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. This is continuation of Part 1 published in the November 2021 issue of the RACA Journal.

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.

Mobile whole-room (bare lamp) UVGI units

Mobile whole-room (bare lamp) units can be used to supplement (not replace) standard physical surface disinfection procedures. In closed, unoccupied sections of hospitals, manual systems are moved from room to room to disinfect surfaces with UV-C lights facing in all directions. Autonomous systems are also available.

Surfaces with thick dust may render the disinfection treatment non-effective. It is important to note that any area in shadow will not receive the benefits of the UV-C radiation so spaces with many objects or non-uniform placement of the unit may create a false sense of surface disinfection coverage, though autonomous units may mitigate this risk due to their ability to move around and increase coverage.

Figure 7 and 8



  • Suitable for all climates and ventilation conditions
  • Achieves high levels of surface decontamination and be used to enhance terminal cleaning of patient rooms upon discharge
  • Can be deployed throughout a facility for targeted disinfection.


  • Potential occupational exposure
  • Very expensive
  • Requires staff training
  • No method to benchmark output
  • Possible material degradation.

Handheld surface disinfectant units

Handheld disinfecting products typically emit inadequate strength UV-C light to reliably disinfect surfaces. Waiving it over a surface for a few seconds does not provide reliable disinfection. Limited directions are included for handheld devices, and it is confusing as to only allow operation when facing down. Unless the mobile device has a safety switch to only allow it to operate when facing downward, there is a safety risk when the human eye is exposed to the UV-C light. Due to safety risks associated with accidental exposure to human skin and eye and the level of training for any individual to use, we do not recommend this method.

Germicidal irradiation dose

For any UVGI application, the ability of UV-C to inactivate micro-organisms is a function of the UV dose. A key characteristic of germicidal lamps of any sort is the UV-C radiation dose.

The Illuminating Engineering Society provides information regarding radiation dose as followsi:

“The UV dose is the exposure time on a given micro-organism or surface multiplied by the UV irradiance and reported in millijoules per square centimetre (mJ/cm²)”. The UV irradiance is the power of electromagnetic radiation incident on a surface per unit surface area and reported in microwatts per square centimetre (μW/cm²).”

The required dose depends on both the pathogen species to be eliminated and the desired degree of reduction. For example, eliminating 90% of Escherichia coli O157:H7, the bacterium that can cause sometimes-fatal food poisoning, requires 1.5 mJ/cm²; doubling the dose eliminates 99%, tripling eliminates 99.9%, and so forth. This is referred to as log10 (‘log-ten’), or more commonly ‘log’, reduction (Table 3).

 Table 3: Log10 Pathogen Reduction

Log10 reduction
(Number of doses)

Percentage pathogen elimination











The International Ultraviolet Association published a compilation of dose requirements for many different pathogens, but viruses on average require a dose of about 20 mJ/cm² for 90% reduction when directly exposed to the UV-C radiation (IUVA undated). Most of the studies referenced in the compilation consider 254-nm radiation from low-pressure mercury vapour lamps, but the required dose from 207-nm and 222-nm excimer lamps should be comparable.

The basics of determining the radiant energy levels to a surface are as followsii:

  • Length of exposure- When disinfecting surfaces, it must be first determined if the target is moving or stationary. This helps to determine if there are any limiting factors associated with the length of exposure time. In most surface disinfection applications time is relative to intensity; increased source intensity can decrease necessary exposure time. It is important to remember that micro-organisms vary, requiring a higher or lower intensity for inactivation, depending on their structureiii.
  • Intensity of source- UV-C lamp and equipment manufacturers normally provide the intensity of a given source (lamp or fixture) at a given distance. A distance correction factor may be needed when calculating a desired dose or intensity for a surface. UV-C energy follows the same inverse square law for intensity as visible light and other electromagnetic sources: the amount of energy at the surface is measured in proportion to the square of the distance from the energy’s source (the UV-C lamp), assuming no loss through scattering or absorption. Temperature and airflow corrections may also be necessary, depending on the location of the application. The intensity of a source is given in power per unit area (i.e., μW/cm²).
  • Distance from source to surface – in a point irradiation application, the distance is relatively easy to calculate. Calculating time requirements and intensity levels for a three-dimensional object or space is more complex. The varying distances from the source are the first challenge, because the object itself creates a shadowing effect, and any shadows from the local environment must be taken into consideration (for example the back surface of an object that did not get direct light). However, portable devices are available that effectively measure the reflected dose from shadow areas and offer quantifiable results.

UVGI lamp technologies

Lamp technologiesiv include continuously emitting low- and medium-pressure mercury lamps, as well as pulsed xenon arc lamps. Studies have shown that these technologies are comparably effective for disinfection. Pulsed sources may be more practical if rapid disinfection is requiredv. Light emitting diodes (LEDs) and krypton-chlorine excimer lamps, which emit in the germicidal range (UV-C), are emerging technologies.

The most practical method of generating germicidal radiant energy is by passage of an electric discharge through a rare gas (usually argon) at low pressures (on the order of 130 to 400 pascals, or 1 to 3 torr) containing mercury vapor enclosed in a special glass tube with no fluorescent coating.

Hot-cathode germicidal lamps are identical in shape, electrical connection, operating power, and life to standard fluorescent lamps. Cold-cathode germicidal lamps are also available in various sizes, usually for shorter, smaller diameter lamps. Their operating characteristics are similar to those of hot-cathode lamps, but their starting mechanisms are different.

Approximately 45% of the input power from such a device is emitted at a mercury-discharge wavelength of 253.7 nm, in the middle of the UV-C band. The second major emission line is at 184.9 nm, but this emission is normally absorbed by the glass, since—if emitted through the glass, as it is with pure quartz—it would create ozone at levels far above the safety limit.

Other mercury lines in the UV-B and UV-A regions are present at much lower emitted-power levels and not considered important in germicidal action. These spectral distribution spikes can, unfortunately cause safety concerns.

Figure 9: Intensity of different UV Lampsvi

It is important to take the application condition of the lamp into account, as the surface temperature and airflow speed in contact with the lamp tube affects the lamp output.

The ballast selection affects the lamp output and serviceable life, and may also create audible noise (electromagnetic), EMI/RFI and affect power quality.

Tech13Figure 10: Effect of lamp surface temperature on UV output.

Effectiveness of UV-C to inactivate SARS-COV-2

Various articlesvii suggest that the SARS-CoV-2 virus (responsible for Covid-19) can be inactivated by UV-C at 254 nm if directly illuminated by UV-C at the effective dose level. UV light exposure is a direct antimicrobial approachviii,ix and its effectiveness against different strains of airborne viruses has long been establishedx,xi

The National Emerging Infectious Diseases Laboratories (NEIDL)xii at Boston University in the US have conducted research in conjunction with the lighting company Signify to validate the effectiveness of UV-C light sources on the inactivation of Covid-19. Their research applied a dose of 5mJ/cm², resulting in a reduction of the Covid-19 virus of 99% in 6 secondsxiii. Based on the data, it was determined that a dose of 22mJ/cm² will result in a reduction of 99.9999% in 25 seconds.

It is worth mentioning that the performance of far UV-C light (207-222 nm) was comparedxiv with UV-C light (254 nm) and found these results:

  • Very low doses of far-UV-C light can inactivate the airborne human coronavirus. Tests have shown that a dose as low as 1.2 to 1.7 mJ/cm² of 222-nm light has inactivated 99.9% of the airborne human coronavirus.
  • Safety studies have shown that far-UV-C light cannot penetrate either the human stratum corneum (the outer dead-cell skin layer), nor the ocular tear layer, nor even the cytoplasm of individual human cellsxv,xvi,xviii,xix,xx,xxi. Thus, far-UV-C light (222 nm) cannot reach or damage living cells in the human skin or the human eye. Together with these safety studies mentioned and studies with aerosolised influenza-A (H1N1)xxii, these results suggest the utility of continuous low-dose-rate far-UV-C light (222 nm) in occupied indoor public locations such as hospitals, transportation vehicles, restaurants, airports, and schools, potentially representing a safe and inexpensive tool to reduce the spread of airborne-mediated viruses.

Therefore, low-dose-rate far-UV-C exposure can potentially safely provide a major reduction in the ambient level of airborne coronaviruses including SARS-CoV-2, while staying within the current regulatory dose limits.

UV-C safety

UVGI lamps pose a health hazard to the eyes and skin. Only trained maintenance staff with UV specific training should be allowed to work on UV-C systems or work in any area where UV-C is present. All UV-C systems require periodic inspection, maintenance, and lamp replacement to ensure proper system performance. Whenever maintenance is performed on UV-C systems, the appropriate safety guidelines, listed below, should be carefully followed. Training topics should include at least the following:

  • UV-C exposure hazards
  • Electrical safety
  • Lock-out/tag-out (for in-duct units) Health hazards of mercury
  • Rotating machinery (for in-duct units)
  • Slippery condensate pans (for in-duct units)
  • Sharp unfinished edges (for in-duct units)
  • Confined-space entry (if applicable) (for in-duct units)
  • Emergency procedures

The Centre for Disease Control and Prevention (CDC) and National Institute for Occupational Safety and Health (NIOSH) published a recommended exposure limit (REL) for occupational exposure to UV radiationxxiii.

The REL is intended to protect workers from the acute effects of UV exposure, although photosensitive persons and those exposed concomitantly to photoactive chemicals might not be protected by the recommended standard. Exposures exceeding CDC/NIOSH REL levels require that workers use personal protective equipment (PPE) consisting of eyewear and clothing known to be non-transparent to UV-C penetration, and which covers exposed eyes and skin. UV inspection, maintenance, and repair workers typically do not remain in one location during their workday, and therefore are not exposed to UV irradiance levels for 8 hours.

Threshold Limit Value® (TLV®) consideration should be based on real-time occupancy of spaces treated by UV-Cxxiv. This recommendation is supported by UV monitoring data from First et al. (2005), which showed that peak meter readings poorly predict actual exposure of room occupants.

Individuals working on UV-C must be protected to prevent UV hazards to the eyes and skin. UV-C should not be used to disinfect the hands since mild erythema (sunburn) occurs when skin is exposed to UV-C. UV-C is almost entirely absorbed by the outer dead layer (stratum orneum) and outer skin (outer epidermis), with very limited penetration to the deeper cellular layers of skin where new cells are constantly created. which is why UV-C fixtures are typically installed overhead (or called upper-room), preventing UV light exposure to humans.

The human eye is the organ most susceptible to UV-C because it has no outer dead protective layer. When the UV-C source is overhead, the eyes receive very little exposure during normal activities, just like sunlight when the sun is overhead. It appears there are no known long-term consequences from an accidental UV-C overexposurexxv.

The CDC has provided guidelines for the use of UVGI lamps in upper rooms and AHUs as a supplemental control measure for air disinfectionxxvi,xxvii,xxviii

To ensure the safe use of UVGI lamps for air disinfection, follow these guidelinesxxix:

  • All lamps – Workers should place warning signs near upper-room UVGI lamps and on AHU access panels where internal UVGI lamps are installed. Activation switches should be clearly labelled and protected with switch guards to prevent accidental activation by unauthorised personnel. If exposure cannot be avoided, workers should wear plastic or glass face shields to protect the eyes and face, nitrile gloves or work gloves to protect the hands, and full-coverage clothing with tightly woven fabrics to protect all other exposed skin. Many fixtures incorporate a safety switch that breaks the circuit when fixtures are opened for servicing and should contain baffles or louvers appropriately positioned to direct UV irradiation to the upper air space. Baffles and louvers must never be bent or deformed.
  • Upper-room UVGI lamps – Proper installation is critical to ensure the safe use of these lamps. It is important that ceiling reflectance at the UV-C wavelength be studied before installing in a new location, since downwelling UV-C could be increased. Regular work activity should not resume in rooms with upper-room UVGI lamps unless qualified measurements have confirmed that the potential radiant exposures in the lower room are within the 8-hour exposure limit. Except in very large rooms, the emitting lamps should not be visible to occupants in the lower room.
  • AHUs with internal UVGI lamps- Access panels for AHUs with internal UVGI lamps should be interlocked with automatic shutoff switches to prevent accidental exposure to UV radiation. An inspection window that blocks germicidal UV energy (e.g., plastic or glass) should be installed to allow workers to see whether the UVGI lamp inside the AHU is operating.
  • During commissioning and before operation of the UV-C installation, hand-held radiometers with sensors tuned to read the specific 254 nm wavelength should be used to measure stray UV-C energy and should be used in upper-room systemsxxx.
  • UV irradiation should never replace sterilisation of surgical instruments.


UV-C is now used as an engineering control to reduce transmission of pathogenic organisms.

Primary applications include

  • Using upper room UVGI systems

Secondary applications include

  • In-AHU UVGI (coil, filter and drain irradiation)
  • In-duct and in-AHU UVGI (airstream disinfection)
  • Mobile air cleaning UVGI units
  • Mobile whole room (bare light) UVGI units

Where feasible, a whole-building approach to UV should be considered.

Various articles suggest that the SARS-CoV-2 virus can be inactivated by UV-C at 254 nm if directly illuminated by UV-C at the effective dose level. Further studies need to be done about the usage of far UV-C for disinfection, but it appears that low-dose-rate, far-UV-C exposure can potentially safely provide a major reduction in the ambient level of airborne coronaviruses including SARS-CoV-2.

Stringent safety requirements, considerations and guidelines should be followed when using UVGI to reduce health hazards to the eyes and skin.

Contact qualified professionals for additional information and implementation of UV-C lighting.

Please note that the information in this article was compiled in 2020, and any information that became made available after 2020 is not necessarily included in this article.


This white paper was prepared by: Pieter de Bod, Laura Cowley, Lilian Rodriguez Fu.



Centres for Disease Control and Prevention (CDC)


Compared to sterilisation, a less lethal process of inactivating microorganisms.


The molecule inside cells that contains the genetic information responsible for the development and function of an organism.


Being subjected to infectious agents, irradiation, particulates, or chemicals that could have harmful effects.

Germicidal UV (GUV)

The practice of using UV-C to disinfect.


Being, using, or containing a filter usually designed to remove 99.97% of airborne particles measuring 0.3 micrometres or greater in diameter passing through it.


Power of electromagnetic radiation incident on a surface per unit surface area, typically reported in microwatts per square centimetre (μW/cm₂).

Mycobacterium tuberculosis

Is a species of pathogenic bacteria in the family Mycobacteriaceae and the causative agent of TB.

Mega Joule (mJ)

A unit of work or energy, equal to one million joules.

Microwatt (μW)

One millionth of a watt.

Nano meter (nm)

One billionth of a meter.


A particle representing a quantum of light or other electromagnetic radiation. A photon carries energy proportional to the radiation frequency but has zero rest mass.


Ribonucleic acid

SARS-CoV-2 severe acute respiratory syndrome coronavirus 2.

 It is a virus that causes respiratory illness in humans.


Tuberculosis is a disease caused by germs that are spread from person to person through the air. TB usually affects the lungs, but it can also affect other parts of the body, such as the brain, the kidneys, or the spine. A person with TB can die if they do not get treatment.

Ultraviolet germicidal irradiation (UVGI)

Ultraviolet radiation that inactivates microorganisms. UVC energy is generated by germicidal lamps that kill or inactivate microorganisms by emitting radiation predominantly at a wavelength of 253.7 nm.

Ultraviolet radiation

Optical radiation with a wavelength shorter than that of visible radiation. The range between 100 and 400 nm is commonly subdivided into UVA: 315 to 400 nm UVB: 280 to 315 nm UVC: 200 to 280 nm Vacuum UV 100 to 200 nm.

UV dose

Product of UV irradiance and specific exposure time on a given microorganism or surface, typically reported in millijoules per square centimetre (mJ/cm²).


Distance between repeating units of a wave pattern, commonly designated by the Greek letter lambda (λ).



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