Like I said, the category defines the loss of containment and risk of exposure, the present 7 being "no containment". There is no level 8.
Bear in mind when reading about the spread of radiation from the plant, especially across the ocean, that radioactive material is really easy to detect. It is true that there is no such thing as a "safe" level of radiation, but it's not binary by any means. In the immediate aftermath, radiation levels in California were detected at "thousands" of times higher than normal. However, we can detect radioactive particulate in the air - "fallout", the real threat of radioactive leaks - in the parts-per-billion. If the normal radiation level is "nil", then thousands of times normal is still basically nil.
Fallout is the key, radioactive dirt and dust being spread on the wind. It gets exponentially less "dangerous" as it spreads, and doesn't last forever. Every living person on Earth has particulate from Chernobyl in the bodies right now. That being said, the immediate area around Fukushima is definitely dangerous - they have an advantage of the prevailing winds mostly blowing out to sea, but it's still a serious problem, and the agricultural lands around the plant should probably be written off.
Likewise, when you hear about safe "evacuation" zones and exclusion zones, it should be obvious from the fact that they're always circular that it's really just guesswork. If you've ever lit off fireworks, and thought you were standing too close but were probably safe three steps back, that's the same basic logic. It's really a question of wind-spread and, for lack of a better word, political cover and educated guesses at safety. After all, it's always safer to be farther away.
Live report I'm watching says that the official estimate of the radioactive material leak from Fukushima, presumably by the U.S. Navy vessels in the area since they're equipped for it but it wasn't clear, now says that Fukushima has over the course of the event released about 10% of the radioactive fallout of Chernobyl. The problem is, Chernobyl was basically a one-time explosion and then a bad fire, while this thing is smoldering and boiling with no end in sight, so there's no upper limit on how bad it could eventually get. That it's only managed 10% after a couple weeks sounds good, but who knows how it's going to end up.