(CNN) -- While workers trying to prevent a meltdown at the Fukushima Daiichi nuclear plant should not suffer short-term health effects if they stay below radiation limits set by the government, they should be monitored for health effects over the long term, experts said Wednesday.
Japanese authorities have said 19 workers so far have been exposed to 100 millisieverts of radiation. The nation's health ministry has set a cumulative limit of 250 millisieverts per year before workers must leave the plant. No figures were available on Wednesday showing how quickly the workers are approaching that limit.
People in industrialized nations are typically exposed to about 3 millisieverts of radiation per year. A CT scan of the abdomen is about 10 millisieverts, according to Harvard Medical School.
It is doubtful that the workers are experiencing any health effects currently, said Dr. James Cox, a radiation oncologist at the MD Anderson Cancer Center in Houston. Symptoms would start to be seen at between 250 and 1,000 millisieverts, he said.
"If they really are not exposed to a whole body dose greater (than) the current emergency limit of 250 millisieverts, then they would almost certainly have no symptoms," said David Brenner, director of the Center for Radiological Research at Columbia University. In addition, "the chances are pretty good that there would be no radiation-related health consequences to any of them, short- or long-term."
While the effects would be dependent on the dose -- those exposed to 1,000 millisieverts, for instance, would face a much higher risk in the short term -- workers' blood counts should be monitored to ensure they aren't lowered by the radiation, said Dr. Wally Curran, a radiation oncologist and head of Emory University's Winship Cancer Center. Workers should also be monitored for any gastrointestinal distress, he said.
If blood counts are lowered by the radiation, they can rebound once the radiation exposure ceases, Curran said. But still, the body doesn't forget the exposure, noted Cox.
Each worker carries a dosimeter to check their radiation exposure, plant owner Tokyo Electric Power Co. said Wednesday. The workers who reach 100 millisieverts are asked if they want to continue working, officials said; to a person, they have opted to continue.
Workers who work in areas of higher radiation are on the job for fewer hours, the company said. Some can work as long as 15 hours a day, depending on the area where they are working.
At much higher exposure levels, workers could begin displaying symptoms, and ultimately could require a bone marrow replacement, Curran said. "That's not going to happen at 100 millisieverts."
However, even at levels below the emergency limit, workers might face an very slightly elevated risk of cancer, Brenner said. He provided rough numbers as an example, saying a 50-year-old who received a whole body dose of 200 millisieverts might face a lifetime risk of dying from cancer of 20.2%, rather than 20% without it.
But, Curran notes, overall, one in four people are diagnosed with some form of cancer in their life. While the long-term health effects are also dose-dependent, "it would be medically wise to monitor these people carefully," he said.
A whole-body dose cannot be compared to a dose received by people undergoing radiation therapy, as the radiation is targeted to a specific location, said Dr. Timothy Fox, head of Emory University's Division of Medical Physics.
One possible effect in the intermediate term -- two to five years down the road, Curran estimated -- would be an increased risk for cataracts. The lens of the eye is one of the body's structures more sensitive to radiation, he said.
As far as future effects on workers' fertility or the health of their future children, the outlook is generally good, Curran said, adding that most of the data on the subject has been obtained from studying those exposed to radiation when atomic bombs fell on Hiroshima and Nagasaki, Japan.
Men and women who receive low doses of radiation, such as radiation therapy, are usually counseled to use birth control for six months to be on the safe side, he said. Some reduction in sperm counts can be seen after exposure. No evidence has been seen regarding an increased risk to the health of future children.
"A high enough dose would render someone infertile," he said, but at that level a person would experience other significant health problems.
CNN's Sabriya Rice, John Bonifield and Ashley Hayes contributed to this report