Er arbeitete in einem unterirdischen Raum mit nur zwei Ventilationsschächten, was seiner Gesundheit nicht bekam.
As far as I know, "was" means "what" or is the contracted form of "etwas"
But they wouldn't make sense here, so I guess the meaning should be something like:
He worked in a room underground..., and that was bad for his health
Yes, you are right. In German, sentences are often much longer than in the English language. I think a common translation would be to end the first sentence after "Ventilationsschächten" and start a new one at 'was'. 'Was' would become 'that' here.
"He worked in a room in the underground with only two ventilation shafts. That was bad for his health."
As already proposed by Emanuel, you can also make a long sentence connecting both parts with "which".
It is a indefinite relative pronoun here, so basically it means "which".
Was is a relative pronoun that refers to the whole preceding sentence. Basically one can replace it by (und) das
Er arbeitete in einem unterirdischen Raum mit nur zwei Ventilationsschächten. Das bekam seiner Gesundheit nicht.
Example:
Meine Schüler haben viele Fehler gemacht, was mich sehr enttäuscht hat. Meine Schüler haben viele Fehler gemacht und das hat mich sehr enttäuscht.
