Opening windows can bring in pollen, dust, humidity, noise, and a lot of energy loss during cold winters and hot summers.

In a bedroom it might be worse than the elevated CO2 problem.

Nope. Opening windows is very often disallowed - whether socially, or you'd need a hammer, or the space doesn't have windows. Or opening windows would have other downsides - letting in rain, or too-hot/too-cold air, or pollution, or ...