Good article!

I think the key is the fact that plants breathe in carbon dioxide and breathe out oxygen, the reverse of what humans do. Therefore the effect of having a lot of plants around, is much the same as the invigorating effect that we experience when we get out of the city, and go to the lake or to the countryside, where the air is much cleaner than in the city, with all its pollution.

Remember also that most articles or books that you find on relieving stress will have at least some emphasis on deep breathing, even though they may not specifically mention that the biggest factor is actually the quality of the air we do breathe in. While good quality air will produce good result, poor quality will produce poor results.