Officials in Japan's capital Wednesday advised parents not to give city tap water to infants after tests showed it had elevated levels of radioactive iodine - a problem attributed to a nuclear plant damaged in the March 11 earthquake and tsunami.
Radiation exceeding legal limits also has been found in 11 types of vegetables and milk in prefectures surrounding the damaged Fukushima Daiichi nuclear power plant, prompting some prefectures to stop shipping these products. The United StatesÂ is preventing the import of milk, milk products, fresh vegetables and fruit from four Japanese prefectures, though certain products could be allowed inÂ if tests show them to be safe, aÂ Food and Drug Administration spokeswoman said.
Below are brief explanations of how the radiation can get into food and water and how dangerous the Â food/water contamination in this instance might be.
Traveling from nuclear plant to food, water and milk
Radioactive particlesÂ escaping from the Fukushima Daiichi plant (see this interactive for how and why this is happening) bind to dust, traveling in the air for a distance before coming to ground, according to CNN chief medical correspondent Dr. Sanjay Gupta. The particles, suchÂ as cesium-137 and iodine-131,Â contaminate farm produce and water simply by falling on them.
The large surface areas of leafy vegetables, such as spinach, make them likely to collect greater amounts of particles than many other produce types, said Marko Moscovitch, professor at Georgetown's Department of Radiation Medicine.
The main way these particles get into milk is when they fall on the grass eaten by cows.
What are the risks of consuming the food, milk and water?
Experts say little is known aboutÂ how eating radiation-contaminated food affects people in the short- and long-term. But experts who have spoken with CNN say that the contamination levels reported so far appear to pose very little risk.
Dr. James Cox, an oncology professor at the MD Anderson Cancer Center in Houston, said he believes the radiation levels measured in these products pose a "nonexistent" immediate risk to humans, and "very low" long-term risk.
Spinach tested in a prefecture south of Â Fukushima showed radiation up to 27 times greater than the legal limit. Gupta, however, said a person "would have to eat the contaminated spinach from Japan every day for one year to get the same amount of radiation you would get from one chest CT (computed tomography) scan."
A chest CT scan would expose a person to about 7 millisieverts of radiation. The U.S. Nuclear Regulatory Commission says that an average person gets about 3.1 millisieverts annually from natural sources, and an average American - thanks in part to medical diagnostic procedures and other man-made sources of radiation - gets about 6.2 millisieverts per year.
Even low radiation doses can damage or alter the DNA of irradiated cells, the NRC says. And the radiation protection community "conservatively assumes that any amount of radiation may pose some risk for causing cancer and hereditary effect, and that the risk is higher for higher radiation exposures," the NRC says.
But Gupta andÂ Moscovitch say it's highly unlikely that the radiation reported so far in Japanese food would pose a risk to human health.
"(The radiation is) not negligible my any means. But impact on human health? Not likely," Gupta said Wednesday night on the CNN program "In the Arena."
Read more about what Cox - an expert on the effects of radiation on the survivors of Nagasaki and Hiroshima - has to say about the risks in this story, which also addresses the consumption of contaminated milk following the Chernobyl nuclear disaster in 1986.
The concern about infants and the contaminated water
Tokyo officials recommended withholding tap water from infants after government samples taken Tuesday night found 210 becquerels of radioactive iodine per kilogram of water - two times higher than the limit that the government considers safe for infants.
The amount of iodine detected was lower than the level considered safe for adults: 300 becquerels per kilogram. A liter of water weights 1 kilogram. A becquerel is a measurement of radioactive intensity by weight.
The level set for infants is "very conservative," Cox said,Â but elevated radiation levels are considered a problem for small children, because their thyroid glands are more susceptible to radioactive iodine.
"Erring on the side of caution for the extreme degree for children makes good sense," Cox said. For adults, "as far as the immediate health risk, something that would make people sick, I don't think that would come close to it."
Can radioactive contamination be removed from water?
The World Health Organization says standard water treatment procedures - including coagulation, sedimentation and filtration - might remove "significant amounts of radioactive contaminants." Other options including blending contaminated water with noncontaminated water to dilute the radioactive particles, the organization says.
CNN's Thom Patterson, Elizabeth Landau, Danielle Dellorto, Miriam Falco, Madison Park and Jason Hanna contributed to this report.