Anyone who recalls the famous Legionnaires Disease outbreak in a Philadephia hotel in 1976 knows the fear that a deadly disease outbreak can create when the source is unknown. We're suffering through a similar fear due to the Covid-19 virus outbreak.
A recent article on AccuWeather.com is the reason this came to mind. The Legionnaires outbreak in 1976 was pegged to festering bacteria in the hotel's air conditioning system which was being pumped into the air in the conference rooms, causing sickness in 221 people and 34 deaths. While Legionnairs is a bacteria and Covid-19 is a virus, the form of spread is similar through airborne transmission.
So I started thinking about airborne transmission inside occupied homes for sale. Any home with a forced air heating and cooling system (most every) would be capable of transmitting the Covid-19 virus when operating, if the home is occupied by someone with the virus. While in most cases, the Covid-19 virus will die within 72-96 hours if left to dry naturally, if it's recently in a home it may be circulating in the air. So the question came up, "Can a high efficiency/high density air filter trap the small size (125 nanometers or 0.125 micrometers) of the airborne virus?"
The short answer is No.
The long answer is "partially". Even the highest pleated MERV rated filter material available for HVAC systems (MERV 13) is not dense enough to "catch" a floating virus. "But those cheap N95 masks we've been wearing are claimed to protect us from the virus", you say. Those masks, if they are honestly made to the N-95 requirements, have multiple layers of varying material, so as the virus goes through, it will zig and zag eventually hitting one of the layers (95% of the time). HVAC filters are one layer of material, and while they may also catch one of those funky suction cups of the virus, some may slip through.
The American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) has posted an information guide for HVAC building enginners. Some of the statements more relevant to home systems are posted here:
"[A] few actions related to HVAC systems are suggested, in case some spread of the virus can be affected:
•Increase outdoor air ventilation (use caution in highly polluted areas); with a lower population in the building, this increases the effective dilution ventilation per person.
•Further open minimum outdoor air dampers, as high as 100%, thus eliminating recirculation (in the mild weather season, this need not affect thermal comfort or humidity, but clearly becomes more difficult in extreme [hot and cold] weather).
•Improve central air filtration to [a] MERV-13 [filter,] or the highest compatible with the filter rack, and seal edges of the filter to limit bypass [of unfiltered air]. [Note, the flow and filtration between a typical one inch filter and some of the 4-5 inch filters is significant.]
•Keep [fan] systems running longer hours, if possible 24/7, to enhance the two actions above.
•Consider portable room air cleaners with HEPA filters. [Note, HEPA filters are fine enough to catch these small virus particles, but their air resistance would nearly stop the air flow of a typical furnace or air handler, due to the filter size, so it is not feasible to install them in this equipment.]
•Consider UVGI (ultraviolet germicidal irradiation). [ Note, some homes have these UV systems added into the supply or return ducts, but we have no way of knowing their impact on and protection of the overall air supply.].
This is a 'game' of chance, and the fewer individuals who come in close contact with each other, the lower the probability for spread of the disease. Since symptoms do not become apparent for days or weeks, each of us must behave as though we are infected. Like all hazards, risk can be reduced but not eliminated... We all have a role to play to control the spread of this disease. HVAC is part of it and even more significant are social distancing, hygiene and the influence we can have on personal behavior."
While wearing a mask and gloves, and washing hands is our best defense against the spread of this virus, it's important to remind buyers and sellers of the importance of a well functioning HVAC filtration system, and annual cleaning of the A/C condensate pan (where live organisms can survive and grow).