<P> An additional device, which may help measuring turbidity in shallow waters is the turbidity tube . The turbidity tube condenses water in a graded tube which allows determination of turbidity based on a contrast disk in its bottom, being analogous to the Secchi disk . </P> <P> Turbidity in air, which causes solar attenuation, is used as a measure of pollution . To model the attenuation of beam irradiance, several turbidity parameters have been introduced, including the Linke turbidity factor (T). </P> <P> Governments have set standards on the allowable turbidity in drinking water . In the United States, systems that use conventional or direct filtration methods turbidity cannot be higher than 1.0 nephelometric turbidity units (NTU) at the plant outlet and all samples for turbidity must be less than or equal to 0.3 NTU for at least 95 percent of the samples in any month . Systems that use filtration other than the conventional or direct filtration must follow state limits, which must include turbidity at no time exceeding 5 NTU . Many drinking water utilities strive to achieve levels as low as 0.1 NTU . The European standards for turbidity state that it must be no more than 4 NTU . The World Health Organization, establishes that the turbidity of drinking water should not be more than 5 NTU, and should ideally be below 1 NTU . </P> <P> The U.S. Environmental Protection Agency (EPA) has published water quality criteria for turbidity . These criteria are scientific assessments of the effects of turbidity, which are used by states to develop water quality standards for water bodies . (States may also publish their own criteria .) Some states have promulgated water quality standards for turbidity, including: </P>

What is the acceptable level of turbidity in drinking water