text: The degree of inactivation by ultraviolet radiation is directly related to the UV dose applied to the water. The dosage, a product of UV light intensity and exposure time, is usually measured in microjoules per square centimeter, or equivalently as microwatt seconds per square centimeter (μW·s/cm) = 10 mW·s/m = 0.01 W·s/m, the latter might be better, giving two-digit values for the ones in this article-->. Dosages for a 90% kill of most bacteria and viruses range between 2,000 and 8,000 μW·s/cm. Larger parasites such as Cryptosporidium require a lower dose for inactivation. As a result, US EPA has accepted UV disinfection as a method for drinking water plants to obtain Cryptosporidium, Giardia or virus inactivation credits. For example, for a 90% reduction of Cryptosporidium, a minimum dose of 2,500 μW·s/cm is required based on EPA's 2006 guidance manual.