Charles: During recessions unemployment typically rises  Thus, during a recession air pollution due to automobile exhaust decreases, since fewer people commute in cars to jobs and so cars emitting pollutants into the air are used less  Darla: Why think that air pollution would decrease
During a recession fewer people can afford to buy new cars, and cars tend to emit more pollutants as they get older
It calls into question the truth of the premises that Charles uses to support his conclusion.