Some older buildings have had their systems retrofitted or fixed to ensure maximum air changes and good “mixing” of that air into the rafters away from people, though most constructed in the past 20-plus years were already capable of high-quality ventilation.

“At the time, those things weren’t done, per se, for pandemic-based scenarios,” said Ryan Sickman, global director of sports at the Gensler architecture firm. “But they were done for very similar things. It was cleaner, it was removing bacteria from the air, it was removing particulates from the air. It was providing for a vast number of people clean air, and that’s an important part of the experience.”

Of course, it takes more energy to run those systems, but it’s considered worth it to bring fans and their dollars back. The NHL has 12 pages of arena protocols outlining air change and other requirements; 18 of its 31 teams allow fans or plan to soon. In the NBA, it’s 17 of 30, though that number could quickly increase to 20.

“Subject to our protocols and what local government is mandating, we think we can be safe and protective,” NHL Commissioner Gary Bettman said.

Coronavirus cases being down and vaccination rates up have made the conditions right for easing fans back in, said Shandy Dearth, director of undergraduate epidemiology education at IUPUI’s school of public health in Indianapolis. She said if the U.S. were in a surge right now, it would be hard to mitigate risk of infection.