Plasma cytokine levels in the four groups of participants.

Circulating levels of cytokines were measured in the plasma of the four groups of participants using a customized MILLIPLEX Cytokine Human Ultrasensitive magnetic bead panel (EMD Millipore). The differences between smoking and nonsmoking participants were not significant. But increased levels of circulating IL-1β, IL-6, IL-8, IL-17 and TNF-α were observed in the smokers of both groups compared to the corresponding non-smoking group.