题目：Does air pollution increase weight gain and obesity? Evidence from China
We provide the first study on estimating the causal effect of air pollution on weight gain and obesity. Using the China Health and Nutrition Survey (CHNS), which provided detailed longitudinal health and economic information for 13,226 adult individuals over 1989-2011, we find significantly positive effect of air pollution, instrumented by thermal inversions, on body-mass index (BMI). In particular, a one standard deviation (26.19 µg/m3) increase in the average concentration of PM2.5 in the past 12 months increases the BMI by 0.55 standard deviations, and further increases the probability of being overweight (BMI>=25) and obese (BMI>=30) by 0.57 and 0.31 standard deviations respectively. Our paper contributes to the literature by identifying a new cause of obesity, and shed light on the potential cost of air pollution.