Heat and humidity on early-life outcomes: Evidence from Mexico

I provide evidence on the detrimental effect of in utero exposure to heat and humidity on children’s health at birth in a middle-income country, Mexico. Humidity affects the body’s ability to regulate heat via perspiration and may thus exacerbate the adverse effects of high temperatures. I link temperature and humidity exposure during pregnancy to individual outcomes regarding 25 million births and stillbirths from 2008 through 2021 using Mexican administrative records. The results show that high wet-bulb temperatures adversely affect birth outcomes. Specifically, each additional day per month with a wet-bulb temperature above 24°C (equivalent to about 40°C at 25% humidity) reduces birth weight by 1.21% and increases the likelihood of preterm birth by 2%. I find that the combined effects of humidity and high temperature on birth outcomes are greater than that of high temperature alone, suggesting that the damaging effects of high temperature can be underestimated when humidity is not accounted for. I also present evidence that the adverse effects of heat on health at birth can be mitigated by adopting air conditioning.