The accident has been given a Level 7 rating on the INES scale, the same as Chernobyl, the worst nuclear accident ever. Still, the situation in Fukushima is far from reaching those proportions:
Fukushima isn’t there yet. So far, most of the material in the core, including the longest-lived isotopes, seems to have stayed there. Far less material entered the atmosphere (only 10 percent of what was released by Chernobyl), and most of that drifted over uninhabited areas of the Pacific. The biggest release occurred directly into the ocean, where it poses less of a threat to humans in the short time before it is diluted into background levels. There have been people exposed to dangerous levels of radioactivity, contamination of nearby land, and threats to the food and water supply. But each of these, so far at least, has been on a smaller scale than in the Ukraine—Fukushima is bad, but it hasn’t yet become Chernobyl-level bad.
They are considering the accidents in units 1, 2 and 3 as a single event, and giving a global rating based on the estimated total release of radioactive materials into the atmosphere.