I found this on the web and added Wikipedia’s description of the dangers.
From Popular Mechanics
The biggest radioactive risk right now comes from the byproducts of fission. Tokyo Electric Power Company, which runs the reactors, has reported releases of both iodine-131 and cesium-137, the two primary radionuclides that nuclear fission creates. According to Hutchinson, strontium-90 has also been detected, and the presence of cesium and strontium indicates fuel melting.
Iodine-131 moves through the atmosphere more easily than cesium-137, but it has a half-life of only eight days, according to Classic. That means it would be all but gone within weeks. Cesium-137, on the other hand, attaches itself to particles or debris. That means that eventually cesium-137 will fall out of the air onto the ground, and there it will stay until it decays. The isotope’s half-life is about 30 years, so it would be a long time before an area it traveled to would be free from radiation. Depending on the level of radiation, the area would have to be sectioned off or the material dealt with by a hazardous waste disposal team. According to the Environmental Protection Agency, exposure to radiation from cesium-137 near a nuclear accident site could significantly increase the risk of cancer. Trace amounts of cesium-137 are already in the environment worldwide, mostly because of nuclear weapons testing in the 1950s and ’60s, but most of that has decayed.
The effects of exposure to Iodine-131 from Wikipedia
Iodine in food is absorbed by the body and preferentially concentrated in the thyroid where it is needed for the functioning of that gland. When 131I is present in high levels in the environment from radioactive fallout, it can be absorbed through contaminated food, and will also accumulate in the thyroid. As it decays, it may cause damage to the thyroid. The primary risk from exposure to high levels of 131I is the chance occurrence of radiogenic thyroid cancer in later life. Other risks include the possibility of non-cancerous growths and thyroiditis.
The risk of thyroid cancer in later life appears to diminish with increasing age at time of exposure. Most risk estimates are based on studies in which radiation exposures occurred in children or teenagers. When adults are exposed, it has been difficult for epidemiologists to detect a statistically significant difference in the rates of thyroid disease above that of a similar but otherwise unexposed group.
The risk can be mitigated by taking iodine supplements, raising the total amount of iodine in the body and therefore reducing uptake and retention in tissues and lowering the relative proportion of radioactive iodine. Unfortunately, such supplements were not distributed to the population living nearest to the Chernobyl nuclear power plant after the disaster, though they were widely distributed to children in Poland.
The effects of exposure to Cesium-137 : from Wikipedia -
Caesium-137 is water-soluble and chemically toxic in small amounts. The biological behavior of caesium-137 is similar to that of potassium and rubidium. After entering the body, caesium gets more or less uniformly distributed through the body, with higher concentration in muscle tissues and lower in bones. The biological half-life of caesium is rather short at about 70 days. Experiments with dogs showed that a single dose of 3800 μCi/kg (approx. 44 μg/kg of caesium-137) is lethal within three weeks.
Accidental ingestion of caesium-137 can be treated with the chemical Prussian blue, which binds to it chemically and then speeds its expulsion from the body.