Some properly designed, and well-maintained, UVGI installations have proven highly effective, as in certain hospitals, and some studies perfomed in schools. CDC guidelines recommend the use of UVGI only with the simultaneous use of HEPA filters and high rates of purge airflow. The germicidal effects can also be species-dependent. Laboratory tests have achieved extremely high rates of mortality under idealized conditions. The presence of moisture or particulates provide protection for microbes.
Dust settling on light bulbs can reduce exposures, maintenance is necessary.
One especially effective application of UVGI is the control of microbial growth in air handling unit cooling coil and filter assemblies. The constant exposure has been found to be very effective at controlling fungal growth, either because the spores are inactivated, or perhaps because mycelial growth cannot be sustained under continuous exposure. Certain types of UVGI designs seem to provide a much higher rate of disinfection than standard models operating at nearly identical spectrums, the difference being the result of improvements in the electrical power controls and regulation of internal plasma temperature, resulting in the generation of a more constant energy density at a distance from the light source. Viruses are especially susceptible to UVGI, more so than bacteria, but are also very difficult to filter. A combination of filtration for bacteria and spores, with UVGI for viruses may be an optimum combination if all components are sized appropriately.