The WHO in its July 2020 scientific brief has outlined airborne transmission as one of the possible modes of transmission for the SARS-CoV-2 (COVID-19) virus. In particular, the scientific brief says that aerosol transmission in inadequately ventilated spaces over a prolonged period of time with an infected person cannot be ruled out. Aerosols are defined as suspension of fine solid particles or liquid droplets in air. Broad consensus between WHO and US’ Center for Disease Control has defined particles of size under 5 μm (approximately 1/60th the size of grain of salt) as aerosols and those greater than 5 μm as droplets.
Humans can produce aerosols by coughing and talking. Certain aerosol generating procedures, especially in industrial and healthcare facilities, increase the number of aerosols in a closed room, thereby increasing the risk of transmission. The WHO brief came immediately after an open letter was published by 239 scientists in 32 countries which highlighted 3 measures to mitigate airborne transmission of the virus – the measures primarily focusing on improving ventilation through air flow control.
There have been documented cases of outbreaks in indoor crowded spaces – specifically an outbreak associated with air conditioning in a restaurant in Guangzhou, China and a cluster of coronavirus disease associated with fitness dance classes in South Korea. In another recent University of Florida study, live coronavirus from aerosols was collected at a distance of 7 to 16 feet from patients hospitalized with COVID-19, farther than the two meter guidelines issued for physical distancing norms in India. A recent study of over 7000 cases found that all outbreaks involving three or more people occurred indoors.
The Center for Disease Control (CDC) has confirmed “Indoor spaces are more risky than outdoor spaces where it might be harder to keep people apart and there’s less ventilation”. For indoor conditions, the social distancing rules of six-foot minimum may not be sufficient, as aerosols may travel longer distances even in still air. Once the virus is present in the indoor environment there are only 2 ways to remove it – bringing in fresh air from outside (dilution) or removing the virus from inside the building (filtration). In the US, specific guidelines have been issued for office buildings functioning during the COVID-19 pandemic. Improving ventilation of air in closed areas is critical in ensuring COVID-19 preparedness in office buildings. CDC guidelines have suggested increasing the percentage of outdoor air in HVAC (Heating, Ventilation and Air Conditioning) systems to as high as 100%. This, of course, would have a significant effect on the GHG (GreenHouseGas) emissions footprint of that facility.
Natural ventilation is recommended by keeping windows open to increase outdoor air dilution of indoor air. The ASHRAE association of professionals, a leading body for setting standards related to indoor air quality has suggested guidelines for reducing particle concentrations (due to aerosols) in a closed environment to reduce the risk related to COVID-19, which include- evaluating filter efficiency and its location in the HVAC system; air flow rate through the filter and monitoring the size of the aerosols in the environment. Experts have warned against getting stuck in a “hygiene theater” – where we are scrubbing our way to a false sense of security – without tracking the air we breathe.
Air changes per hour (ACH), a measure of the number of times the air changes per hour within a closed space, is a critical metric to track the efficiency of the ventilation systems in removing airborne transmission. The CDC guidelines for Environmental Infection Control in Health-Care facilities has found an ACH of 2 hrs would take up to 207 minutes while an ACH of 50 should take 8 minutes for 99.9% removal of airborne contaminants. Most patient-care areas in hospitals and health-care facilities have an ACH of 10 – 12. In a research study on the topic of indoor air quality in an intensive care unit of a hospital in a metropolitan city, undertaken by researchers from CEPT University, Ahmedabad had found ICU facilities in a Mumbai-based hospital to have an ACH of 20 in the patient-care areas. The ISHRAE association in India has issued the COVID-19 guidance for Air Conditioning and Ventilation which has recommended an ACH of 10-15 for industrial facilities. Measuring CO2 levels inside a room is also a good proxy parameter to assess the air flow circulation in the room. CO2 levels go up inside a room from exhaling of the people present inside. A well-ventilated room will have around 800 ppm CO2 (outdoor levels tend to be around 400ppm). Any higher than that would indicate the room is under-ventilated and more at risk for transmission.
The evaluation of ACH as a metric for COVID19 preparedness of a building involves measuring the air flow rates provided either by the HVAC system or by the natural ventilation of the room. The essential parameter for ACH is the air flow measured in terms of CFM (cubic feet per minute). ACH then is the CFM of the air flowing into the room divided by the volume of air in the room. Increasing the air flow would directly impact the aerosol particulate matter concentrations in the room. In the study done in the Mumbai-based Healthcare facility, increasing the air flow from 145 to 375 CFM reduced the PM2.5 concentration levels in an ICU from 67 to 37 ug/m3. Increasing the airflow in the HVAC system however would have a significant impact on energy consumption of the facility. A demand control based ventilation which is adaptive to the conditions in the room can help make the system more responsive without compromising the safety offered by higher air flow. Differential airflows in a room with higher airflow in places where people are expected to be sitting for longer durations can significantly lower the overall risk for airborne transmissions of viruses. Most residents of a city, especially those who are looking to buy or rent property in metro cities are closely familiar with the “per sq foot” price of commercial or residential real estate. Commercial prices in cities like Mumbai and Delhi tend to be over Rs. 80,000 per square foot. In the post COVID-19 era, as we begin to unlock and restart our offices and buildings, knowing metrics such as ACH of the building and the airflow CFM in a room would be additional parameters that we would need to track when looking to buy or rent a home or office. With the spread of the virus not looking like ending anytime soon, the better we are able to understand our indoor air quality, the lower would be the risk we would be subjecting ourselves as well as our loved ones. A generation ago, one of the primary criteria our grandparents would consider while buying or renting a house would be the natural “cross ventilation” offered by the room for good quality air flow. Looks like COVID-19 is going to make us go a full-circle from having highly sophisticated ventilation and air conditioning systems to going back to looking for well ventilated homes with ample fresh outside air.