Exhaled breath condensate (EBC) of patients with inflammatory diseases has a decreased pH. This could make EBC-pH an interesting tool for studying work-related inflammatory processes, provided that normal work activities would not interfere with the results. We consequently tested whether EBC-pH was influenced by moderate exercise. Fifteen healthy nonsmoking subjects exercised for 30 min on a treadmill by walking at 60% of predicted maximal heart rate. Four EBC samples were obtained: one to learn the technique, one before exercise, one immediately after exercise, and one 60 min later. EBC-pH was significantly increased after exercise compared to before (mean of 8.27 vs. 8.20, p _ 0.001). It remained significantly increased after 60 min (8.25, p _ 0.02). The increase was strongest for the subjects with lowest pH. All pH measures were significantly correlated with each other. Light physical activity increases EBC-pH. The persistence of this increase after the end of the exercise poses a serious challenge if one wants to use this technique for health surveys or for diagnostic purposes. It could prevent the observation of a lowered EBC-pH that results from an inflammatory response. [Authors]