DUSTSHINE GRASES FUMIGATION. Experiments conducted in exposure chambers produce differing conclusions concerning the sensitivity of grasses to SO2 pollution. Several possible reasons have previously been identified for the discrepancies between the results obtained by different workers. We here draw attention to an important one which has been overlooked, namely the way in which exposure concentrations are defined. The principles that emerge apply equally to fumigation experiments with other pollutants and on other types of plant.