The cloudiness or haziness of a fluid caused by individual particles (suspended solids) that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids can contain suspended solid matter consisting of particles of many different sizes. While some suspended material will be large enough and heavy enough to settle rapidly to the bottom container if a liquid sample is left to stand (the settleable solids), very small particles will settle only very slowly or not at all if the sample is regularly agitated. These small solid particles cause the liquid to appear turbid. Turbidity is used as a water quality indicator. Turbidity is measured in “NTU” (Nephelometric Turbidity Units). Water that looks clear in a glass of water will usually have NTU < 2.  Water that looks cloudy will have NTU between 2 and 10. Water with NTU > 10 is very muddy.