How much silica exposure is dangerous?

[justteenguy]

Hello! I was wondering about the numbers regarding silica dust exposure..for example is spending an hour in a poorly ventilated room with high quartz dust content enough to be considered a risk for the acute form?
What about 2 hours?
Does the visible dust have to be present at least a bit or it's possible that ALL of the respirable is invisible?
Also is it true that the higher the exposure the faster the onset of symptoms?
And the last question, what is an average weight of a DRY lung?

Thanks!