<prompt> Soiling index is a measure ofSoil pollutionWater pollutionNoise pollutionAir pollution  <response>: Smoke or soiling index: A known volume of air is filtered through a white paper under specified conditions and the stain is measured by the photoelectric meter.Smoke concentration is estimated and expressed as microgramscubic metre of air as an average level over a period of time.Park 23e pg: 736 