UV light is a form of electromagnetic energy that originates from the sun, existing beyond the spectrum of visible light. This radiation possesses enough energy to interact with and alter the fundamental biological materials of living organisms, including bacteria, viruses, and molds. Harnessing this energy allows for a physical process of inactivation where the light alters the microscopic structure of a pathogen. UV technology is a powerful, non-chemical method for disinfection and sterilization by rendering microorganisms unable to reproduce or cause infection.
Classifying Ultraviolet Radiation
The ultraviolet spectrum is divided into three categories based on wavelength, which correlates to the energy and biological effect of the radiation. The longest wavelength is UVA (315 to 400 nanometers (nm)), which makes up the vast majority of UV light reaching the Earth’s surface. UVB radiation (280 to 315 nm) is responsible for sunburn and vitamin D synthesis. The shortest and most energetic band is UVC (100 to 280 nm), which contains the germicidal properties used for disinfection.
UVC radiation does not naturally reach the planet’s surface because the Earth’s atmosphere completely absorbs it. Germicidal UVC must be artificially produced using specialized lamps, such as low-pressure mercury-vapor lamps, which commonly emit light at 254 nm. This wavelength is highly effective because it is near the peak absorption range for the genetic material of microorganisms. UVA and UVB energy levels are generally too low or their wavelengths too long for efficient microbial inactivation.
The Molecular Mechanism of Bacterial Inactivation
Bacterial inactivation begins when UVC radiation is absorbed directly by the genetic material (DNA or RNA) within the microbial cell. The pyrimidine bases—specifically thymine and cytosine—are the primary targets for this energy absorption. This absorption causes a photochemical reaction between adjacent bases on the same strand of the genetic helix, forming a lesion known as a pyrimidine dimer.
The most common lesion is the cyclobutane pyrimidine dimer (CPD), where two neighboring thymine bases become fused. The formation of these dimers introduces a physical kink or irregularity into the DNA or RNA structure. This structural damage prevents the cell’s replication machinery from accurately reading the genetic code. By preventing transcription and replication, the microorganism is rendered unable to multiply, neutralizing its ability to cause infection.
Practical Applications for Disinfection and Sterilization
The chemical-free nature of UVC light makes it a valued technology for robust microbial control across numerous fields.
Water Purification
One widespread use is in water purification systems, where UVC lamps are placed in flow-through chambers to disinfect drinking water and wastewater. This avoids the need for chemical additives like chlorine, which can leave behind residual byproducts.
Air Treatment
UVC is extensively used in air treatment, commonly in heating, ventilation, and air conditioning (HVAC) systems. Lamps are installed inside ductwork to continuously inactivate airborne pathogens as air circulates. Upper-room ultraviolet germicidal irradiation (UVGI) is a related application where fixtures are mounted high on walls to treat air above the occupied zone, providing continuous sanitation in hospitals and clinics.
Surface Disinfection
Surface disinfection is a significant application, particularly in healthcare settings where automated UVC devices sterilize operating rooms and patient areas. These systems also sterilize medical equipment and non-porous surfaces in laboratories and food processing plants.
Factors Influencing Effectiveness and Safety Considerations
The success of UVC disinfection depends on operational factors, particularly the total dose of radiation delivered to the target microbe. This dose combines UVC light intensity and the duration of exposure. Increasing the distance between the light source and the target surface significantly reduces intensity, decreasing effectiveness. Any material that creates a shadow, such as dust or organic matter, will shield the bacteria and prevent the light from reaching them.
Microorganisms possess a biological defense mechanism called photoreactivation, which can repair some pyrimidine dimers using the enzyme photolyase when exposed to longer-wavelength light. Disinfection systems must deliver a sufficient UVC dose to overwhelm the organism’s repair capacity and ensure permanent inactivation.
Safety Protocols
Due to the danger of UVC radiation to living tissue, safety protocols are enforced for traditional 254 nm UVC systems. Direct exposure can cause painful, temporary eye damage (photokeratitis) and skin burns. Consequently, these germicidal lamps are used only in shielded environments, such as within an air duct, or in unoccupied spaces. Newer technology, referred to as Far-UVC (around 222 nm), is being developed because its reduced penetration depth makes it less harmful to human skin and eyes, potentially allowing for safe use in occupied areas.