Box plots of particle number concentrations among healthy subjects and among COPD patients according to stage.

<p>Healthy subjects (n = 11) and patients with COPD stage 2 (n = 2), stage 3 (n = 5) and stage 4 (n = 6) were included. Horizontal lines represent the median, diamond shapes represent the mean, boxes represent the interquartile range and whiskers represent the range.