Getting some estimates with a simplistic model. It's probably ok for looking at relative risks. You may have to sign up (free) to use the site.
the high level is you want to have a lot of ventilation - outdoors is best. Playing with the sliders on the top set of graphs is more informative than looking at the fixed examples below.
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The model provides a rough estimation of risk, Jimenez cautions. (Of course, no model can explain exactly what will happen in reality.) Still, it can provide valuable clues to the relative risks of different activities. The risk also depends on the prevalence of the disease in your area, which users can input into the model to change the potential number of infected people in a given group.
As with any model, the calculations must make some assumptions, such as requiring the air to be mixed, so the virus is dispersed throughout the room. (This is why the model does not account for close proximity with other people.) That's not always the case in the real world, but it is appropriate for many situations, says Shelly Miller, an expert in indoor air pollution at the University of Colorado Boulder, who led the modeling effort to characterize the potential aerosol spread during the Washington choir practice.
The model underscores the importance of widespread use of masks and the risks of COVID-19 transmission in crowded rooms and poorly ventilated conditions—and in any of these settings, time is key, says Linsey Marr, a civil and environmental engineer at Virginia Tech who specializes in airborne transmission of viruses and provided feedback on the model. The longer anyone spends in a poorly ventilated or crowded space, the greater the predicted risk of falling ill.
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Here's a more detailed model, again only aerosol risks, but the simpler model is better for those of us trying to get a handle on risky activities as opposed to worrying about the specifics of room design and specifics about occupants and their activities.