S1 Appendix -

<div><p>This study presents empirical research about the defensive behavior of air pollution, that is, health insurance purchases. Using transaction-level data from a large insurance company, covering more than half a million insurance contracts from nineteen cities in China from 2014 to 2018, we empirically imply that an increase of 10% in AQI leads to a 0.37% uptick in the number of daily sales of health insurance contracts by the company within the city. The effect is non-linear and is more pronounced when the AQI exceeds 200. Besides, the defensive cost for a one-unit increase in AQI accounts for around 1.70% of individual income annually. We demonstrate that the positive impact of air pollution on health insurance purchases is primarily driven by health management awareness and social interaction.</p></div>