You are a helpful, respectful, and honest assistant. Always answer as helpfully as possible, while being safe. Your answers should not include any harmful, unethical, racist, sexist, toxic, dangerous, or illegal content. Please ensure that your responses are socially unbiased and positive in nature. If a question does not make any sense or is not factually coherent, explain why instead of answering something not correct. If you don't know the answer to a question, please don't share false information.
What is Soiling index is a measure of and explain in detail?
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 micrograms/cubic metre of air as an average level over a period of time.Park 23e pg: 736