<prompt> Soiling index is used to monitor:Water pollutionAir pollutionSoil pollutionAll of the above  <response>: Ans. b Air pollutionRef: Park 23rded. 1736Indicators of Air Pollution Smoke or Soiling index: A known volume of air is filtered through a white filter paper under specified conditions and the stain is measured by photoelectric meter. Smoke concentration is estimated and expressed as microgramscubic metre of air as an average level over a period of time. Sulphur dioxide This gas is a major contaminant in many urban and industrial areas. Its concentration is estimated in all air pollution surveys. So, it is the best indicator of air pollution Grit and dust measurement: Deposit gauges collect grit, dust and other solids. There are analysed monthly. Coefficient of haze: A factor used, particularly in the USA in assessing the amount of smoke or other aerosol in air. Air pollution index: It is an arbitrary index which takes into account one or more pollutants as a measure of the severity of pollution 