With the winter months now here in North America, outdoor gatherings are becoming difficult to hold.
Unfortunately, indoor spaces can present significant challenges that make it harder to prevent the spread of COVID-19. While we await widespread vaccine distribution, making indoor spaces safer will be key to preventing another spike in cases.
Much has been written about how HVAC systems can contribute to the spread of COVID-19, but what if we could prevent spread by modifying our existing systems? The result could make our workplaces, homes, and communities healthier for years to come.
How Does HVAC Spread Respiratory Viruses?
Studies have taught us that COVID-19 spreads primarily through respiratory droplets that pass through the air when an infected person breathes, coughs, sneezes, or talks.
Normally these droplets do not hang in the air for very long. Due to their size, they typically fall to the floor after a few seconds. However, because COVID-19 particles are so tiny, they can travel on aerosolized microdroplets.
These are much smaller particles (less than 100 micrometers) that can hang in the air far longer. Aerosols can even travel across rooms beyond the social distancing guidelines of six feet., or be carried on strong air currents caused by HVAC systems.
In an early field study of a restaurant in Guangzhou, China, HVAC systems were shown to increase the risk of guests becoming infected across tables. Although the study was limited, results suggested that the microdroplets were carried along with the airflow. Due to a lack of ventilation, these particles likely remained in the air even once the infected guest had departed.
The good news is that additional research has shown us that good ventilation and improved HVAC (when combined with other precautions) can significantly decrease the likelihood of indoor transmission.
System Modifications Can Prevent Spread
Effective modifications to HVAC systems can be as simple as replacing a few parts or as complex as reworking the entire ventilation system.
Each indoor space will have unique constraints that allow for some upgrades and prevent other solutions from being feasible. Whether you are re-working an existing space or constructing a new one, it’s important to focus efforts on two main aspects of design: airflow and sanitation.
Preventing Strong Air Currents
Because aerosols can be carried on air currents much farther than six feet (studies suggest they can travel up to thirty feet), it is important to minimize strong air currents to decrease the chance of indoor spread.
Instead, the ideal design will utilize a slow and steady stream of air. Since many systems have fixed-speed motors, exchanging these for variable speed motors will allow you to decrease the speed and pressure of the airflow.
HVAC layouts that utilize a ductless design can also help curb strong directional currents. However, if ducts are already in place, you will want to redirect the flow so it doesn’t create cross-room currents.
Gaining more control over the direction of the air flow can be a simple way to prevent the spread of COVID-19 indoors. This will also allow other precautions implemented within the environment to remain effective.
First, you will want to be sure that all ducts and vents are cleaned and maintained. Repair any gaps, leaks, or broken seals to ensure no air escapes in unintended directions. This will help focus the stream of air into a laminar flow (one direction and speed, with little turbulence or crossover).
You can also replace old vent covers and hoods with ones that create laminar airflow. This can also make the strategic placement of dividers more effective and can be a cheaper solution than many other modifications.
If the air is generally flowing in one direction with no turbulence, you can implement barriers and doors that stop air from spreading to certain spaces. In this case, the pathways for any possible spread also become more predictable & controllable.
If possible, keep this steady stream of air directed away from people. In larger buildings, the HVAC technician in charge of the upgrades will want to consider how air flows between each room.
In any case, the most ideal direction is vertical. This can be accomplished through the use of air gates, doors, and air curtains. You may also consider incorporating more hoods or floor vents to capture and redirect air that might otherwise escape or reflect off floors or walls.
Air Turnover Rate
In addition to controlling the direction of air currents, you will also want to increase the amount of fresh air coming into the space. One of the pitfalls evident in the restaurant study was that the same air was being recirculated continuously.
By cycling in fresh air and cycling out old (and perhaps contaminated) air, you can decrease the proportion of dangerous particles present in the air and decrease the likelihood of exposure. With fewer contaminated particles in the air, this can also lower the chances of contracting a more severe infection. This is the same tactic used by the University of Colorado Boulder.
There are two main ways to accomplish this:
The simplest way to increase air turnover rate is to increase the number of outlets for the indoor space. If the ducting system is already in place, this may require an extensive re-working of the existing structure, requiring new vents and duct paths.
However, if there are already existing solutions, such as dampers that can be opened to allow more air to flow out of the building, it’s an easy improvement to make.
Run Units For Longer Periods
You can also leave HVAC systems running fans continuously, even when they aren’t needed for temperature control. While this certainly requires more energy, it is an effective method if reconfiguring the ducting system is not an option.
While increasing the fresh air ratio in the room will help dilute the proportion of infectious particles, it can be even more effective to remove contaminated particles each time air is recirculated.
While this won’t be possible with the vast majority of HVAC systems (including PTAC units) due to the high air pressure required for effective filtration, if your system can support high degrees of filtration, this is the best possible solution to help combat the spread of covid-19 indoors.
MERV 11 Or Greater
At a minimum, filters with a rating of MERV 11 will capture 65-80% of particles 1-3 microns in size. This alone should stop a large portion of particles traveling on droplets. However, COVID-19 particles themselves can be much smaller than droplets. Because of this, New York state requires a minimum filtration rating of MERV 13 for some re-opening businesses.
For context, a filter with a MERV 16 rating is essentially equivalent to an N-95 mask and can be effective for preventing recirculation of COVID-19 particles.
If possible, HEPA filters offer the best filtration available. These filters have been shown to stop particles less than 0.1 microns from recirculating, but they do require a significant amount of air pressure. For this reason, it is not always possible to implement them into your existing design.
Whichever filter you choose, you will also want to be sure that your system is completely sealed and maintained by HVAC professionals. You may also want to consider installing pre-filters for additional protection in high-risk areas.
Originally used in hospitals during the 1930s, far-UVC light can also be effective in creating safer air for indoor spaces.
UVC lights have been shown to destroy the outer protein coating of the SARS-Coronavirus, which suggests it may have the same effect on the SARS-COV-2 virus. This type of light differs from regular UV light in that it does not penetrate skin or living human cells.
While UVC light has been used in ducting systems for sterilization, it is only effective when the air is exposed to the light for an extended period of time. Because we also want to increase airflow, this is not the ideal construction.
In order to be effective, the light needs to be installed in such a way that it can continuously project over a room of people – usually overhead. When using a far-UVC lamp, be sure to have it installed by a professional so that it does not cause harm to anyone exposed.
Commonly found in hotels, hospitals, and senior living facilities, PTACs may also provide some useful benefits for making indoor spaces safer.
Because PTACs are already ductless, this can prevent the accidental creation of strong air currents that carry contaminated particles across rooms. Due to their individual design, the risk of inter-room contamination through a centralized HVAC system is no longer a factor.
If you’re considering investing in a PTAC unit, it’s worth knowing that some higher-end models such as the Friedrich FreshAire models increase turnover rate while adding some additional filtration. This is one option if you are looking for a complete system rehaul.
As we approach a new season in the pandemic, new challenges are bound to arise for indoor gatherings. While many of these upgrades can be expensive, especially for struggling businesses, the health safety of our communities relies on each of us doing whatever we can to decrease the risk of spread.
Many experts believe that COVID-19 may not be the last major respiratory virus we have to battle in the years to come. Moving forward, we will need to rethink the way we design our HVAC systems so that if we face another pandemic again, our environments will be set up to slow the spread of infection in our communities.