As dental practices and other offices with shared work spaces reopen, Jianshun “Jensen” Zhang, PhD, of Syracuse University’s Department of Mechanical and Aerospace Engineering offers a three-step plan for improving indoor air quality (IAQ) and helping to prevent the spread of COVID-19 indoors. The plan would work for schools as well, he said.
“Classrooms and open-space offices present a special challenge because of their relatively large occupant density, which can lead to a higher chance of person-to-person cross infection in the space via airborne transmission as well as through direct or indirect contacts,” said Zhang, a professor and director of Syracuse’s Building Energy and Environmental Systems Laboratory.
“As schools and businesses are making plans to reopen, how can the risk of such cross-infection be minimized or prevented?” said Zhang.
COVID-19 is a respiratory illness that can spread from person to person. The virus that causes it spreads typically through respiratory droplets from coughing, sneezing, or talking. Some people without symptoms can spread the disease without knowing they have it. Zhang believes that source control, ventilation, and air cleaning can prevent COVID-19’s spread in indoor spaces.
Source control is the first and most important among all IAQ strategies, Zhang said. For preventing the spread of the coronavirus, that means detecting, tracking, and isolating infected persons and preventing transmission from asymptomatic virus carriers.
“The latter is more challenging in open space office or classroom settings in which air is typically well mixed,” said Zhang. “To reduce the number of virus-containing aerosols emitted to the space from any possible virus carriers present, universal face masking, as well as hand sanitizing before entering the space is essential.”
The next step in air quality is ventilation with a focus on supplying enough clean outdoor air to rooms and offices and effectively diluting the concentration of pollutants.
“Mechanically ventilated classrooms and offices typically have about 20% of their air supplied from outdoors, and the rest is recirculated air. This is done to save heating and cooling energy while maintaining acceptable levels of IAQ,” Zhang said.
“To reduce the risk of the SARS-CoV-2 virus infection, the outdoor ventilation rate should be increased to the maximum operational capacity of the building ventilation system, which can be two or more times of that under the normal operation mode per the existing standard,” he said.
Plus, any recirculated air needs to be filtered with high-efficiency particulate (HEPA) filters or MERV 14 filters to minimize cross-contamination, Zhang said.
Proper air distribution also is essential for making sure the filtered air is reaching the people where they are, he added, noting that this is an area that so far “has been largely neglected in existing guidelines or recommended practices for reducing the SARS-CoV-2 virus infection.”
Most classrooms and open plan offices in the United States use mixing ventilation (MV) for fresh air delivery, he said. Air diffusers in MV are typically located close to the ceiling level, but Zhang recommends that air supply should be brought in through ground ventilation since filtered air from the ceiling mixes with the exhaled breath of people who could be asymptomatic carriers of the virus.
To prevent filtered air from becoming contaminated, Zhang said, it should be pumped into a room through ground-level vents. Zhang recommends using displacement ventilation, which simply delivers the air supply at the floor level but exhausts it through vents in the ceiling.
Air cleaning strategies involve applying air filtration or purification within a building or rooms or at a personal level, such as a properly worn mask. But among all three, there must be high efficiency filters and sufficient airflow, Zhang said.
At the building level, HEPA filters in the recirculated or mixed air duct can reduce the cross contamination between rooms and increase the total clean air delivery rate (outdoor plus filtered air) for diluting the virus concentration in the ventilated space.
Standalone room air cleaners with HEPA filters also can be used as a supplementary measure to further reduce the concentration of virus in the occupied space. Research has shown a range of clean air delivery rate (CADR) from 170 to 800 m3/h (or 100 to 470 CFM) with a median cost of $361 based on a comprehensive survey of off-the-shelf air cleaners available from the most popular online shopping sites, Zheng said.
The results were consistent with an earlier laboratory study in which six portable air cleaners were tested for both particulate and volatile organic compounds removal performance. Zhang said that an air cleaner with a CADR of 722 m3/h (425 CFM) can double the clean air supply for 25 people in a classroom or open plan office.
Zhang said that this can be considered as a cost-effective supplementary measure for rooms where total ventilation airflow rate is insufficient. However, for spaces with displacement ventilation (DV), a room air cleaner should only be used with caution so the desirable airflow pattern of the DV isn’t disturbed.
These IAQ strategies can be used and implemented across multiple scales from an entire building to a room to an individual’s cubicle or personal space, Zhang said.
The study, “Integrating IAQ Control Strategies to Reduce the Risk of Asymptomatic SARS CoV-2 Infections in Classrooms and Open Plan Offices,” was published by Science and Technology for the Built Environment.