Hygiene Ventilation: How Effective Could the Passive House Approach Be in the Fight Against Covid-19
As we head into August, cities across Canada are opening up again. In many places, buildings have severely reduced their capacities, masks are mandatory, and everyone must remain “socially distanced” at 6ft apart. Together we are trying to figure out what our lives will look like as we move into phases 3, 4, and beyond while an effective vaccine remains pending. Every aspect of the way we live our lives needs to be scrutinized, not just in relation to the current pandemic, but also as we think about combating future viruses. Innovative thought leadership is advancing from every sector, including the building industry which has been abuzz with the term “hygiene ventilation,” since the pandemic began.
Hygiene ventilation focuses on using 100% fresh air to ensure that viruses are not recirculated in enclosed areas, whether commercial buildings or homes. Current research proposes that only 15-20% of air is fresh when using traditional air conditioners, suggesting then that anywhere from 80-85% of potentially contaminated air is being recirculated. Unfortunately, traditional HVAC systems aren’t made to accommodate an intake of 100% fresh air and adapting them to perform such a task would require 4-6x the energy. But Passive House presents itself as the environmentally and economically sustainable option, one that has already been employing hygiene ventilation as one of its basic tenets in the form of balanced ventilation.
Although Covid-19 is thought to be spread via droplets from the nose and mouth, it has been confirmed that the virus can indeed be airborne. Unfortunately, there seems to be a gap in research when it comes to how Covid-19 can be spread and how it is actually being spread. The problem is there is no conclusive research determining to what degree airborne transmissions or aerosol droplets have contributed to the current pandemic. The main difference between respiratory droplets and aerosol droplets is size. Aerosol droplets are lighter and invisible compared to respiratory droplets and our current measures, such as wearing a mask, would not provide an efficient enough defense against them. Although it seems like an obvious conclusion to draw that a balanced ventilation system, drawing in 100% fresh air, while also constantly expelling stale air, would diminish the spread and recirculation of any virus, without knowing what percentage of cases are transmitted by droplets suspended in the air, it is difficult to actually determine just how effective a common component in passive house buildings like an Energy Recover Ventilator (ERV) would be in the fight against Covid-19 specifically.
When it comes to ventilation flow rates and their relationship to airborne transmissions, a study in China also noted there was a lack of strong scientific evidence in recommending a minimum ventilation flow rate for infection control, although the CDC recommends 12 air change per hour (ACH) for hospitals. The same study did however confirm that The Wells-Riley equation, an equation that has been widely used to predict outbreaks of airborne infections in enclosed areas, has been extremely successful, indicating that ventilation rates can reduce infection risk significantly, with greater results yielded from natural ventilation than mechanical ventilation. This suggests that ERVs that allow for the control of various ventilation rates are particularly useful.
ERVs can also provide an additional layer of defence against aerosol droplets as they transfer moisture. A lot of research has determined that indoor humidity levels between 35 and 55 percent reduce the transmission of viruses, because the higher the relative humidity the faster the virus falls to the floor, which may help explain why flu season coincides with the increase in dry air in the winter. Air that is dried out further by indoor heating systems. And when it comes to Covid-19, research shows that just a 1% reduction in humidity is related to a 6% increase in cases.
A specialized ERV with a focus on viral elimination, has also popped up on the market. The CERV-UV Ultraviolet Air Purification. It uses a 18W UVGI (Ultraviolet Germicidal Irradiation) light source, without producing any ozone. That’s a huge innovation, as ozone is naturally occurring by-product of ultraviolet light and it has been speculated that ozone may cause or aggravate respiratory ailments. CERV-UC Ultraviolet Air Purification claims to possess a viral kill efficiency of 85%, a percentage that is based on the UV air purification research of Professor William Bahnfleth at Penn State University. And while 85% sounds like a passing grade, again it is difficult to know just how effective that is without comparing it to an existing viral dose, that is, how many particles it takes to cause an infection, of a specific virus. According to REHVA, the Federation of European Heating, Ventilation and Air Conditioning Associations, small droplets of Covid-19 (<5 microns), generated from a cough or sneeze, can stay airborne for hours. A Coronavirus particle is only 0.8 to 0.16 microns in diameter, so there could be anywhere from 6- 32 viral particles in each 5-micron droplet that is suspended in the air. And there can be anywhere from 40k to more than two million droplets released from a sneeze, fewer than 100,000 from a cough, and 3,000 from loudly speaking. Unfortunately, the “viral dose” for Covid-19, has also not been determined. If the viral dose for a given infection is extremely low, then hypothetically 15% could still get you infected. But if that 15% is removed from the building via a balanced ventilation system without lingering in the air too long, it could reduce the likelihood of infection.
It is important for all of us to do everything we can to prevent the spread of this current and future pandemics. It seems the best approach we have going forward will be a layered one, applying everything we now lowers risk, including wearing masks, social distancing, opening windows where we can, adding more hand washing stations at entrances to buildings and even incorporating balanced ventilation systems, especially as this technology and the research needed to support that technology continues to develop.