A German research institute once commented on this question (in English) two years ago:
Are The Fumes You Smell During 3D Printing Harmful or Even Toxic?
Basically I agree with them, you should always ventilate the room well!
As far as I know, there are still no serious studies on inorganic or organic fine dust particles and which substances have an effect on us. We breathe in, a number of substances, industrial and car exhaust fumes, every day. Composites that evaporate from our household furniture (this sometimes takes decades). Or the cigarette smoke from the person sitting next to us.
And yet none of us walks around with a CBRN protective mask. Because none of this knocks us down directly. In the long term, we will see how much dose makes the poison.
So with ABS and ASA, I wouldn’t want to stay in the room for long. Unless it’s well ventilated and there’s only one printer running. (This material can lead to poisoning, even if the printers have filters.)
In PETG, caprolactam is suspected of causing eye irritation and respiratory problems. If the dose is too high. But it is far less harmful and can at least not let us fall over directly, than like ABS/ASA can.
But yes, life is still trial and error.
And I guess we find out.