Responses of Salmonella typhimurium LT2, Vibrio harveyi, and Cryptosporidium parvum to UVB and UVA radiation

Conventional solar disinfection (SODIS) processes rely on UVA radiation due to exclusion of shorter wavelengths by common SODIS containers. Because of this, these processes are slow and could be improved by inclusion of UVB radiation, which has been reported to be more effective for microbial inactivation than UVA on a photon basis, but is typically present at lower spectral irradiance. To examine the potential for microbial inactivation resulting from exposure to solar UVB radiation at sea-level, experiments were conducted to define the UVB/UVA action and effectiveness spectra for Salmonella typhimurium LT2, Vibrio harveyi, and Cryptosporidium parvum, which are representative of three of the most prevalent waterborne pathogens globally. For each organism, the action spectrum was similar in shape to its corresponding DNA absorption spectrum, thereby suggesting that inactivation of these organisms by UVB irradiation was largely attributable to DNA damage. Modeling and measurements of ambient solar UVB spectral irradiance were compared, indicating a trend of model over-prediction of spectral irradiance by up to 20% on cloudless days. Effectiveness spectra for organism/location pairs were calculated as the product of the action spectra and calculated spectral irradiance to identify the most effective wavelengths for inactivation. For the organisms studied, maximum predicted effectiveness appeared at wavelengths between 318 and 330 nm. At 320 nm, the simulated inactivation of C. parvum in the top 20-cm of an outdoor swimming pool (mid-latitude location in summer) after one hour of exposure was approximately 6-log₁₀ units. These results suggest that solar UVB irradiation could yield substantial inactivation of C. parvum in outdoor recreational waters, where these protozoan parasites are responsible for a large fraction of the disease burden among swimmers.