By Laura Cowley PE, LEED BD+C, Lilian Rodriguez Fu IALD, LC, and Pieter de Bod Pr.Eng., LEED BD+C

Ultraviolet light (UV) technology is a non-chemical approach to disinfection and has been used and researched for decades.Ultraviolet light (UV) technology is a non-chemical approach to disinfection and has been used and researched for decades.

This paper investigates using UV-C for disinfection of room air and surfaces, and its ability to effectively inactivate airborne viruses like the SARS-COV2 responsible for the Covid-19 pandemic. The paper explores different lamp options, potential applications, and safety considerations when using UV-C light for disinfection.

UV radiation is a type of electromagnetic radiation, present in the range between visible light and X-rays (refer Figure 1) and is divided into UV-A (315 nm to 400 nm), UV-B (280 nm to 315 nm), and UV-C (100 nm to 280 nm). UV radiation is undetectable by the human eye but when it falls on certain materials it may cause them to fluoresce.

UV radiation is artificially created using UV lamps. A germicidal lamp is an electric light that produces UV-C light.

The practice of using UV-C to disinfect is called Germicidal UV (GUV). The use of Germicidal Ultraviolet (GUV) lamps and lamp systems to disinfect room air and air streams dates to about 1877 when Arthur Downes and Thos Blunt discovered the ability of sunlight to prevent microbial growth, and the ability to show that the ability to inactivate microorganisms is dependent on the dose (intensity x time) and wavelength of radiation, and the sensitivity of the specific type of organism.

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Early applications used UV light prior to antibiotics and vaccines, and were used for air-disinfection in schools, hospitals, and other public buildings.

In 1904 the first UV quarts lamps were invented, and it was discovered that the UV lamp can be used as an antiseptic or germicidal.

In 1922 Mr Matthew Luckiesh released a book on Ultraviolet Radiation. Matthew Luckiesh was a physicist and the director of General Electric’s Lighting Research Laboratory.

In 1934 UV light was used to disinfect air to reduce measles in Philadelphia (USA) schools.

1985 – Rise in TB in the United State of America caused a renewed interested in the use of UV Germicidal Irradiation (UVGI).

It was realised that UV light cannot kill viruses, but only deactivate them.

The application of UV-C is becoming increasingly frequent as concerns about indoor air quality increase. UV-C is now used as an engineering control to interrupt the transmission of pathogenic organisms such as Mycobacterium tuberculosis (TB), influenza viruses, mould, and potential bioterrorism agents.

Effectiveness of UV-A, UV-B and UV-C for disinfection

UV-A and UV-B are generally not effective to inactivate viruses. However, UV-C is known to be effective in inactivating viruses and disinfecting surfaces,, water and air by killing bacteria, mould spores and fungi.

UV Germicidal Irradiation (UVGI) is UV radiation at wavelengths between 100nm and 280nm. Germicidal lamps generate UV-C radiation inactivating microorganisms predominantly at 253.7 nm.

Figure 1 : Ultraviolet Spectrum

On a microscopic level, UV-C photons photo-chemically interact with the RNA and DNA molecules in a virus or bacterium, causing mutations that prevent replication. This leads to the death of almost all bacteria and render these microbes non-infectious.

UV-C applications

UV-C radiation wavelength is between 200nm and 280nm. Most UV-C lamps discharge optimally at 254nm. Far-UV-C lamps discharge at 207-222 nm. UV-C light applications include use in occupied and unoccupied spaces for airborne and surface disinfection, but are subject to stringent safety requirements, installation criteria, and limitations.

Various software programs calculate the effectiveness of surface disinfection, which is dependent on factors like the amount of UV-C light energy emitted, process time, the type of surface the light is disinfecting and desired cleanliness.

 Table 1 : Summary of UVGI applications


Airborne Disinfection

Surface Disinfection

Upper Room UVGI


In-Ahu UVGI (Coil, Filter And Drain Irradiation)


In-Duct & In-Ahu UVGI (Airstream Disinfection)


Mobile Air Cleaning UVGI Units


Mobile Whole Room (Bare Light) UVGI Units


Clients interested in a UV-C system should consider cost of ownership and operating expenses when selecting a system. Major costs include the initial equipment and installation costs, maintenance costs (primarily lamp replacement), and energy costs (increased direct cost of lamp operation plus increase in heating and cooling energy consumption).

Air treatment systems and room surface disinfection systems have the objective of improving the safety, health, and productivity of building occupants through reduced incidence of infectious disease and sick building complaints. Although many studies exist to support claims of UV-C’s effectiveness in these applications, it is difficult to express the resulting benefits in economic terms. A conservative approach to economic evaluation is to compare the costs of alternative approaches such as dilution ventilation and particulate filtration that have the same effectiveness.

Other things to note are that the UV-C energy can be detrimental to most organic materials. UV rays in general will degrade paint, may damage plants, cause plastics to change colour, and degrade air filters based on their composition.

Upper room UVGI

Upper-room UV-C’s primary objective is to interrupt the transmission of airborne infectious pathogens within the indoor environment.

Various upper-room UV-C devices generate a controlled UV-C field above the heads of occupants, and to minimise UV-C in the lower occupied area of the room. Upper-room UV-Cs are typically installed in fixtures suspended from a ceiling or mounted on a wall at 7ft or above.

Upper room UV-C placement is more suited for spaces where people congregate and where potentially infected persons may transmit the virus to others (for example medical waiting rooms or homeless shelters). Common corridors potentially used by unknown infected persons in a medical facility would also benefit from upper-room UVGI fixtures. Upper-room UV-C also covers situations where untreated re-circulated air might enter an occupied space.

The fixtures are typically shielded with louvers or baffles to block radiation below the horizontal plane of the fixtures, and should be appropriately spaced to accommodate the area, shape, and height of the space in which air is to be disinfected.

Tech02.pngFigure 2a: Sectional view of upper room louvered UV-C fixtures

Disinfection effectiveness of upper-room UVGI devices improve when air in the upper and lower zone is mixed. Clean irradiated air in the upper room zone is replaced by infected air from the lower room zone.

Visual Lighting, an upper-room computer-based software, can calculate the average fluence (the radiant exposure) in the upper room. Additionally, computational fluid dynamics (CFD) is being used to understand the interaction between airflow and upper-room UV-C.

Figure 3a and 4


Upper-room UV-C fixtures are selected based on the floor-to-ceiling height. Ceiling heights above 9.8ft (3m) may allow for more open fixtures, which may be more efficient because they may allow for a larger vertical irradiation zone. In occupied spaces with lower ceilings (less than 9.8ft (3m)), various louvered upper-room UV-C devices (wall-mount, pendant, and corner-mount) are available for use in combinations and are mounted with at least 6.9ft (2.1m) from the floor to the bottom of the fixture.

The fixture should be mounted so that its UV energy is distributed parallel to the plane of the ceiling. Device construction, adjustment, and placement protect occupants from excessive ultraviolet energy. For example, in high-risk areas such as corridors of infectious disease wards, a maximum UV irradiation of (1 μJ/s)/0.155 in2 (0.4 μW/cm²) at eye level is an acceptable level based on an engineering guide. No long-term health effects of UV-C exposure at these levels in the lower occupied part of rooms are known.

Figure 4 shows a typical installation position and illustrates typical UV-C intensity level in a room. Many UVGI fixtures produce visible light to help indicate when the fixture is turned on.

A UV-C installation that produces a maintained, uniform distribution of UV irradiance averaging between 30 and 50 μW/cm² is effective in inactivating most airborne droplet nuclei containing mycobacteria and is presumably effective against viruses as well.

Beyond UV-C irradiance, effectiveness of upper-room UV-C is related to air mixing, relative humidity, and the specific characteristics of the pathogenic organisms being addressed. Effectiveness can improve greatly with well-mixed air, so ventilation systems that maximise air mixing receive the greatest benefit from upper-room UV-C.

Upper room UV-C is already proven 80% effective against tuberculosis (TB) spread in two hospital-controlled studies,.


  • Suitable for all climates and ventilation conditions
  • Achieves high levels of equivalent air changes/hour (equivalent ACH)
  • A permanent built-in solution for room air disinfection
  • Disinfects large volumes of room air when combined with forms of mechanical ventilation, natural ventilation, or hybrid types to mix the air; and have a secondary benefit for surface decontamination
  • Can be safely deployed and effectively used with proper installation, commissioning, operation, maintenance, and facility staff training
  • Mitigates airborne spread of tuberculosis 
  • Available in a variety of open and louvered fixtures for different room geometries


  • Installation and maintenance costs
  • Potential occupational exposure from improper installation
  • Requires a minimum room height of 2.4 m (8 ft)
  • Possible degradation to exposed paint, plants, surfaces, and filters
  • Requires expertise to design, commission, install, operate, and maintain.

In-AHU UVGI (coil, filter and drain irradiation)

The principal design objectives for installing UV-C lighting systems in air handling unit’s (AHUs) are to prevent the growth of bacteria and mould on system components like coils, filters and drain pans.

A large dose of UV-C light can be delivered with a low UV-C irradiance because of the constant exposure time to the mentioned AHU components. Coil surface irradiance levels in the order of 1 μW/cm² are effective, though 50 to 100 μW/cm² is more typical.

Cooling coil treatment systems have the two-fold objectives of maintaining coil performance and minimising energy use by reducing air-side flow resistance and increasing the overall heat transfer coefficient relative to a conventionally maintained, mechanically and chemically cleaned coil.

Figure 2b and 3b


A field study in a hot environment experienced a 22% reduction in pressure drop and 15% increase in air-side heat transfer coefficient after less than two months of surface treatment system operation.

In most cases, the lowest maximum velocity in a system occurs inside an AHU, increasing the effectiveness of in-AHU UV-C lights due to increased exposure time. In-AHU UV-C additionally treat air from many spaces and simultaneously irradiate cooling coils and condensate pans, increasing overall benefits of the system.

At an air velocity of 500 fpm (2.54 m/s) a typical irradiance zone 7.8ft (2.4m) in length achieves a 1 second exposure. Generally, in-duct or in AHU systems should be installed in a location that can provide a minimum of 0.25 second of UV exposure; otherwise, system cost and power consumption will be excessive. Proposed ASHRAE Standard 185.1 provides a testing method for UV-C lights in-AHU and in-duct applications to inactivate airborne microorganisms.


  • Suitable for all climates
  • Enclosed system (exposure to humans reduced)
  • Treats re-circulated air from a centralised location
  • Prevents mould and biofilm on cooling coils


  • Potential occupational exposure when safety procedures are not followed
  • Expensive to install
  • Requires UV-save filters
  • Possible material degradation (example filters)

In-duct and in-AHU UVGI (airstream disinfection)

The principal design objective for UV-C in duct or AHU is to distribute UV energy uniformly in all directions throughout the length of the duct or AHU, and to deliver the appropriate UV-C dose to air moving through the irradiated zone with minimum system power.

It should be noted that it is not very effective to prevent transmission of an airborne virus from an infected person to a non-infected person in the same room.

Figure 4

Tech07Figure 4: In-duct UVGIFigure 4: In-duct UVGI

To determine the dose value requires analysis of the entire system.

Typically, single pass ducted systems are used. In-duct air disinfection systems should be designed to have the desired single-pass inactivation level under worst-case conditions of air temperature (minimum and maximum temperatures in duct) and velocity (maximum allowable duct velocity) in the irradiated zone.

The worst-case performance to reflect the combined effect of the (a) number/power of UV-C fixtures (b) air residence time (which is inversely proportional to air velocity) (c) lamp/ballast characteristics (including the effect that the wind chill factor and lamp depreciation has on the lamp). Lamps may be located anywhere in an air duct system provided their position allows for good maintenance access.

Figure 5 and 6



  • Suitable for all climates
  • Enclosed system
  • Treats re-circulated air
  • Treats cooling coils to keep clean of mould
  • Recommendations and guidelines available


  • Potential occupational exposure and lamp distance
  • Expensive to install and maintain in resource-limited settings
  • Degrading filter media
  • Possible material degradation
  • Mobile air cleaning UVGI units

Mobile air cleaner units disinfect re-circulated air in occupied spaces and serve the greatest benefit in confined areas where UV-C cannot be used effectively. Larger air cleaners are more effective than small cleaners in small rooms, but both units are impractical and ineffective to high levels of disinfection compared to upper room UV-C in open spaces.

When HVAC equipment does not meet ASHRAE recommendations for ventilation and filtrations, calculations should be performed to select the appropriate mobile air cleaner. Potential calculations include those for contaminants to be controlled, space size and layout, noise, air distribution, ventilation, and the amount of clean air needed. 


  • Suitable for all climates and ventilation conditions
  • Enclosed unit
  • Portable


  • Quality can vary
  • Can require costly filter media (if HEPA)
  • Possible material degradation