The Staten Island ferry takes commuters to New York's financial district days after the collapse of the twin towers. [Ruben Sprich/Reuters]

PROBLEM: When an entire population undergoes a stressful situation, public health suffers in sometimes unexpected ways. After the 9/11 attacks, U.S. women were more likely to miscarry their male fetuses, which are thought to be more fragile than females. Aside from the obvious trauma, measured in deaths, fear, and war, it's likely that such events affect us in more subtle ways, too.

METHODOLOGY: Michael Pesko, at Weill Cornell Medical College, took data from a survey conducted by the Centers for Disease Control and Prevention that included a total of over 1.6 millions Americans between 1994 and 2004. Within that nationally representative sample, he looked for population changes in self-reported stress and smoking rates surrounding the 9/11 terrorist attacks.

RESULTS: In the last three months of 2011, self-reported stress increased by 11 percent, regardless of how close to the attacks people lived. And between then and 2003 there was a 2.3 percent increase in smoking: from 950,000 to 1.3 million former smokers are estimated to taken the habit back up. People who had never smoked, on the other hand, did not start. While stress levels returned to normal soon after the attacks, the uptick in smoking persisted.

IMPLICATIONS: Pesko assumes that the re-uptake of smoking was an effort at relieving the stress of the terrorist attacks; that nicotine is so addictive might explain why it remained a problem long after the more acute effects of the attacks had passed. Even taking into account increased tax revenue from more people buying cigarettes, his "back-of-the-envelope" calculation estimates that the cost to the government from the increase in smokers could have been anywhere from $530 to $830 million.

The full study, "Stress and Smoking: Association With Terrorism and Causal Impact," is published in the journal Current Economic Policy.