 The "dust" in the vent was made out of walnut powder, even though  the crew had received a memo saying that Kal Penn (qv) is  extremely allergic to nuts. Production had to be stopped for the day  that they shot the scenes with Kumar falling out of the vent in the  police station while Kal Penn (qv) had to go to the hospital. The  powder was then remade with chocolate powder for the next round of  shooting. (from DVD commentary)