Disinfecting Air with UV-C vs. Filtering Air with HEPA Filters

With the ongoing pandemic continuing to affect the world we live in, more people are trying to improve the indoor air quality of their homes and businesses. HEPA filters and ultraviolet light are two common methods of air purifying. While both of them have the same goal of improving indoor air quality, the technology behind them is quite different and the type of air pollutants they can remove differ. No air purifier is perfect and without flaws. Both HEPA filters and UV-C light air purifiers come with their own set of benefits and disadvantages.

A HEPA filter is a “high-efficiency particulate air” filter made up of strands of randomly aligned synthetic fibers or glass. HEPA filters have been used by hospitals and in research environments for a long time. They are designed to trap particles that can come from pollen, pet allergens, viruses, mold and bacteria. The standard for HEPA filters is based on the (MPPS) most penetrating particle size of 0.3 microns. In order for a filter to be designated as a HEPA filter, it must meet international standards (ISO) to remove 99.95% of particles. In the United States, the standard is removal of 99.97% of particles. One important thing to remember about HEPA filters is that particles like viruses and bacteria are only trapped by the filter. If you do not replace them, then the air cleaning effectiveness goes down, and unclean filters can become a hotspot for dangerous pathogens. For this reason, it is recommended that proper PPE be used when replacing the filters. 

Traditional sanitation procedures typically do not include Air Handling Units or their accompanying ductwork. Formaldehyde used to be the most prevalent decontamination method used to attain a 6-log sporicidal kill of HEPA housings. This method was effective, but the process typically took over 12 hours and held considerable safety concerns, as formaldehyde is a carcinogen known to leave residues. When decontaminating with chlorine dioxide gas, it is often times very easy to include the ductwork and air handling system (even HEPA housings) in the scope of the project. CD gas penetrates through HEPA filters as if they are not there, and being a dry gas, it is able to navigate the bends of the ductwork system without condensing and getting "stuck."

Ultraviolet light air purifiers remove harmful pathogens from the air by actually destroying viruses and bacteria, not just capturing them. When a micro-organism is exposed to UV-C, the nuclei of the cells are altered due to photolytic processes. This process prevents further replication and causes cell death. The AirGlow is an in-duct ultraviolet light disinfection system that can be installed in any HVAC system. The AirGlow reduces and/or eliminates the growth of bacteria, mold and spores. It can also prevent the spread of airborne transmitted diseases including the flu and SARS-CoV-2 virus. As air passes by the quartz glass bulbs of the AirGlow, the travelling air is disinfected, and harmful organisms that may have been present are killed. To improve energy efficiency, the AirGlow can be positioned parallel to the cooling coils. When used on cooling coils, the Airglow is used to reduce biofilms that can accumulate on the coils. Clean coils can deliver a 30% increase in cooling capacity which in turn reduces energy consumption and costs.

HEPA filters clean the air with the use of filters located inside the air purifier. As polluted air passes through the device, HEPA filters capture many of these harmful pollutants and keep them trapped inside. HEPA housings should be decontaminated prior to filter changeout. Alternatively, ultraviolet light air purifiers use certain UV wavelengths to literally destroy airborne pathogens. Both of these devices ultimately share a common goal – clean indoor air – but one does not necessarily substitute the other.

Decontamination of HEPA Housings

HEPA housings can undergo a decontamination process for multiple reasons.  Most frequently, HEPA housings are decontaminated prior to filter changeout. They can also be decontaminated as part of a yearly routine or during construction/renovation.  HEPA housings can be on the supply or exhaust side of an HVAC system for a facility. On the supply side, they are purifying the incoming air to maintain sterility for a clean facility. On the exhaust side, they are purifying the air exhausting a facility that works with biologically hazardous organisms to prevent their escape.  

Formaldehyde used to be the most prevalent decontamination method used to attain a 6-log sporicidal kill. This method was effective, but the process typically took over 12 hours and held considerable safety concerns. Formaldehyde is a carcinogen known to leave residues behind.  Both of these attributes are concerning, especially if a HEPA housing is on the supply side of the room.

Hydrogen peroxide vapor is another decontamination method utilized for HEPA housing decontamination.  Due to adsorption issues into the HEPA filter itself, aerating HEPA housings can take considerably longer and typically lasts overnight.  Adsorption into the filter material can cause uneven concentration amounts on either side of the filter too, potentially limiting the success of the decontamination.

Chlorine dioxide has become a more optimal decontamination method, especially when considering HEPA housings.  Chlorine dioxide gas works faster, with overall cycle times between 1.5-3 hours.  Part of this is because chlorine dioxide does not leave a residue and the aeration time is shorter.  For exhaust HEPA housings, aeration is accomplished by simply turning on the exhaust blower and opening the “infeed” and “exhaust” dampers on a HEPA housing. This method aerates a HEPA housing in under a minute. For supply HEPA housings, this is accomplished by using a carbon scrubber to break down the CD gas. This method aerates a HEPA housing in under an hour.

To read more about decontaminating HEPA housings, please click here.