Some plastics are more toxic than others. PLA is low on the toxicity scale, although it may emit hormone-mimicking nano-particles when heated. ABS is worse, though. SLS printers melt particles of plastic or metal with lasers, so they require positive ventilation as well. Other types of printers have problems of their own - the photo-reactive resins used in SLA or DLP printers aren't something you want in your kid's mouth or on their skin, while powder-bed printers (like the Z-corp machines) use proprietary powders and binders of unknown composition, so it's hard to say how toxic they might be, although no powder is good to breathe. Probably the safest type are the LOM printers, like the Mcor Iris, which use regular paper as a feedstock, laminate them with glue and print on them with inkjet inks. They aren't cheap, though...