Exposure time is based on how long you are exposed , not the residence time of the pollutant. In other words, it's how long you are inside the box, not how long a pollutant molecule stays in the box.

Acute exposures results in quick, deleterious effects on the body. An acute exposure is usually correlated with high concentrations of a toxin because there will probably not be a fast response unless the concentration is high (and if there is no response, we would not need to say anything about the exposure).

A chronic exposure usually builds up gradually to something, so it might not seem as bad, but the deterioration is still there.