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HVAC: Vigorous ventilation can cause spike in viral concentrations

2021/05/19 > Back

We know proper ventilation is key to preventing the spread of COVID-19 and other pathogens, and having an open floor plan where you can open up windows would quickly improve indoor ventilation. If you can’t open your windows or doing so won’t ventilate the entire area, it may be time to invest in an HVAC system (Heating, ventilation, and air conditioning).

HVAC is a mechanical ventilation system that controls indoor temperature and indoor air quality, designed according to the characteristics and use of buildings, and because buildings vary, there are many different ways to improve ventilation and filtration effects to reduce the spread of air pollution in buildings.

Most studies have looked at particle levels in just one room, and for a one-room building, increased ventilation is always useful to reducing viral concentration, but for a building with more than one room, air exchanges can pose a risk in the adjacent rooms by elevating virus concentrations more quickly than would otherwise occur. To understand what’s happening, consider how secondhand smoke is distributed throughout a building. Near the source, air exchange reduces the smoke near the person but can also distribute the smoke at lower levels into nearby rooms.

For the buildings with more than one room, researchers looked at the effects of three factors: different levels of filtration, different rates of outdoor air into the building air supply, and different rates of ventilation or air changes per hour. They found the following ways are beneficial for reducing coronavirus:

1. More clean outdoor air into indoor: Replacement of one-third of a building’s air per hour with clean outdoor air reduced infection risk in downstream rooms by about 20% compared to the lower levels of outdoor air commonly included in buildings. More outside air is clearly a good thing for transmission risk, as long as the air is free of virus. Increase the external intake to 100% or the maximum allowed air intake, rather than opening the indoor ventilation fan to the maximum.

2. Strong filtration: MERV stands for minimum efficiency reporting value, a common measure of filtration. A higher number translates to a stronger filter. It is recommended to select at least the filter level of MERV 13 or MERV 14, which can reduce the peak concentration of virus particles in the connected room by 93%. If the HVAC device allows it, adjust the MERV higher.

3. The team found that a rapid rate of air exchange (12 air changes per hour) can cause a spike in viral particle levels within minutes in connected rooms, this increases the risk of infection in those rooms for a few minutes to more than 10 times what it was at lower air-exchange rates.

Some of the following recommendations are from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE):

1. Turn off any demand-controlled ventilation (DCV) controls that reduce air supply based on occupancy or temperature during occupied hours.

2. If possible, turn on the HVAC system for at least one week before it is actually used.

3. Level of home Air Cleaner: Many manufacturers use the Clean Air Delivery Rate (CADR) rating system to rate air cleaner performance. Others indicate they use High Efficiency Particulate Air (HEPA) filters. In order to select an air cleaner that effectively filters viruses from the air, choose:

3-1. A unit that the right size for the space you will be using it in (this is typically indicated by the manufacturer in square feet).

3-2. A unit that has a high CADR for smoke (vs. pollen or dust), is designated a HEPA unit, or specifically indicates that it filters particles in the 0.1-1 um size range.

Data source: www.pnnl.gov, ASHRAE, www.cdc.gov

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