Opening a window is a simple yet vital way to improve indoor air quality and keep buildings healthy. Photo by Alec MacDonald.

When air outdoors is unhealthy because of toxic gas, smoke, or tiny particles of diesel soot, people are advised to protect themselves by going indoors and shutting out outside air. However, breathing the air indoors isn’t always very healthy either, even if people aren’t choking, gasping, or getting very sick. For example, natural gas fumes from home heating and cooking can provoke asthma, and overall indoor air quality is typically two to five times worse than outdoors. Now there are additional concerns about the air we share indoors, as research on COVID-19 has made clear that the primary way it spreads is through small-to-microscopic droplets expelled into the air by infected people.

People have turned to using outdoor spaces during California’s sunny, dry summer. This may help them avoid unhealthy indoor air, but it’s temporary, subject to disruption by heat and smoke events, and doesn’t work for everyone all the time. Instead, using lots of that outdoor air to make indoor air healthier is recommended for better air management in buildings in order to counteract indoor air pollution and deter the transmission of coronavirus.

The coronavirus pandemic has added urgency to the science and mechanics of creating “healthy buildings.” The term was originally coined as a counterpoint to “sick buildings” in which emissions from materials inside a well-sealed building make occupants ill, but now when experts talk about healthy buildings they’re usually referring to best practices for design and operation that supports human health. Ventilation remains one of those key ways to create and maintain healthy homes, schools, and workplaces.

Harvard’s T. H. Chan School of Public Health demonstrated in 2017 that improving air circulation in offices can have a significant effect on both health and productivity. Likewise, stuffy classrooms impact student performance, according to a recent study by Lawrence Berkeley Laboratory and UC Davis, which corroborated other studies showing that many K-12 schools are not meeting basic air circulation standards. A separate 2019 study published in Scientific Reports demonstrated how poor airflow in classrooms could lead to higher flu transmission.

This research is newly relevant as households, school administrators, employers, churches, and others make plans to move forward while coronavirus is still a threat. The Center for Disease Control’s interim guidance for employers and businesses calls for increasing overall ventilation rates, and also increasing the percentage of outdoor air in systems which recirculate the air in a building.

Deborah Bennett, a public health professor at UC Davis, commented that some buildings could have difficulty following that guidance as businesses and offices re-open. “Commercial buildings don’t always have ventilation that meets code,” she explained, “and even when they do, the code is set to require a minimal air exchange rate” which may not be sufficient to protect against coronavirus. To do that, Bennett affirmed, “you need to bring in more outdoor air.”

The American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) has issued several coronavirus-specific advisories to its members, many of whom are responsible for ventilation systems. They recommend that facilities operators should disable demand-controlled ventilation and open outdoor air dampers to 100 percent as indoor and outdoor conditions permit, and keep systems running longer hours (or constantly, if possible).

Although recirculating air is energy-efficient, it can spread virus particles if they are present. Scientists and engineers agree that dampers should be re-set to eliminate air recirculation, and air conditioning units should operate on outside air only. If this isn’t feasible, bringing in as much outside air as possible, and adding filters or using in-duct ultraviolet irradiation on recirculated air, are recommended alternatives.

In addition to re-setting controls, experts say it’s a good idea to have the ventilation systems inspected and repaired if necessary. “Check for stuck dampers, switch out filters,” advised Bennett. Such maintenance is usually not cost-prohibitive and will bring these spaces into the healthy zone for occupants. In some circumstances, a check-up may find a system is unable to meet the new performance recommendations; replacement could be a necessary investment to allow the building to be utilized in the current environment.

The essential point, according to Bennett, is “making sure you’re maximizing ventilation.” She noted that school ventilation studies have already led some districts to install CO₂ monitors in classrooms to measure whether ventilation is adequate. These monitors are recommended for all school districts as part of a set of guidelines for re-opening classrooms produced by the Coalition for Adequate School Housing. CO₂ monitors can also be used in office buildings and other spaces where stale and therefore possibly contaminated air may build up.

What about homes? Again, the key is outside air. In a research letter published in the scientific journal Environment International, the authors wrote, “In residential houses and apartments, normal practices (e.g. segregating infected individuals, opening windows and doors, and using portable air-cleaning devices when practical) to ensure healthy indoor air, should stay in place.”

However, ensuring healthy indoor air may not be as simple as opening doors and windows. Outdoor air quality may be poor. As Bennett conceded, “it’s a judgment call — in a building next to a highway it’s probably not the best idea, but if the outdoor air is fairly clean, go ahead and do it.”

Temperature and humidity also play a role. Residents will be reluctant to open their homes to hot dry air, or cold damp air in winter, so their HVAC systems will need to be effective as well. Jeffery Laing, project manager for BayREN’s Single Family Program focusing on electrification, explained that “electric heat pump [heating and cooling] systems which tie into ducts will be filtered to the recommended MERV-13 level at least.” The MERV rating system indicates a filter’s ability to catch particles; a MERV-13 filter captures 90 percent of most particles and 50 percent of the tiniest particles, as small as .03 microns. However, Laing admitted, “the ducts themselves can pull in 30 to 80 percent of their air from other areas of the home. In that respect, the mini-split non-ducted systems may be healthier.” The non-ducted systems simply recirculate air within a living space instead of sending it through ducts that include filters, but they are less likely to pick up contaminants from gas appliances or garage spaces. In small personal spaces, portable air-cleaners can also help; filters should be MERV-13 or the even more efficient HEPA filters, and they must be kept clean.

Reducing other toxic indoor contaminants, such as fumes from gas-fueled appliances, can also contribute to a home where residents are better protected against a virus which attacks through the respiratory system. Ultimately, Laing commented, “you want outside air, and the issue is how to be sure it’s healthy when it comes inside, and that it stays healthy.”

Adapting to the need for high-volume ventilation in many of the spaces used for daily life will have some side effects. One of those is energy use. Bennett observed that “any time you bring air into a building, you are probably also using energy to either heat it or cool it.” The energy use drops if air is recirculated, because it is already close to the desired temperature, but as noted, this can increase the danger of contamination. In addition, ASHRAE recommends bypassing energy recovery units in ventilation systems, which can leak potentially contaminated exhaust air into the incoming fresh outdoor air supply.

Most changes to ventilation in public spaces will be made based on recommendations and guidelines, in some cases with revised performance standards, rather than through new codes and enforcement. “Codes aren’t going to help us in the near future,” said Bennett, “because they only apply to new construction.”

In most existing buildings, the only regulations that apply are worker safety and public health requirements, although school districts may set additional rules. One government restriction that has already been applied in some areas is reducing the number of people in a space, such as restaurants and salons, which can compensate for less efficient ventilation. Joseph Allen at Harvard’s T.H. Chan School of Public Health refers to this as “de-densification”. “[Y]ou have to think about the space where we have people, how close they are together and what controls are in place,” Allen said in a press conference in May. “There’s no such thing as zero risk in anything we do, certainly not during a pandemic. But if you layer enough of these control strategies on top of each other, you can significantly reduce risk.”

Private property rights apply to homes, so it will be up to residents to create healthy spaces according to the same simple rules underlying the professional advice for other buildings: More airflow is better, outside air is (usually) better than inside air, fewer people breathing in a room is better than many. The proverbial breath of fresh air is the preferred option for staying healthy, indoors as well as out.

Leslie Stewart covers air quality and energy for the Monitor.