There is a study relating rise of CO2 to loss of IQ in indoor situations. The outdoor concentration of CO2 is never and won't be high enough in the future to have a significant effect, but climate change will raise the base level which will increase indoor levels - this may have the effect of making us a bit dumber.

However, I think improvements in insolation are much more significant in this respect.

Phytoncides (antibiotic plant metabolites) are thought to boost immunity in a very direct way: https://pubmed.ncbi.nlm.nih.gov/20074458/

Some cultures practice „forest bathing“ which would probably reap the greatest benefits (besides living in the woods, obviously).

Maybe this sounds gimmicky, but boosting immunity has a lot of downstream effects, from heart health to brain health to structural integrity and longevity. The body can put the resources it otherwise needs to do damage control into useful things like producing enzymes and correcting metabolic shortcomings.

A few minutes with window open and you are back to the outside CO2 levels, so I don't think it's just CO2 (unless you have really high levels of it, sensors are pretty cheap nowadays and can serve as a good reminder to let in some fresh air).

Not sure about pollen but trees produce chemicals that kill pathogens which attack them. Which is why it's healthy to breath forest air especially when you have a cold. Plus they are great at filtering air.

btw 'Think of time in nature as a "multivitamin" — it's best to take it every day' - I don't know how this ends up in articles like this with so much research against it (multivitamins that is), they might have as well been suggesting draining your blood everyday.

Except people don't open their windows because it's cold or the someone in the office doesn't like the breeze or the temp or the sound.....

Loosing blood has many beneficial effects including increasing blood oxygen levels as new blood cells are generated with better mitochondrial activity

If you are talking about office environment then it's done by HVAC system and CO2 is often being monitored.

Re blood, I said every day. Losing blood can have some benefits. Just like taking multivitamin can if you have high deficiency of something it contains, although you likely need higher dosage of that thing then. But you don't want to be doing it every day. I mean, I admit I'm less confident about that statement regarding losing blood that I am regarding multivitamins, but draining blood used to be popular for.. pretty much everything and modern medicine somehow got rid of that.

Yes - so better comparison would be “think of it like leeches- best applied regularly to reduce your circulating blood volume”

Those actually still are (rarely) used in modern medicine because they inject pretty efficient anti-clotting substance.