Ultraviolet germicidal irradiation (UVGI) rids water, air and surfaces of harmful microorganisms such as viruses and bacteria. Sunlight does this naturally to some extent. We know that UV light damages our skin and eyes; it also kills or inactivates some microorganisms.
UVGI systems use concentrated UV light to do so in a controlled manner, emitting shortwave ultraviolet-B and ultraviolet-C radiation at certain wavelengths, namely in the germicidal range between 200 and 320 nanometers -- often via a low-pressure mercury lamp. The UV light damages the cells or DNA of the affected microorganisms, killing them or rendering them unable to replicate. UV light in the higher 320 to 400 nanometer range is not effective against germs.
UVGI has been incorporated into ventilation ducts, heating and air conditioning systems and air disinfection units. It has also been used on entire rooms, preferably while they are unoccupied or everyone is in protective gear. Some systems emit UV light in near-ceiling areas to disinfect the air above peoples' heads in conjunction with vertical airflow mechanisms. High-efficiency particulate air (HEPA) filters or other types of filtration can be used alongside UVGI to remove other contaminants that UV won't kill.
Heavy research on UVGI was done from the 1930s through the 1970s in hospitals and schools, but despite its demonstrated efficacy, UVGI was mostly abandoned, in part due to breakthroughs in immunization, antibiotics advancements and safety concerns about UV radiation.
The increasing prevalence of antibiotic-resistant germs (including drug-resistant strains of tuberculosis) and fear of bioterrorism has renewed interest in UVGI. It's most commonly accepted for water disinfection, but air and surface disinfection uses continue to gain ground. In 2003, the Centers for Disease Control (CDC) sanctioned its use in hospitals in conjunction with air cleaning systems to help control the spread of TB.