Pollution is not permanent - it has a half-life in the air/water/ground. (Assuming exponential decay, which is a decent heuristic.) "Permanent" forms of polluti

Pollution is not permanent - it has a half-life in the air/water/ground. (Assuming exponential decay, which is a decent heuristic.) "Permanent" forms of pollution are just those which have unreasonably long half-lives, like some radioactive waste in the millions of years. So CO2, which is of course a big deal now, isn't actually permanent, but its half-life is on the order of a couple of decades (the decay model is one of the big open problems in climate modeling) - hence the talk about how even if we stop emitting today, the problem will still be around. Whereas methane, a stronger greenhouse gas, has a half-life of only around 5-10 years. Particulates have a much shorter half life before they settle to the ground; and, because they are a local form of pollution, they have a very short half-life before being blown away and diluted. If you've been following California news, this is why the massive pollution caused by recent forest fires went away within a few days of the fires dying out. If you're in any big and polluted city, this is why air quality can vary wildly from day to day with weather conditions and economic activity, leading weather forecasts to include an air quality forecast.