Forest terpenes and stress: Examining the associations of filtered vs. non-filtered air in a real-life natural environment

Human health may benefit from exposure to a class of biogenic volatile organic compounds (BVOCs) consisting of isoprene units, known as terpenes. In this double-blind, randomized crossover trial, participants sat in a forest for two 60-min sessions, one in which terpenes were filtered out of the ambient air they breathed, and another in which they were not, separated by a minimum of an eight-day washout period. The primary outcome was the high frequency (HF) component of heart rate variability (HRV; measured continuously). Secondary outcomes included skin conductance levels (SCL) (measured continuously), self-reported stress and affect (measured every 20 min), blood pressure, heart rate, cortisol and inflammatory cytokines (measured before and after sessions). Serum concentrations of terpenes (measured before and after sessions) were also assessed to investigate the association of absorbed dose with these outcomes. We did not observe a significant association of filter condition with most outcomes; although the trends for affect, systolic blood pressure, cortisol, TNF-α, and CRP were all in the hypothesized direction. We did observe a significant association with interleukin-6, which was -0.19 pg/mL lower in the terpenes-on vs. terpenes-off condition, adjusted for baseline (95 % CI: -0.35, -0.03); and SCL over the session as a whole. A sensitivity analysis of the subset of data from participants who completed both conditions supports these findings and revealed additional significant associations with SCL (95 % CI: -1.87, -0.05); and TNF-α (95 % CI: -2.63, -0.10). To our knowledge, this is the first RCT to filter terpenes from ambient air during forest contact.