A study from the Illinois Institute of Technology, has shown that some 3D printers can emit serious amounts of toxic nanoparticles. Exactly how many nanoparticles were emitted and how toxic they were varied wildly, depending on printer temperature and filament type, but all the emissions levels were high. The lowest emissions, for instance, were still on the order of 20 billion particles per minute. On the high end, over ten times that number of were recorded. These are particles which can hang in the air and be easily breathable, potentially poisoning you while you sleep, as long as you're printing stuff in your bedroom in the middle of the night. And once inhaled, these tiny plastic particles could not only deposit themselves in your lungs, but might even work their way up into your brain by way of your olfactory nerve.
Needless to say, plastic in your brain is probably bad. But the type of plastic you're using for your filament can also effect how dangerous it really is. When heated, acrylonitrile butadiene styrene (ABS) plastic can create some byproducts that have been proven to be toxic in mammals. On the other hand, polylactic acid (PLA) plastics are highly compatible with your body. Which is not to say you should go out and take a bite out of your 3D printer's spool of filament. If you'd like to get into the details of the study, you can find it here. And in the future, maybe try to keep that 3D printer ventilated.
Via Science Direct