 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/cm2).Dosages for a 90% kill of most bacteria and viruses range from 2,000 to 8,000 μW·s/cm2.Larger parasites such as cryptosporidium require a lower dose for inactivation.