It’s still far from clear what the outcome of the nuclear crisis in Japan will be, in part because there is still the potential for a large release of radiation from the plant, so experts are unable to say will impact it will have on Japan.

But here’s what we know so far. According to data gathered by the U.S. during a fly over of the area, the worst contamination so far is limited to a 30-kilometer radius around the plant. The most intense radiation is limited to the plant itself. In Tokyo (220 kilometers from the plant) the average radiation dose rate on Wednesday and Thursday was 474 microsieverts per year (if you’re exposed to that level for a year, you get a dose of 474 microsieverts). In comparison, the average dose for a year from natural sources is about 2,400 microsieverts. If you live in Guarapari, Brazil, you’ll be exposed to about 10,000 microsieverts from natural sources. So, if you’re in Tokyo, the added radiation is not large.

The situation is different close to the Fukushima power plant. On Friday, one monitoring station 30 kilometers from the plant reported dose rates of 140 microsieverts per hour. A year of exposure at that rate would exceed the maximum yearly dose for emergency workers by at least five times (more in some countries). But the measurements taken close to the plant vary widely. A station very close to the one above measured levels of 40 microsieverts per hour–100 less than its neighbor. Stations further than 40 kilometers from the plant reported doses of under 10 microsieverts per hour. Five hours of exposure to that is roughly equivalent to a chest x-ray. A year’s exposure at that level would be more than workers at most nuclear power plants are typically allowed to receive. Inside the power plant, radiation levels as high as 400,000 microsieverts per hour have been reported—workers can only be exposed to such levels for a few minutes at a time. I’m getting these numbers here. For more on health effects of radiation, check here.The New York Times also has a good graphic here.

Before experts can make long-term predictions, several variables need to be accounted for. The first is whether cooling efforts will fail resulting in a massive release of radiation from spent fuel in either the reactors themselves or the cooling pools. (As soon as Sunday, workers may have restored power to the plant and restarted water pumps, which will make cooling easier.) Another is what radioactive materials are emitted, and how the weather affects the distribution of those materials. Wind direction and speed affect how far the materials are spread, and whether they head out to sea (as is largely happening now) or inland toward populated areas. Rain washes radioactive particles out of the atmosphere, which can limit the distribution, but it can also result in relatively heavy concentrations of particles where the rain falls and the water accumulates.

It’s important to measure the amounts of different types of radioactive materials (such as radioactive isotopes of iodine, cesium, strontium, and potassium) because these materials vary in how they can cause health problems. Some only cause harm if they’re ingested or inhaled, and can be protected against by taking safe forms of iodine and avoiding contaminated water. Others can cause damage from a distance by emitting high energy photons. Radioactive materials have already been found in spinach and milk near the plant, the New York Times reports:

Food safety inspectors said that the amount of iodine-131 found in the tested milk was five times higher than levels deemed safe. They said that the iodine found in the spinach was more than seven times higher. The spinach also contained slightly higher amounts of cesium-137.

Iodine-131 and cesium-137 are two of the more dangerous elements that are feared to have been released from the plants in Fukushima. Iodine-131 can be dangerous to human health, especially if absorbed through milk and milk products, because it can accumulate in the thyroid and cause cancer. Cesium-137 can damage cells and lead to an increased risk of cancer.