The thousands of aromas humans can smell can be sorted into 10 basic categories, US scientists say.Prof Jason Castro, of Bates College, and Prof Chakra Chennubhotla, of the University of Pittsburgh, used a computerised technique to whittle down smells to their most basic essence.They told the PLoS One journal they had then tested 144 of these and found they could be grouped into 10 categories.The findings are contentious - some say there are thousands of permutations.Prof Castro said: "You have these 10 basic categories because they reflect important attributes about the world - danger, food and so on."If you know these basic categories, then you can start to think about building smells."We have not solved the problem of predicting a smell based on its chemical structure, but that's something we hope to do."He said it would be important to start testing the theory on more complex aromas, such as perfumes and everyday smells.In reality, any natural scent was likely to be a complex mix - a blend of the 10 different categories, he said.Prof Tim Jacob, a UK expert in smell science at Cardiff University, said: "In the 1950s a scientist called John Amoore proposed a theory which involved seven smell categories based upon molecular shape and size."He eventually withdrew it, to the poorly suppressed glee of his rival R W Moncrieff, who said thjavascript:openNewWindow('pix.php',%20'PhotoGallery','900','700');ere was 'never much solid evidence to support it, and there were difficulties all along the line, but it did stimulate a lot of useful thought'."I'm sure that Castro et al's paper will 'stimulate a lot of useful thought'."