There are a growing number of articles that show a clear connection between increased mental wellbeing, stress relief and even immune system activation. when exercise is conducted in forested green spaces. In the UK and Japan, they study this phenomenon extensively, citing active components that are directly responsible for all these positive effects. That fresh pine smell, for example, is actually alpha and beta pinene. These naturally occurring volatile aromas increase Natural Killer cells, which help us battle things like viruses and tumor cells.
You don't have to smell the woods, to get the benefit though. In the UK, researchers found that just looking at peaceful, natural scenery while exercise increased several health parameters.
So, exercise in nature if you can, but also consider switching from zoning out with a TV show on the treadmill to treating yourself to some seriously beautiful views instead.
Li Q1, Kobayashi M, Wakayama Y, Inagaki H, Katsumata M, Hirata Y, Hirata K, Shimizu T, Kawada T, Park BJ, Ohira T, Kagawa T, Miyazaki Y. Effect of phytoncide from trees on human natural killer cell function. Int J Immunopathol Pharmacol. 2009 Oct-Dec;22(4):951-9.
Li Q1, Kobayashi M, Inagaki H, Hirata Y, Li YJ, Hirata K, Shimizu T, Suzuki H, Katsumata M, Wakayama Y, Kawada T, Ohira T, Matsui N, Kagawa T. A day trip to a forest park increases human natural killer activity and the expression of anti-cancer proteins in male subjects. J Biol Regul Homeost Agents. 2010 Apr-Jun;24(2):157-65.
Humpel N., Owen N., Iverson D., Leslie E., Bauman A. Perceived environment attributes, residential location and walking for particular purposes. Am. J. Prev. Med. 2004;26:119–125. doi: 10.1016/j.amepre.2003.10.005. [PubMed] [Cross Ref]
De Vries S., Verheij R.A., Groenewegen P.P., Spreeuwenberg P. Natural environments—Healthy environments? An exploratory analysis of the relationship between greenspace and health. Environ. Plan A. 2003;35:1717–1731.