The invisible pollutant is rapidly becoming a main cause for concern in many areas of China, as Zheng Jinran reports.

After the heavy, prolonged smog that often shrouds China during winter, the return of blue skies in spring and summer is always welcome.

However, sunny summer days may not be as beneficial as one might imagine: instead, people may become unwell as a result of rising concentrations of ground-level ozone, an invisible airborne pollutant.