> Hiking has the biggest effect though.

I remember reading that pine trees give off a chemical that is a natural human bronchodilator.

One thought of why people love hiking, especially in piney woods, is that the chemical allows humans to process more oxygen which in turn helps them feel more "energized".

I point this out for two reasons:

1. It's a fascinating bit of trivie

2. It highlights that there are MANY confounding variables so it will always be tough to figure out the isolated impact.