Health effects of a forest environment on natural killer cells in humans: an observational pilot study

// Tsung-Ming Tsao 1 , Ming-Jer Tsai 1, 2 , Jing-Shiang Hwang 3 , Wen-Fang Cheng 4 , Chang-Fu Wu 5 , Charles-C.K. Chou 6 and Ta-Chen Su 5, 7 1 The Experimental Forest, National Taiwan University, Nantou, Taiwan 2 School of Forestry and Resource Conservation, National Taiwan University, Taipei, Taiwan 3 Institute of Statistical Science, Academia Sinica, Taipei, Taiwan 4 Department of Obstetrics and Gynecology, National Taiwan University Hospital, Taipei, Taiwan 5 Institute of Occupational Medicine and Industrial Hygiene, College of Public Health, National Taiwan University, Taipei, Taiwan 6 Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan 7 Department of Internal Medicine and Cardiovascular Center, National Taiwan University Hospital, Taipei, Taiwan Correspondence to: Ta-Chen Su, email: tachensu@gmail.com Keywords: natural killer cells; activating NK cells; forest trip; forest environment; urban environment Received: January 05, 2017 Accepted: March 01, 2018 Published: March 27, 2018 ABSTRACT Health effect assessments based on natural killer (NK) cells are an important emerging area of human health. We recruited 90 forest staff members in Xitou, Taiwan and 110 urban staff members in Taipei to investigate the health effects of forest environment exposure on NK cells (CD3 − /CD56 + ) and activating NK cells (CD3 − /CD56 + /CD69 + ) in humans. We also invited 11 middle-aged volunteers in a pilot study to participate in a five-day/four-night forest trip to Xitou forest to investigate the health effects of a forest trip on NK cells and activating NK cells. Results showed that NK cells were higher in the forest group (19.5 ± 9.1%) than in the urban group (16.4 ± 8.4%). In particular, the percentage of NK cells was significantly higher in the forest group than in the urban group among the subgroups of male, a higher body mass index (≥ 25 kg/m 2 ), without hypertension, lower high-sensitivity C-reactive protein, hyperglycemia, without smoking habit, and with tea drinking habit. After the five-day trip in Xitou forest, the percentage of activating NK cells of the invited participants from Taipei increased significantly after the trip to Xitou forest (0.83 ± 0.39% vs. 1.72 ± 0.1%). The percentage of activating NK cells was 1.13 ± 0.43%, which was higher than the baseline value of 0.77 ± 0.38% before the forest trip among the seven subjects who participated in the follow-up study four days after returning to Taipei. This study suggests that exposure to forest environments might enhance the immune response of NK cells and activating NK cells in humans.