Air pollution can cause oxidative stress and adverse health effects such as asthma and other respiratory diseases. The underlying chemical processes, however, are not well characterized.  We present chemical exposure-response relations between ambient concentrations of air pollutants and the concentrations of reactive oxygen species (ROS) in the epithelial lining fluid of the human respiratory tract. In highly polluted environments, ozone and fine particulate matter containing metal ions and organic aerosols can increase ROS concentrations to levels characteristic for respiratory diseases. The work is published in the journal Scientific Reports (http://www.nature.com/articles/srep32916).