The Fulton County School System says it is installing hospital-grade filters in its schools’ heating and air conditioning systems to fight the airborne spread of the coronavirus.

FCS has installed the filters in six of its 11 Sandy Springs schools and expects to have them in all facilities in November, according to FSC spokesperson Shumuriel Ratliff. Ratliff did not respond to a request to identify the local schools with the filters.

FSC returned to optional in-person classes Oct. 14. Since then, it has already had to close two high schools due to COVID-19 diagnoses, though there is no information about how those students or staff members got infected.

Asked why the filters were not installed before the Oct. 14 reopening, FCS’s Facilities Services Department cited supply issues.

“We are committed to updating all filters as quickly as possible,” the department said in a written statement provided by Ratliff. “Filter material is limited due to the worldwide demand on MERV 13 filters as a result of COVID, and we are replacing the filters as they become available.”

“MERV” stands for “Minimum Efficiency Reporting Value,” a scale of how well an air filter removes variously sized particles from the air on a scale of 1 to 16, with higher being better. MERV is a standard of ASHRAE, an international HVAC industry standards group headquartered in Brookhaven.

A MERV 13 filter is at least 85% efficient at removing particles that are 1 to 3 micrometers in size, according to ASHRAE’s website. A coronavirus by itself is smaller than that and could get through such a filter, but it typically is spread through the air when it is exhaled by people within saliva droplets that are around 1 micrometer, according to ASHRAE. That means the filters could capture most of the particles that could spread the pandemic through the air.

Filters with a MERV rating of 13 to 16 are used in such high-sensitivity places as surgical hospitals, according to the website of the National Air Filtration Association, a Wisconsin-based industry group. The grocery store chain Lidl announced Oct. 20 that it is installing filters with a rating of MERV 13 or higher in all of its U.S. stores by the end of the year.

ASHRAE’s website says that for fighting COVID-19, MERV 13 is the minimum standard and that MERV 14 or better is “preferred.” However, the benefits of filters depend on the specific details of an individual building’s HVAC system.

Another standard filter rating is “high-efficiency particulate air” or HEPA, which may be more familiar from a variety of consumer products. According to ASHRAE, HEPA filters are even more efficient than MERV filters. “By definition,” ASHRAE says, HEPA filters must be 99.97% efficient at capturing particles 0.3 micrometers in size.

Since April, the Centers for Disease Control and Prevention have warned about the possibility of COVID-19 spreading through droplets within indoor air, but it took several months for that idea to catch on. Now there is great demand for a variety of systems to filter or disinfect the air, ranging from improved ventilation to virus-killing ultraviolet lights mounted within HVAC systems. A wide variety of local businesses and institutions, from restaurants to MARTA, have upgraded their HVAC systems due to the pandemic.

Experts say that, like precautions, HVAC system upgrades can only reduce the risk of getting COVID-19, not eliminate it completely. HVAC systems in particular vary widely in their capabilities among buildings.