• 1877: Arthur Downes and Thomas Blunt discover the ability of sunlight to prevent microbial growth. It is later shown that the ability of light to inactivate microorganisms is dependent on:
  • dose (intensity X time)
  • wavelength of radiation
  • sensitivity of the specific type of microorganism.
  
  
  
• 1890: Robert Koch demonstrated the lethal effect of sunlight on Mycobacterium tuberculosis, hinting at UVGI's potential for combating similar diseases
  
  
• 1930: Gates publishes the first analytical bactericidal action spectrum with peak effectiveness at 265 nm, very near the 254 nm output of low-pressure Hg germicidal lamps.
  
  
• 1933: William F. Wells presents the concept of airborne infection via "droplet nuclei"-evaporated droplets containing infectious organisms that can remain suspended in the air for extended durations.
  
  
• 1935: Wells and Fair demonstrate the ability of UVGI to efficiently inactivate airborne microorganisms and prove the concept of infection via the airborne route.
  
  
• 1937: Wells et al.* use upper-room UVGI to prevent the epidemic spread of measles in suburban Philadelphia day schools where infection outside the school is unlikely.
  
  
• 1940s to 1950s: Several studies are unable to reproduce Wells et al.'s success in using UVGI to prevent the spread of measles in schoolchildren, contributing to the disillusionment with and abandonment of UVGI for air disinfection. These failures have since been attributed to infections occurring outside the irradiated schools.
  
  
• 1956-1962: Riley exposes guinea pigs to air originating from an occupied TB ward and proves that TB is spread via the airborne route. A group of guinea pigs receiving infected air via a UVGI irradiated duct were not infected, while a group receiving air via a non-irradiated duct were infected.
  
  
• 1969-1972: Riley and colleagues conduct model room studies evaluating the use of upper-room UVGI to reduce the concentration of aerosolized test organisms in the lower room. They also show that air mixing between the upper and lower room is imperative for effective disinfection and confirm that UVGI is less effective at high humidity.
  
  
• 1974-1975: Riley et al. determine virulent tubercle bacilli and BCG to be equally susceptible to UVGI and measure the disappearance rate of aerosolized BCG in a model room with and without upper-room UVGI. Upper-room UVGI is shown to be highly effective in disinfecting the lower room, quantitatively demonstrating the potential of upper-room UVGI to reduce TB infection.
  
  
• 1985-1992: After decades of decline, there is an unexpected rise in TB in the United States, leading to a renewed interest in UVGI for air disinfection.
  
  
• 1990s to present: New in-depth efforts are undertaken, aimed toward quantitatively examining UVGI efficacy and safety and providing guidance for the proper use of UVGI.