Air desinfection by UV-C as an anti-Covid measure

Our clients have been recently seeking advice about various anti-Covid strategies including UV-C air desinfection. We prepared for you a brief summary of available UV-C technologies and its application based on the ASHRAE knowledge. Should you have any questions about anti-Covid strategies for HVAC systems do not hesitate to ask OPTIMAL for consultation.

Ultraviolet Energy (UV-C)

• Ultraviolet energy inactivates viral, bacterial, and fungal organisms so they are unable to replicate and potentially cause disease.
• The entire UV spectrum is capable of inactivating microorganisms, but UV-C energy (wavelengths of 100 – 280 nm) provides the most germicidal effect, with 265 nm being the optimum wavelength.
• The majority of modern UVGI lamps create UV-C energy with an electrical discharge through a low-pressure gas (including mercury vapor) enclosed in a quartz tube, similar to fluorescent lamps.
• Roughly 95% of the energy produced by these lamps is radiated at a near-optimal wavelength of 253.7 nm.
• UV-C light-emitting diodes (LEDs) are emerging for use.
• Types of disinfection systems using UV-C energy:
• In-duct air disinfection
• Upper-air disinfection
• In-duct surface disinfection
• Portable room decontamination
• Requires special PPE to prevent damage to eyes and/or skin from overexposure.
• The Illuminating Engineering Society (IES) Photobiology Committee published a FAQ on Germicidal Ultraviolet (GUV) specific to the COVID-19 pandemic.

For more information, see the ASHRAE Position Document on Filtration and Air Cleaning.

UV-C LEDs

• Have been common in the UV-A spectrum (315 – 400 nm)
• LEDs are starting to be produced in the 265 nm range
• Efficiency is dramatically less than current low-pressure mercury vapor lamps
• Minimal UV output compared to a low-pressure mercury vapor lamp
• For equal output, UV-C LEDs are more expensive than current low-pressure mercury vapor lamps
• Limited availability; not yet practical for commercial HVAC applications

For more information, see the FAQ on Germicidal Ultraviolet (GUV) published by the Illuminating Engineering Society (IES) Photobiology Committee. 

UV-C In-Duct Air Disinfection

• Banks of UV-Lamps installed inside HVAC systems or associated ductwork.
• Requires high UV doses to inactivate microorganisms on-the-fly as they pass through the irradiated zone due to limited exposure time.
• Minimum target UV dose of 1,500 µW•s/cm2 (1,500 µJ/cm2)
• Systems typically designed for 500 fpm moving airstream.
• Minimum irradiance zone of two feet
• Minimum UV exposure time of 0.25 second.
• Should always be coupled with mechanical filtration.
• MERV 8 filter for dust control
• Highest practical MERV filter recommended
• Enhanced overall air cleaning with increased filter efficiency

UV-C Upper-Air Disinfection

• UV fixtures mounted in occupied spaces at heights of 7 feet and above.
• Consider when:
• No mechanical ventilation
• Limited mechanical ventilation
• Congregate settings and other high-risk areas
• Economics/other
• Requires low UV-reflectivity of walls and ceilings
• Ventilation should maximize air mixing
• Use supplemental fans where ventilation is insufficient

UV-C In-Duct Surface Disinfection

• Banks of UV-Lamps installed inside HVAC systems, generally focused on:
• Cooling coils
• Drain pans
• Other wetted surfaces
• UV irradiance can be lower than in-duct air disinfection systems due to long exposure times.
• Goals are:
• Even distribution of UV energy across the coil face
• Generally, 12 to 36 inches from the coil face
• Operated 24/7

UV-C Portable Room Decontamination

• For surface decontamination
• Portable, fully automated units; may use UV-C lamps or Pulsed Xenon technology
• Settings for specific pathogens such as MRSA, C. difficile, both of which are harder to inactivate than coronaviruses.
• >99.9% reduction of vegetative bacteria within 15 minutes
• 99.8% for C.difficile spores within 50 minutes

                                                                                                                     (Rutala et al. 2010)