Risk of transmission goes up indoors, with heavy breathing, and being exposed for longer than 10 minutes. People who exercise, are going to breathe heavily. People on equipment or in a class stay longer than 10 minutes. And gyms are indoors and often not well-ventilated.

The result is that gyms are an excellent space for spreading COVID-19. And measures that reduce the spread for things like walking around a store are not sufficient to reduce it to a safe level in a gym.