James M. Smith is an adjunct professor of environmental health at Emory University's Rollins School of Public Health, and former chief of radiation studies at the Centers for Disease Control and Prevention in Atlanta.

In the aftermath of the earthquake and tsunami that struck Japan on March 11, the Fukushima Daiichi nuclear power station is reporting a critical situation with at least four of its six reactors. Even after nuclear reactors of the type used there are shut down, they will usually remain at very high temperatures and require cooling water to bathe the nuclear core for weeks, perhaps months. About 200,000 residents within 12 miles of the Fukushima stations have already been evacuated.

Recent reports from Japan refer to a third explosion and a fire at a fourth reactor site. There are also reports that those living within 19 miles of the Daiichi plant must remain in their homes ("shelter in place") along with statements that levels of radiation off-site could "impact human health." All of this suggests a situation that is rapidly deteriorating and is of grave concern.

Below I've provided a brief background that may be helpful in understanding some of the issues, particularly regarding public health. I have tried to be careful not to speculate too much, given the very limited information that has come in, some of which appears contradictory.

Monitoring people for radiation exposure

Since we are reading and hearing a lot about people being monitored for "radiation exposure," it would be good to review what is meant by the phrase. There is plenty of room for confusion. It is important to understand the difference between exposure and contamination. On TV, we see technicians in protective clothing who monitor people using hand-held instruments. They are checking for radioactive contamination — i.e., radiation-emitting substances, or "radionuclides" — on skin, hair or clothing. In this case, any contamination detected would likely be due to aerial emissions of radioactivity from the reactor sites. In these situations, most contamination (perhaps 75 percent or more) is found on clothing. Any remaining on the body can be dealt with by showering. Of course, this could be a significant logistical issue in the region under current conditions.

This population monitoring for radioactive contamination is one of the major roles played by public health professionals in these kinds of nuclear incidents. The U.S. Centers for Disease Control and Prevention have produced an online handbook dedicated to the topic of population monitoring.

If someone has been contaminated with radioactivity, then he or she is being exposed to radiation. Keep in mind, however, that people may be exposed to radiation (e.g., from contaminated buildings or the ground) without being contaminated themselves. The hand-held instruments (think Geiger counters) cannot detect whether someone has only been exposed to radiation.

Distribution of potassium iodide tablets

This is standard operating practice around nuclear power plants when a nuclear incident has occurred or could possibly occur. The thyroid is a radiosensitive organ and the idea is that potassium iodide (KI) will saturate the thyroid with "cold" (stable) iodine and prevent uptake of radioactive iodine that could be inhaled or ingested. But remember this: The tablet(s) must be taken within 24 hours before the exposure or within hours afterward to be effective. It is especially important for children and infants.

The potassium iodide tablets that are distributed only work for preventing or ameliorating the uptake of radioactive iodine in the thyroid gland. They play no role for other organs of the body or for any other radioactive substance in preventing or ameliorating radiation exposures or their effects.

The distribution of potassium iodide tablets can be helpful for the current situation in Japan. But if radioactive iodine is released to the environment (see "Fission Products" below), it is also important that milk and dairy products in the region be checked for the contaminant. Historical incidents, such as Chernobyl, have shown that the ingestion of milk contaminated with radioactive iodine was the primary reason for the subsequent elevation of thyroid cancer observed in children.

Risk communication issues

The current crisis at Japan's Fukushima Daiichi nuclear complex could last for weeks. Even after it passes, however, public health officials will likely face difficult risk communication and psychosocial issues for months and years. Communicating risk is always tough in dealing with unfamiliar exposures like radiation. A most challenging problem for Japanese officials will be explaining an individual's risk from environmental radiation exposures, no matter how small those exposures might be. The primary concern is an elevation of the risk of cancer in later years. The exposures to the plant operators and emergency workers could be at high enough levels to cause acute radiation sickness within hours or days. I don't have sufficient information to tell if that has yet happened.

Fission products

If the water in a reactor descends below the level of the fuel rods, the temperature rises and the fuel casing or cladding ruptures, releasing "fission products." Eventually the core begins to melt (the beginning of a partial or complete meltdown). That's the reason for the frantic attempts at keeping water pumped into the reactors. Although a partial meltdown of reactor fuel occurred at Three Mile Island, the containment vessel did not fail.

In the current incident, it appears a partial meltdown may have taken place in one or more reactors. In that case, we would expect any environmental radioactivity to include fission products. These are simply the radionuclides created when uranium has been split in the chain-reaction fission process. The products are contained within the fuel and its cladding, but can be released from the reactor core to the environment whenever there is rupture of the uranium fuel rods and breaching of the reactor containment. Also, reactor operators can periodically relieve pressure inside the containment vessel by performing controlled releases of vapors that contain fission products.

It is important to remember that there can be melting of the fuel rods (a partial or complete meltdown), without any significant release of the fuel or fission products if the reactor vessel and shielding remain intact (many inches of steel and several feet of concrete).

Those fission products of concern include isotopes of elements such as iodine, cesium and strontium. Iodine-131 has a half-life of eight days and cesium-137, 30 years. Both were released in mega quantities at Chernobyl in 1986. The cesium-137 continues in the local environment around Chernobyl and remains a radioactive contaminant of concern, which it will for many more years.

What about the explosions?

We are not talking about nuclear explosions. Rather, what has occurred apparently three times and could occur again is a hydrogen explosion. It can happen when the water level within the reactor core drops so low the nuclear fuel rods overheat. At a high enough temperature within the core, the zirconium cladding on the rods interacts with steam to rapidly form zirconium oxide and hydrogen gas. In a high enough concentration, the hydrogen is either flammable or explosive. It may escape through piping and meet air (thus, oxygen) in an auxiliary building, where it then ignites or explodes, or might simply explode within the containment vessel itself.

What about a fire associated with 'spent fuel'?

Uranium fuel rods have a limited useful life. When the amount of uranium-235 has decreased somewhat due to the fission process, they are pulled from the reactor and stored in a pool of water nearby. The water keeps the rods from overheating, and also serves as a radiation shield from the very high levels of radioactivity within each spent fuel rod. The pools are normally robust structures made of thick, steel-reinforced concrete walls with stainless steel liners. If water is lost from the pool — as apparently has occurred at Fukushima Daiichi Unit No. 4 — extremely high levels of heat and radiation can be released.

Assessment of the event on an international scale

The Japanese authorities have classified the event early on as a level 4 (accident with local consequences) on the International Nuclear and Radiological Event Scale. That assessment could go higher. The scale is used to communicate the safety significance of events associated with sources of radiation throughout the world. The scale runs from 0 (a deviation from the norm, but no safety significance) to 7 (major event). The 1979 Three Mile Island incident was a level 5. The 1986 Chernobyl disaster was a level 7. (Unfortunately, little has been written about a catastrophe in the southern Urals of Russia that happened in 1957 and rated a level 6.)

Websites for more information

Nuclear disasters and public health
As Japan struggles to avert nuclear disaster, a radiation expert explains how public health could be affected.