Actually, the more nastiness, the higher the background count. A positive background is the result of excess energies in the area which "mucks up" the magic in the area. The background, however would be aspected to whatever type of magic the were using, and give that style of magic a bonus and everything else a penalty equal to the magnitude of the background.
As far as keeping the background contained, wards wouldn't really hold anything in, but the background also wouldn't necessarily be immediately noticeable until you got close. I've seen lots of sources that using a sliding scale of magnitude for backgrounds. The closer you get the stronger it becomes. An example would be a lair of a toxic mage. Now, the mage turned toxic because he had prolonged exposure to a high level of pollution/radiation, so the area the lair is in is already a background 2 just in general. As you approach the building, the immediate area ramps up to 3, because it is simply more polluted. Entering the building there is no appreciable change, but going downstairs into his work-space, the background increases to 4.
I could very easily see a background of up to 4 or 5 being fairly self-contained and not seeping out to an enormous degree. It is also worth mentioning that a background of 1-2 is fairly normal in less "happy" places.
