How cyclones can cause excess mortality years after they strike

Calculating excess mortality

Stanford researchers Rachel Young and Solomon Hsiang tracked the effects of 501 historical storms that hit the US between 1930 and 2015. Specifically, they analysed how mortality rates within a state changed after it was hit by a TC.

“A big storm will hit, and there’s all these cascades of effects where cities are rebuilding or households are displaced or social networks are broken. These cascades have serious consequences for public health,” Hsiang, professor of environmental social sciences at Stanford’s Doerr School of Sustainability, said.

But these effects are hard to calculate in detail. This is why the researchers chose the excess mortality method. “If mortality systematically rises after TCs, conditional on other factors, we may infer that it was plausibly caused by the TCs,” they wrote. This approach allowed the researchers to study the long-term impacts of TCs on mortality, without having to model every single pathway.

Drawing from pre-existing analysis on the long-term economic impacts of TCs, the researchers decided to study changes in monthly mortality rates in states for a period of 20 years after they were hit by a TC. They took into account the effects of previous TCs, and other miscellaneous factors such as state health care systems and demographics into their analysis.

Long-term impacts

The study found that states’ “mortality systematically increased for about 172 months (14.3 years) following a TC”.

This excess mortality” resulted in “7,000–11,000 excess deaths” per TC on average, much more than what official tallies record. Between 1930 and 2015, these excess deaths amounted to anywhere between “3.6 million to 5.2 million” extra deaths —more than the combined fatalities caused by motor vehicle accidents, infectious diseases, and war in the same period.

“The results show deaths due to hurricanes persist at much higher rates not only for months but years after floodwaters recede and public attention moves on,” Young told The New York Times.

Why the long-term impacts? The researchers point to a few notable factors.

First, economic loss caused by TCs may lead to families exhausting their financial resources, leaving less money available for future healthcare. Second, TCs also lead to families dispersing, and thus weaken social networks that are crucial to maintain one’s health. Third, in the aftermath of TCs, government spending on healthcare may be diverted towards recovery.

These factors particularly impact infants, children, and young adults, the researchers found, resulting in a quarter of all infant deaths in the US.