Immediate health risks for people living near Japan's Fukushima Daiichi nuclear plant appeared minimal Saturday, but experts cautioned that radioactive vapors could cause long-term problems ranging from birth defects to cancer if the situation worsens.

Radiation emitting from a nuclear plant, such as the one that became damaged following the earthquake in Japan and may be leaking vapors, is much more harmful to humans than radiation found in nature or x-rays, say experts. That is because the process that splits uranium using high heat creates over 100 new chemicals that can cause damage when they enter the air or food supply, said Joseph Mangano, executive director of nonprofit Radiation and Public Health Project, which researches the effect of radiation on public health.

"Once in the body these particles act like a wild bull in a china shop," said Mr. Mangano. "They bang around among the healthy cells and kill or injure them."

Reports out of Japan suggest that the situation might not yet be harmful to the population's health. (Japanese officials say that the latest explosion at the nuclear power plant didn't come with a significant leakage of radiation.) Officials have said that the levels of radiation around the facility are eight times above normal, and 1,000 times more than normal in the facility's control room.

Based on a widely used definition of average normal exposure for humans —360 millirems per year of radiation from rocks, cosmic rays and manmade sources—even those levels would be safe for humans, said Ron Chesser, director of the Center for Environmental Radiation Studies at Texas Tech University. Radiation doses would have to rise 250,000 times beyond those background levels to cause damage to human cells, he said.

Still, Dr. Chesser said that "there is no doubt that there is a serious health situation there" because of how quickly problems could escalate. "Every precaution needs to be taken."

Health risks could include specific types of cancers, stillbirths and acute radiation syndrome if a full meltdown occurs, experts said. Of particular concern are three chemicals that can be released from nuclear facilities that mimic substances the body naturally uses, such as iodine and potassium. These chemicals are radioactive Iodine, Cesium and Strontium-90, said Dr. Chesser.

Problems to the thyroid are of particular concern. The thyroid gland doesn't differentiate between radioactive iodine and the normal kind, which it uses to produce hormones and function normally. Adolescents, whose bodies are quickly growing, are at particular risk. In addition, the effects can be long term: Dr. Chesser said that following the nuclear explosion in Chernobyl, thyroid cancers started to significantly increase only seven years after exposure.

To mitigate that risk, people can take iodine tablets that saturate the organ and make it unnecessary to absorb the radioactive iodine. Dr. Chesser also recommended that people stay indoors as much as possible.

Other health risks include bone cancer and compromised immune systems if the poisonous particles damage bone marrow. Fetuses and children are particularly susceptible to these risks since their immune systems are weaker, as are those of elderly people, experts said. Birth defects can arise after radiation exposure if the fetus is exposed to the toxic substances.

In an extreme case, people exposed to very high levels of radiation, such as after a nuclear bomb like the one dropped on Hiroshima, can suffer from radiation sickness, or acute radiation syndrome, said Mr. Mangano. Symptoms include fever, nausea, vomiting and skin rashes.