UV air filters

Ultraviolet (UV) light is a form of radiation with very short wavelengths—too short to be perceived by the human eye. This form of energy is called "ultraviolet" because it has shorter wavelengths than violet light, which has the shortest waves of any kind of visible light.
Although UV radiation can be dangerous, and you can often hear reports about the dangers of UV rays coming from the sun, ultraviolet light is not always detrimental to humans and has a variety of uses. Some air purifiers include specialized lights that produce ultraviolet rays as a way to kill living pollutants.
 
Unlike filters, which collect pollutants as they pass through the purifier, the purpose of ultraviolet lights in air cleaners is more specific: they are meant to kill organic matter, such as bacteria, mold, and viruses. Thus the Environmental Protection Agency refers to air purifiers that use UV lights as "Ultraviolet Germicidal Irradiation Cleaners," the term "germicidal" meaning "germ-killing."
The principle behind UV air purifiers is fairly simple: UV radiation in sufficient doses is deadly to many living things, including some that are common constituents of indoor air pollution. When these objects pass through the purifier, they are exposed to the UV radiation released by the light, and as a result, according to makers of UV purifiers, they are destroyed. This is said to neutralize the damage such pollutants cause.
 
It should be evident from this description that UV lights target only one kind of indoor air pollution. They do nothing at all to prevent many other common types of pollution, such as gaseous Volatile Organic Compounds or particles such as dust. However, most air purifiers that use UV lights do so as a secondary feature, complementing other methods of air purification such as HEPA filters.
EPA Studies - Effectiveness of UV Air Purifiers
Based on the available test information, the actual success rate of UV purifiers is mixed. According to the EPA's official guide on indoor air cleaners, well-designed ultraviolet air cleaners are able to:
"[Air purifiers] reduce the viability of vegetative bacteria and molds and to provide low to moderate reductions in viruses but little, if any, reduction in bacterial and mold spores." - United States Environmental Protection Agency (August 2009)
 
It also states that various factors influence the effectiveness of UV purifiers, including
UV irradiation dose
System design and application
?System operation characteristics
?Microorganism targeted for deactivation.
 
Thus not all UV air purifiers are the same, and UV lights will be more or less useful depending on the environment they are placed in.
Furthermore, tests have shown that UV cleaning alone does not reduce dust mite problems or allergic reactions to mold—the latter because mold is still dangerous to humans even if it is dead. The Environmental Protection Agency states that the only solution to mold is to have it removed. Ultimately, the EPA declares that further testing is necessary in order to better understand just how useful ultraviolet air cleaners really are.
 
 

Conclusion

Well designed ultraviolet air purification systems can reduce vegetative bacteria, molds, and some viruses
Little if any reduction in bacteria or mold spores
Ineffective against particles like dust or dust mites
Further research is needed to measure absolute effectiveness