The following is a recent email exchange between David Pollock and Art Hunter (both members of the CaCOR Board of Directors).
From: David Pollock
Dec 15, 2020, 10:24:38 PM
Please circulate and take note as useful
From: Art Hunter
Dec 16, 2020, 5:32:25 AM
Thank you David.
The article clearly states what every aerodynamicist knows. Being in a confined “tunnel” or corridor leaves no option for the exhaled air but to maintain some high concentration. Inside your living room is a little bit better as there is a greater amount of diluting air but you have to go outside to have lots of clean air to mix with the exhaled cloud to enormously reduce the concentration. Add in a little wind and the mixing is far more rapid.
Let’s go back to your living room and someone in your bubble returns from grocery shopping and is unwittingly infected and is shedding the virus. Every room this person moves into is now contaminated and every surface is contaminated. Everyone in that bubble is now at risk as contagion is possible. Fortunately, most homes have central heating with a fan that blows air into every room via ducting. This reduces the concentration of the virus and the threat is reduced. Opening windows and doors (for whatever reason for whatever time the door is open) brings in fresh diluting air. Further, only those homes designed to be very airtight have a Heat Recovery Ventilator to maintain internal air quality by delivering about 1 air change per hour of fresh air from outside. This means that the entire volume of air in your home is intentionally changed every hour and opening doors for ingress and egress is a bonus diluting factor. Most homes also have owner controlled ventilation fans and ducts to extract air from bathrooms and the kitchen to the outside. Further, older homes really are leaking a lot of air and could naturally have 5 or more air changes every hour. Yes, heating this incoming air is expensive so people put additional seals around windows and doors to slow the leakage rate. Each home is different but the process is the same.
Knowing the airflow in your home is aerodynamically complex is the first step. Next, you look at these 5 air changes per hour and acknowledge that without your furnace you would soon have the same internal air temperature as exists outside. So, your furnace is heating air 5 times the internal volume of your house every hour. This is a perfect arrangement for a simple Ultraviolet band C (UVC) radiation intervention. In my home furnace, I bought a UVC light designed for insertion into the cold air return duct of a home furnace. It is sized for the air stream heading to the heat exchanger and exposes the virus to about 3,500 times the dosage required to render the virus ineffective. The lamp is on 24/7 even if the fan is not running.
Operationally, when there is an unavoidable penetration of our home by a foreign person (cleaning staff, appliance service calls), I put the furnace air fan on full speed and confirm previous closed ducts into the spare bedroom and other unused rooms are set closed, leading to an increase in the number of air changes per hour in the core areas to, say, 10. Add in the natural leakage of this older house of about 4 air changes, then roughly there are 14 air clean air changes per hour. Let’s say it is every 4 minutes. The design of the furnace ducting is to heat and distribute the now UVC treated clean air from the furnace into the rest of the house efficiently. This is great news. Mixing clean air (from the furnace) with contaminated air (still existing in the living area) and then moving that reduced concentration mixture to be hit with UVC again will reduce the potential virus concentration lower with each pass through the furnace. External door openings are a bonus in reduction of the viral loading of the air you are breathing. Thus, after the foreign person has left the building, I leave the fan running for about an hour prior to returning it to normal (automatic) mode.
This is the way an aerodynamicist views clouds of exhaled air, contaminated or not.
Elevators and cars are dangerous places due to lack of distancing and poor ventilation. If there is no option, get in and out as fast as you can as a risk reduction procedure. Reduction of continuous exposure time becomes important.