Most homes in the US are are poorly ventilated. There is no mechanical system supplying outside air indoors and exhausting indoor air outside. The way homes are typically ventilated is by opening a window and/door, or by air leaking in (or infiltrating) through unintentional openings and cracks in the building shell. Homes recirculate indoor air through coarse filters when the heating or cooling system is operated. Newer homes and energy efficient homes (such as Passive Homes) often have mechanical ventilation systems called energy recovery ventilators or heat recovery ventilators.
Why is this important? Because the risk of infection with SARS-CoV-2 increases substantially when you are sharing the air in a poorly ventilated space with an infected person. The majority of transmissions happen within the household. And then super spreader outbreaks drive the pandemic forward. And a key common feature to both household and super spreader transmissions is low outside air ventilation rates and very low loss rates of viral aerosol particles indoors (losses happen by surface deposition, virus inactivation, air cleaning, outdoor air ventilation).
The WHO released a recent video describing why ventilation is important and suggested that 6 air changes per hour (ACH) was a target Air Exchange Rate (AER). I understood this recommendation to be a goal for schools, offices, restaurants, commercial spaces. This paper by Gao et al. 2009 shows how increasing the AER from 1 ACH to 6 ACH can decrease the airborne infection attack rate from 100% to 70%, and the percentage of daily infectors decreases to below 10% (see figure below).
We have been focusing a ton of attention on increasing outdoor air ventilation rates in mechanically ventilated buildings by running the HVAC systems as closed to 100% outside air as possible and improving the air cleaning on the recirculated air by upgrading the filtration efficiency or adding ultraviolet lamps to the ducts. And we have been recommending supplemental air cleaning where possible and needed.
Now we need to focus on homes – this is where we will spend much of the colder winter months in the northern climates. I wonder if people are thinking that it is more risky to go out to eat at a restaurant than it is having people over for dinner to your home. The only problem is the AER in your home is very low and if you have a group of friends over for dinner for a couple of hours, airborne virus particles (aerosols) can build up inside if someone comes over that is infectious, and increasing the risk of infection. One friend shared with me that early on in the pandemic they had a large dinner party to celebrate a birthday in their social circle and everyone at that dinner got COVID-19. Due to a combination of poor ventilation, long exposure time and lots of aerosol being released by talking and laughing.
Here is a brief summary of some of the papers that provide estimates of home AERs (all numbers expressed as air changes per hour or 1/h). Note that all studies report AERs < 1 air change per hour. (This was the loss rate we estimated for the Skagit Chorale Valley outbreak that led to 87% infection rate).
Shrestha et al. 2019 https://doi.org/10.3390/su11092667: This was a study funded by the EPA from my research group. We visited 226 low-income homes in Denver metro area. The annual average infiltration rate (similar to AER but with no natural ventilation through windows/doors and measured using a blower door test) was 0.64 ACH (see figure below). Our results also show, similar to other studies, AAIR tends to increase as age of home increases and as volume increases. Energy efficient homes had AAIRs 17% lower than non-energy efficient homes.
Beko et al. 2016 http://dx.doi.org/10.1016/j.buildenv.2016.05.016 studied different methods to measure AERs. Their study showed home air exchange rates vary daily and seasonally and in the winter they were always below 1 ACH (see figure below). they also investigated how opening the window changed the AER and found that the size of window opening was linearly related to AER. With open doors within the home, the air was well mixed within the same floor, not so between floors.
Yamamoto et al 2010 https://doi.org/10.1111/j.1600-0668.2009.00622.x measured AERs in three different states and found that the median AER across three urban areas in TX, CA, and NJ and over all seasons was 0.71 air changes per hour (n=509). In TX, the AER was lower in summer cooling season (0.37) compared to winter (0.63). In CA, the AER was higher in summer (1.1) compared to winter (0.61). And in NJ AER was similar across seasons (see figure below).
Murray and Burmaster 1995 https://doi.org/10.1111/j.1539-6924.1995.tb00338.x conducted the most extensive effort to date documenting residential AERs and provides values for different regions of the country and for different seasons. They report that the median AER was 0.51 ACH, and the mean AER was 0.76 (n=2844) for all homes studied (see figure below).
What does all this mean? It means that to lower your risk of getting or transmitting the coronavirus during the winter when many of us live in cold climates and we spend a ton of time in our homes, and we want to have friends and family over, or go to someone’s house for dinner like we used to…we need to remember that generally homes are poorly ventilated. So what are the tools we can use to lower risk?