A classification system automatically generates an indication that a training feature vector case is a pollutant based on a mis-labelled reference image. It excludes the case from the set, builds a k-nearest neighbour confidence classifier, and then classifies the case using this classifier. The case may be marked as suspect if its classification does not match what is determined, or if they do match, if the confidence level is below a threshold. The system can automatically remove or re-label all suspect cases.