Ultraviolet light has a long history as a disinfectant, and the SARS-CoV-2 virus, which causes COVID-19, is readily rendered harmless by UV light. The question is how best to harness UV light to fight the spread of the virus and protect human health as people work, study, and shop indoors.
The virus spreads in several ways. The main route of transmission is through person-to-person contact via aerosols and droplets emitted when an infected person breathes, talks, sings, or coughs. The virus can also be transmitted when people touch their faces shortly after touching surfaces that have been contaminated by infected individuals. This is of particular concern in healthcare settings, retail spaces where people frequently touch counters and merchandise, and in buses, trains, and planes.
As an environmental engineer who studies UV light, I’ve observed that UV can be used to reduce the risk of transmission through both routes. UV lights can be components of mobile machines, whether robotic or human-controlled, that disinfect surfaces. They can also be incorporated in heating, ventilating, and air-conditioning systems or otherwise positioned within airflows to disinfect indoor air. However, UV portals that are meant to disinfect people as they enter indoor spaces are likely ineffective and potentially hazardous.
WHAT IS ULTRAVIOLET LIGHT?
Electromagnetic radiation, which includes radio waves, visible light, and x-rays, is measured in nanometers, or millionths of a millimeter. UV irradiation consists of wavelengths between 100 and 400 nanometers, which lies just beyond the violet portion of the visible light spectrum and is invisible to the human eye. UV is divided into the UV-A, UV-B, and UV-C regions, which are 315-400 nanometers, 280-315 nanometers, and 200-280 nanometers, respectively.
The ozone layer in the atmosphere filters out UV wavelengths below 300 nanometers, which blocks UV-C from the sun before it reaches Earth’s surface. I think of UV-A as the suntanning range and UV-B as the sunburning range. High enough doses of UV-B can cause skin lesions and skin cancer.
UV-C contains the most effective wavelengths for killing pathogens. UV-C is also hazardous to the eyes and skin. Artificial UV light sources designed for disinfection emit light within the UV-C range or a broad spectrum that includes UV-C.
HOW UV KILLS PATHOGENS
UV photons between 200 and 300 nanometers are absorbed fairly efficiently by the nucleic acids that make up DNA and RNA, and photons below 240 nanometers are also well absorbed by proteins. These essential biomolecules are damaged by the absorbed energy, rendering the genetic material inside a virus particle or microorganism unable to replicate or cause an infection, inactivating the pathogen.
It typically takes a very low dose of UV light in this germicidal range to inactivate a pathogen. The UV dose is determined by the intensity of the light source and duration of exposure. For a given required dose, higher intensity sources require shorter exposure times, while lower intensity sources require longer exposure times.