PRF1 and chronic obstructive pulmonary disease: They found significant decreases in the expression of perforin (natural killer cells(NK cells), NK-like, CD8+ T cells) (p < 0.05) and granzyme B expressions (NK cells, NK-like, cluster of differentiation (CD)8+ T cells) which were associated with the pathogenesis of chronic obstructive pulmonary disease (COPD) after 1 day spent in a forest environment, while there was no differences detected in the urban intervention group [31].