As lockdowns lift and people head back to office buildings and schools, many are worried about being infected with COVID-19 through airborne particles known as aerosols in these enclosed spaces.

But here’s some good news: Something as simple as opening a window can be a big help.

When an infected person coughs or sneezes, large droplets containing the coronavirus fall to the ground, an observation that influenced the development of the 6-foot social distancing standard. But tiny droplets can linger longer and travel farther. Singing, talking loudly and even breathing heavily can send these droplets flying.

Removing virus aerosols from a room involves a process called air exchange, in which fresh air enters and replaces stale air. The easiest way to do that? Open a window or door. Even better, create a cross-breeze by opening windows on two sides of a room

But that may not always be possible or practical, especially in office buildings or in hot or cold climates. That’s why the focus now is on air-conditioning systems.

Some buildings have systems that can be adjusted to take in more air from the outdoors, but you likely have no control over those. At home, though, it’s another matter.

Start by upgrading your air-conditioning unit’s filter. Look for one rated MERV 13 or higher. High-efficiency particulate air, or HEPA, filters are good options, but not all systems can handle them. Interestingly, older homes have an advantage over energy-efficient new ones because they aren’t sealed as tightly. That means fresh air can leak in through cracks in and stale air can leave the same way.

And don’t forget that your best filter is a face mask. Wear one everywhere to help keep the aerosols, and the virus, at bay.