Non-linear increase of respiratory diseases and their costs under severe air pollution

China is experiencing severe and persistent air pollution, with concentrations of fine particulate matters (PM_2.5) reaching unprecedentedly high levels in many cities. Quantifying the detrimental effects on health and their costs derived from high PM_2.5 levels is crucial because of the unsolved challenges to mitigate air pollution in the following decades. Using the daily monitoring data on PM_2.5 concentrations and clinic visits, we found a non-linear increase of respiratory diseases, but not for other diseases (e.g., digestive diseases) under severe air pollution. We found an increase of respiratory diseases by 1% for each 10 μg m^-3 increase in PM_2.5 when the annual average daily PM_2.5 concentration was less than 50 μg m^-3; while this ratio was doubled (around 2%) with the daily PM_2.5 concentration larger than 50 μg m^-3. Under severe air pollution (PM_2.5 concentration >150 μg m^-3), the respiratory diseases increased by over 50% compared to that in clean days. Children are more sensitive to the severe air pollution. The increase of clinic visits, especially for adults, was observed mainly in bigger (>500 beds) hospitals. Re-allocating medical resources (e.g., doctors) from big hospitals to community hospitals can benefit the respiratory patients due to air pollution. The total medical cost of clinic visits of respiratory diseases derived from PM_2.5 pollution was estimated at 17.2-57.0 billion Yuan in 2014 in China, accounting for 0.5-1.6% of national total health expenditure. Because these medical costs only represent a small part of total health cost derived from air pollution, the reduction of associated health costs would be an important co-benefit of implementation of air pollution preventive strategies.