Air Quality and Birth Outcomes: The Mount St. Helens Eruption - A Natural Experiment

The eruption of Mount St. Helens in Washington State on May 18th, 1980 resulted in a massive and unanticipated particulate air pollution shock. I use the incidence of the ash cloud fallout across Washington as a natural experiment to estimate the effect of a particulate shock on birth outcomes and infant mortality. I find that while there is no statistical effect on infant mortality, there were statistically fewer low birth weight babies born. The measured effect indicates about 230 fewer babies were born as a result of exposure while in the womb. These results suggest that about 1 in 10 pregnancies were terminated from the 20th percentile group of the weight distribution, increasing to almost 1 in 5 in the lowest 5th percentile. The effect is found to be the strongest in the early stages of pregnancy. Using these findings I estimate that the cost of a single particulate shock of typical magnitude on a metropolitan area of median size is $3.7 million. This quasi-experimental treatment is unique since the ash particulate is bioreactively inert and uncorrelated with other pollution. It clearly identifies the pernicious nature of all particulates, not just particulate categorized by source. In addition, the discrete timing of the event identifies the most vulnerable window for suspended particulate shocks on expectant mothers.