Improved indoor HVAC system can help prevent airborne respiratory viruses

Virus & crowd picture

In a scientific debate, WHO has acknowledged that in the case of COVID-19, airborne transmission could not be ruled out in crowded, closed or poorly ventilated settings. Scientists across the world are looking for more evidence as it may affect many guidelines in the crowded indoor environment. Previously, WHO has recognized the studies and evidence detailing the transmission of COVID-19 from symptomatic people to others who are in close contact through respiratory droplets, either by direct contact with the infected person or by close contact with contaminated objects and surfaces. 

This infographic shared by BBC explains the difference between the droplet and airborne transmission. Respiratory droplets are very small, often invisible to the eye, with a particle size ranging from less than 0.5 microns to 15 microns, and there are standard measures to help mitigate these droplets within indoor HVAC systems. Before you jump and start making changes in your HVAC system it is important to understand the size of the COVID-19 virus. Some of these droplets are too heavy to remain in the air and will fall on nearby floors or surfaces. Fomites collect droplets contaminated with COVID-19 and a susceptible host touching such a surface would get infected.

Scientists have observed that 95% of particles in the air are often smaller than 100 μm (μm = micrometer), among these are submicron particles that are smaller than 1 micron. Droplets in the range of 0.5–20.0 μm lingering in the air are more likely to be retained in the respiratory tract and produce the infection. Due to the size of the virus (diameter of 0.015 µm to 0.4 µm), they are earmarked with the same category of pollutants that are defined as PM1. 

 
WHO BBC airborne transmission
The size of the particle that needs to be filtered in the indoor environment defines the choice or selection of the efficiency of the air filtration solution. For example, pollen or bacteria seems gigantic in size when compared with the COVID-19 virus, hence the protection that is good enough for seasonal allergies, may not be sufficient to fight against harmful pathogens like the COVID-19 virus. To compare it further with the human hair that is 60 to 120 microns in diameter will give a fair understanding of how small the COVID-19 virus is.
Size of the virus diagram

What can be done?

Improving the ventilation system, managing the airflow and air filtration is the key to reduce the spread of COVID-19 in the indoor environment. 

1. Increase the flow of outside air  

  • supply clean outdoor air
  • increase outdoor air ventilation rates 
  • minimise recirculating air

Where possible determine if the flow of outside air can be increased and turn off or reduce the recirculation air. Modifications in the HVAC system such as installing an airflow control system that will enable the fresh air supply to be maximized and the recirculation air to be minimized. If you need to use recirculation air, upgrade to high efficiency filters that are capable of capturing particles <1 micron in size (ISO ePM1 80% and above). Along with this ensure that your indoor fan motor in HVAC has sufficient power to handle the upgrade.

2. Use air filters with ISO ePM1 filter class 

  • high efficiency air filtration, subject to fan capacity limitations
  • portable room air cleaners/purifiers 
  • follow regular maintenance protocol
  • humidity control 

The high efficiency air filter can trap the particles responsible for the spread of the virus and they become inactive while captured on the filter membrane. Upgrading to high efficiency filter depends on the pollution level in the air. Higher quality filter may require a pre-filter to ensure the reasonable operational life time and fans should be able to handle the increased pressure loss over the filter step.
Camfil recommends an ISO ePM1 filter class according to ISO 16890 (ePM1 80% or higher) for HVAC systems with air handling units (AHUs), however, due to increased resistance to airflow and space restrictions, this may not be practical for every commercial HVAC system. Consider using air cleaners for fast and easy retrofit in case of a sub-standard ventilation system for improvement in air filtration. It is also a way to rapidly boost the air quality of an already good functioning system when there is an increased risk that demands even better protection. An air filtration expert can do the proper analysis and suggest the solution based on the indoor enlivenment and existing HVAC system to minimize the impact of harmful pathogens such as COVID-19.

The efficiency of high performance air filters (HEPA filters) is measured at MPPS (most penetrating particle size) which means this is the lowest efficiency of the filter. For smaller or larger particles that filter will perform even better. MPPS is typically between 0,1-0,2 micrometers in size. Bacteria and viruses are often smaller than that but typically attach themselves to larger particles. It’s also important to understand that air filters do not actively kill living organisms. They capture and hold them within the matrix of the filter. Once the virus is captured it will stay in the air filter and eventually dry out and die (refers to as inactive). Studies on COVID-19 virus indicates that the virus does not survive longer than 3 days on open surfaces. The following diagrams will explain the efficiency of the selected air filtration in the fight against submicron particles such as vehicle emissions, bacteria, viruses, and other harmful pollutants. 

EPM10 efficiency diagram
EPM1 50 efficiency diagram
E10 85 efficiency diagram
E12 995 efficiency diagram
H13 999 efficiency diagram

Compare to other filter classes, H13 filter class air filters captures 99.95% of contaminants classified as submicron particles such as the COVID-19 virus. Along with upgrading to a more efficient filter class, ensure that the supplier you chose can provide the performance test reports. Also, consider portable in room air cleaners or purifiers in reducing the viral load where supplementary localized control is required.

High humidity leads to the loss of the infectious influenza virus from simulated coughs. An indoor humidity level of between 40-60%RH has been scientifically proven to combat airborne flu infections. This ideal humidity level shortens the time airborne flu remains infectious.

An upgraded HVAC system with high efficiency filters to minimize the impact of harmful pathogens in the indoor environment is a must in case of droplet and airborne transmission. If the transmission is airborne, optimized airflow in the HVAC system wherever possible is crucial. To discuss your air filtration requirement and assess your indoor air quality situation with Camfil expert, please locate the nearest office and connect with our specialist.

 

Sources:

Can HVAC systems help prevent transmission of COVID-19? - Published by McKinsey

WHO rethinking how COVID-19 spread in the air – Published by BBC

High humidity leads to loss of infectious influenza virus - Journal

Transmission of COVID-19 virus by droplets & aerosols – Study & Journal