SO2 = Sulphur Dioxide. WS = Wind Speed (m/s). WD = Wind Direction (º). T = Temperature (ºC). RH = Relative Humidity (%). MAE = Mean Absolute Error. RMSE = Root Mean Squared Error. Best Results were obtained considering only Wind Speed as the external variable. Results using atmospheric variables, are similar to results obtained considering SO2 concentration alone. Due to this fact, the experiment of predicting with 12 and 24 hours ahead was made with SO2 concentration as the only input to the GRNN. Results are obtained for different number of atmospherica variables as inputs to the GRNN for a 1-hr ahead prediction. Results are also obtained for different hours ahead, but only considering the SO2 concentration levels as inputs to the GRNN.