An ordinary person needs to go to China, breathe in smoke from coal instead of fresh air, walk like hedgehogs in a fog like the Chinese wear masks every day)) then he will understand everything.
The benefits of outdoor recreation are not so much that the air is clean (this is where the catch lies), but how it helps to strengthen the immune system. The benefit lies in the fact that, surprisingly, the air in forests contains poisons and vapors secreted by mushrooms, plants. They are in a small dose and strengthen our immunity, being something like a natural vaccine.
Air has two main "positive" indicators: cleanliness and freshness. Purity is the absence of harmful impurities, and freshness is the saturation of oxygen with electrical particles, that is, the ionization of its molecules. The chemical composition depends on the environment: factories, cars, railways and so on strongly affect it. Ionization is affected by the amount of sunlight and green spaces. The more charged particles in the air, the easier we breathe. The more all kinds of impurities in the air, the worse we breathe, although we do not feel many harmful elements, they can only be "detected" when measured with special devices.
That is, we can independently assess only the freshness of the air. And this is already good: in stale (poorly ionized) air, the infection spreads more easily and it badly affects the nasopharynx, it dries too much. But it is possible to fully assess the purity of the air only with the help of professionals. And so - we follow the rule "by eye": a lot of greenery is always good.