Exposure of steel and zinc specimens at 32 test sites in Scandinavia shows that the pollution by sulfur dioxide (SO²) and by chlorides in marine locations is of decisive importance in the corrosion attack. The differences in the corrosion rate at the six rural sites in Southern Scandinavia are very slight despite substantial variations in other environmental parameters. Dose response relations for the corrosion attack on steel and zinc, including SO² concentration, chloride deposition rate, and time of exposure have been developed. The results may serve as a basis for a quantitative description of corrosivity classes in systems for classification of the corrosivity of atmospheres of the type which is now under development by the International Standardization Organization.