There are four main constituents of diesel engine exhaust which are of environmental concern NOx, Sox, CO₂, and particulates (soot).Until recently co was seen as a harmless naturally occuring gas neither controlled nor legislated against. Concern about NOx grew out of city smog problems. Sulphur has long been seen as a problem with fossil fuels. Columns of smoke, once seen as a positive sign of economic activity, are now frowned on at sea or ashore.