<prompt> All are the indicators of air pollution exceptSoiling indexMcArdles indexSuspended paicle countSO2 concentration  <response>: Air pollutants can be of several types : Primary pollutants: are emited directly SO2, NO2, CO, Hydrocarbons, Paiculate matter, CFCs, Ammonia, Radioactive materials, Metals like lead , cadmium, copper, Volatile organic compound Secondary pollutants: are formed by interaction between primary pollutants Ground level ozone, peroxyacetyl nitrate, Paiculate matter formed from primary polluntants Chemicals indicators of air pollution: Sulphurdioxide: Best indicator of air pollution Smoke or soiling index: Air strain on a filter paper measured through photoelectric meter Grit and dust measurement Coefficient of haze Air pollution index BEST Biological indicator of air pollution: Lichens. Ref: Park 25th edition PGno: 770-772 