The coronavirus pandemic taught Americans to wear good masks, avoid big crowds and test often to stop the spread of viral illness. Also crucial, but not fully understood early in the crisis, is that indoor air quality is key to reducing viral transmission. Fortunately, the Centers for Disease Control and Prevention released on Friday pathbreaking new guidance to improve ventilation in buildings, with specific targets that can make a difference the next time a novel virus strikes — or even when a bad flu is going around.

Although it wasn’t widely understood when the pandemic began, virus particles spread person-to-person when exhaled in both larger droplets that fall to the ground and smaller aerosols that hover for hours. Clouds of virus particles do not disperse easily in stagnant indoor air.

Most buildings in the United States were constructed to meet minimal air quality standards unsuited to mitigating respiratory disease. According to Joseph G. Allen, an associate professor and director of the Healthy Buildings program at Harvard’s T.H. Chan School of Public Health, this “sick building” problem long predates the pandemic but the pandemic experience is helping to accelerate change. It showed that simple improvements such as bringing in more outside air, upgrading filtration and enhancing ventilation slow disease spread. “Healthy” buildings can also reduce the transmission of other respiratory ailments, including influenza and RSV; can help reduce asthma exacerbations and allergy symptoms; and can improve cognitive functioning for people working and studying in them.

The CDC previously recognized the importance of improving air quality, but on Friday the public health agency published revised guidance that for the first time prescribed a specific target of five air exchanges per hour. This means supplying, or exhausting, the amount of air in a space five times over the course of an hour. The target can be met in different ways — with a combination of outside air and improved filtration and ventilation.

Most buildings in the past were equipped with filters known as MERV-8 that are designed to protect equipment and capture about 20 percent of airborne particles. The CDC has now called for buildings to install MERV-13 filters that can capture 80 to 90 percent of particles.

Although substantial pandemic-era funding remains available, a CDC survey of U.S. schools conducted from August to December last year found only a patchwork of ventilation improvements. Many schools were built to accommodate only three air exchanges per hour.