Quoted from mattosborn:
I've been printing ABS for years (in an enclosure), but I've never bothered with any special ventilation. I think the fume concerns are overblown. I don't hang out in the room with the printer when it's running, so I'm not too worried about it.
Sincerely, I believe it's a mistake. There are dozens of scientific papers out there (as in real scientific papers, not website type analysis) that show that ABS 3d printing produces ten times as many nano particles than PLA (which is already bad in itself). Plus the Nano particles are smaller.
When you print within an enclosure, all nano particles are floating in the enclosure. When you remove the enclosure, they scatter in the neighboring environment. Some stick to surfaces, but it's fair to say in any case that the amount of particles floating in the air will increase significantly the likelihood of inhaling those particles.
And you really really don't want that. We lack data and time to determine whether this is rally harmful, but all studies on Nano particles effect point toward serious risks
Unless you don't care about long term effect.
Remember asbestos? Took several dozen years to realize it's disastrous side effects.