I agree w/ @linseymarr that lower than 800 ppm is often needed, but we also need recognition that what is a "safe" CO2 concentration in on environment may not be safe at all in another, and vice-versa. https://twitter.com/linseymarr/status/1352013264549531654

The actual max acceptable value depends on # of people present, typical time spent in environment, mask requirements, respiratory minute volume, and effectiveness of controls.

For 25 people spending 2 hrs in the same indoor space, a rough estimate based on a quanta generation rate similar to that in Restaurant X is CO2 < 695 ppm for an infection probability of < 5% w/o masks or controls. Assuming worst (no masks or controls) provides a safety factor.

For many restaurants it seems like 700 ppm(ish) is probably more appropriate than 800 ppm, but perhaps still too high for workers. For less people present, the situation is worse (greater rebreathed fraction contribution from an infector) for same CO2 concentration.

Designing for those who are present for much of the time (workers) with running time-averaged CO2 concentrations (which makes more sense than instantaneous) would bring an additional safety factor to customers.

Perhaps rebreathed fraction "audits" are needed for different establishments to determine color-coded tags that identify maximum acceptable CO2 concentrations (600, 700, 800, etc.) One size fits all is easy but not representative of what is safe in different indoor environments.

That is "one" and not "on". Sorry.