Four months after an earthquake and tsunami caused one of the greatest nuclear disasters in history, information about the health and environmental impacts of the multiple meltdowns in Fukushima, Japan is still far from complete.

The First Few Days

The magnitude 9.0 earthquake registered as the one of the largest tremblers in history and spawned a 14 meter tsunami that struck the Fukushima Daiichi nuclear power plant, designed to withstand a six meter tsunami. Over 24,000 people are dead or still missing from the quake and tsunami, according to according to the New York Times .

Three of the six reactors at the Daiichi plant shut down immediately following the earthquake. Reactor number 4 was de-fueled, while reactors 5 and 6 were already offline in a state of cold shutdown for maintenance. Backup generators immediately came online to circulate cooling water within the plant.

Approximately one hour after the earthquake, the tsunami struck, disabling the generators housed in a basement and destroying the fuel tanks. Over the next hours and days, water levels dropped, exposing fuel rods in the reactor containment vessels and nearby storage pools outside the containment vessels. Several hydrogen explosions occurred at the plant caused by a buildup of steam which engineers had attempted to vent. This steam contained hydrogen and radioactive material, mostly tritium and nitrogen-16 (radioactive isotopes), which escaped to the atmosphere. It is now widely believed that three of the reactors have suffered meltdowns and some fuel breached the inner most layer of the containment vessel.

Current Estimates of Radiation

The accidents were rated at Level 7 – the highest level and equivalent to the 1986 Chernobyl meltdown’s rating – on the International Nuclear Event Scale . In June, Japan’s nuclear safety agency estimated 770,000 terabecquerels of radiation leaked from the plant in the week after the tsunami – approximately 20 percent as much as the estimate for Chernobyl. (A becquerel is a measurement of the rate of radiation emission.) The plant's reactors had released up to 10,000 terabecquerels of radioactive iodine-131 per hour into the air for several hours after they were damaged. Emissions since have dropped to below one terabecquerel per hour.

Large amounts of radioactive water have also been released into the Pacific Ocean, but the amount and impacts are unclear. Scientists from Woods Hole Oceanographic Institute are collecting samples. Radioactive iodine and cesium from the Japanese nuclear plants were detected in Japan and the United States but were at extremely low concentrations that do not present health hazards, according to the World Health Organization .

Status of the Nuclear Plant

Power to the plant has been restored and Tokyo Electric Power Company (TEPCO) has been attempting to cool the fuel by pumping a mix of fresh and salt water into the reactors. TEPCO has begun a two-step restoration process . The initial phase is to stabilize the plant by maintaining and enhancing cooling systems, preventing the spread of radioactive materials and beginning initial cleanup of the site. The next phase of restoration will be to attain a cold shutdown with a core temperature less than 100° Celsius (212° Fahrenheit) by January 2012.

Prior to decontamination of the plant, more than 100,000 metric tons of radioactive water held in the basement must be removed and filtered. A recently installed filtration system has been shut down multiple times due to mechanical problems. TEPCO is also concerned that salt water used for emergency cooling will accelerate corrosion within the plant.

Human Health Impacts

No radiation-linked deaths have been reported but 21 plant workers have been affected by minor radiation sickness, according to Japanese officials. Approximately 85,000 residents within a 30 kilometer (18.6 mile) radius remain in shelters. Several districts, some as far as 37 miles from the facility, are also evacuated due to elevated radiation levels, according to a June 30 Agence France Press article . The U.S. government urged American residents within 50 miles to evacuate. Since the disaster, Japan raised the legal exposure limit for people, including children, from one to 20 millisieverts (mSv) per year – matching the safety standard for nuclear industry workers in many countries. Millisieverts are a measure of radiation absorption into the body per hour of exposure; a typical Chest X-ray is approximately 0.1 mSv.

The Fukushima Prefecture government has begun a 30 year effort to monitor the health of residents exposed to radiation. Initial results showed radioactive cesium and iodine in the urine of 15 residents in two locations affected by fallout from the damaged nuclear plant, 35 to 40 kilometres away. According to the Japan Times , samples taken from the same 15 people at the beginning and end of May showed that their cumulative individual exposure is approaching the maximum annual allowable dose of 20 millisieverts. "It will be difficult for people to continue living in these areas," said Nanao Kamada, head of the investigation team.

Around 45 percent of children in Fukushima Prefecture surveyed by the local and central governments in late March experienced thyroid exposure to radiation . Yet in all cases, exposure occurred in trace amounts that did not warrant further examination, officials of the Nuclear Safety Commission said in late June. Authorities have removed top soil from school yards, washed down the walls of school buildings and cleared mud from gutters to protect children from radiation exposure. In Fukushima City , 45 miles from the plant, abnormal radiation levels prompted officials to issue school children dosimeters to monitor radiation levels.

According to the World Health Organization , some agricultural products and seafood in the direct vicinity of Fukushima have been contaminated at levels above the regulatory limits set by the Japanese Government, and control measures are in place to prevent their distribution.

The Future of TEPCO

TEPCO is facing financial challenges in the face of demands for unlimited compensation. The government has proposed a plan to pay claims but the prospect of passing the parliament is unclear. TEPCO’s stock price has dropped almost 90 percent , and on June 20, Moody’s cut its bond rating to junk status. This followed a similar downgrade by Standard & Poor’s in May. The Japanese government has drafted plans to break up the Tokyo power monopoly and nationalize its nuclear operations, according to Reuters.