ASHRAE recommends three main methods to help prevent coronavirus in buildings: increased ventilation with outdoor air, high-efficiency filtering, and ultraviolet germicidal irradiation (UVGI). We have covered each method individually in previous articles:
- Outdoor air ventilation
- Air filtering with MERV 13 filters or better
- Ultraviolet germicidal irradiation
However, there are many other disinfection methods available. The guidance provided by ASHRAE focuses on the three above, but all methods have applications. With a professional assessment of your building, you can identify the best prevention measures for COVID-19.
Make your building healthier and safer with indoor air quality measures.
Electronic Air Filters
MERV and HEPA filters clean the air by capturing particles mechanically. Their effectiveness depends on filtering efficiency, the airflow moved through the filter, the particle size you need to capture, and the filter’s location in the HVAC system.
There are also electronic air filters, which use electric charge to capture particles, as their name implies. Some electronic filters apply direct charge to pollutant particles, while others produce ions that interact with them.
- The particles are then captured by plates with an opposite charge, called precipitators.
- Charged particles can also be captured more easily by mechanical filters, and they are drawn to room surfaces due to their charge.
The removal efficiency of a filter refers to the fraction of particles removed from the air. Portable electronic filters normally use the Clean Air Delivery Rate (CADR), which is the product of airflow rate and removal efficiency. For example, if the total airflow is 500 cfm and the removal efficiency is 80%, the CADR is 400 cfm.
Gas-Phase Air Cleaners
These air purifiers use sorbent materials like activated carbon to capture ozone, volatile organic compounds (VOC) and odors. Sorbent beds are not efficient for particle removal, and they are intended for gaseous air pollutants. However, gas-phase air cleaners can complement mechanical filters, since ozone and VOC molecules are too small to be captured by filtering. Some MERV filters have their fibers impregnated with carbon or another sorbent material, and they can accomplish both functions.
Photocatalytic oxidation (PCO) air cleaners use a metal oxide semiconductor that is activated by an ultraviolet light source. This causes a reaction that captures organic particles in the air by adsorption. Titanium oxide is a common material for PCO air cleaners.
Ozone can kill viruses, bacteria and fungi. Unfortunately, ozone is also highly irritating for the human respiratory system, and asthma patients can suffer flare-ups with exposure. Ozone is often regarded as a helpful substance, but that only applies for the atmospheric ozone that protects us from radiation. At ground level, ozone is considered an air pollutant.
According to the National Institute for Occupational Safety and Health (NIOSH), ozone levels above 5 parts per billion are dangerous for humans. Since much higher concentrations are needed to achieve disinfection, ozone purification is only recommended for unoccupied spaces.
Vaporized Hydrogen Peroxide
Vaporized hydrogen peroxide (VHP) is a powerful disinfection agent, and it can destroy dangerous pathogens like anthrax and avian influenza. However, to use this method safely, the space must be unoccupied and completely sealed. This includes sealing doorways, HVAC ducts, and the spaces around plumbing and electrical fixtures.
VHP requires a long exposure time to inactivate pathogens effectively, and high concentrations are hazardous for humans. For this reason, VHP disinfection is only recommended for areas that can be sealed and disoccupied. After disinfection, the remaining vapor must be scrubbed for surfaces, and all air in the space must be purged completely. A VHP concentration of 75 ppm is immediately dangerous for humans, and the disinfection procedure uses 140-1400 ppm.
There are many types of chemical disinfectants, and their effectiveness varies. However, these substances are tested by the US Environmental Protection Agency, so the best recommendation is reading their label carefully. If a chemical disinfectant has not been tested by the US EPA, avoid using it.
Disinfectants in the US EPA List N have not been tested specifically for the new coronavirus (SARS-CoV-2). However, these chemicals have been tested successfully with other coronaviruses, and also viruses that are harder to kill. Therefore, the EPA expects that List N disinfectants will be effective against the 2019 coronavirus. List N disinfectants can be used on surfaces like countertops and doorknobs.
Visible Light Disinfection
Visible light disinfection uses a wavelength of 405 nm, which is also known as “Near UV”. This method can be integrated with lighting systems, providing continuous disinfection of the air and room surfaces. However, the effectiveness of this method against viruses is not well documented, and this includes COVID-19.
In spite of its name, this disinfection method is different from UVGI. While UV radiation destroys the DNA of pathogens, visible light disinfection causes chemical reactions that kill them, similar to bleach.