in August 2011 during a 5.8 earthquake that struck the region. Utilities have until 2016 to make the changes.