An estimated one million former smokers took up cigarettes again after the 9/11 attacks on America, a new study suggests. The stress of the events of that day led to the resurgence of smoking among people who had previously quit, the researchers report.

Researchers at Weill Cornell Medical College in New York concluded in the two years after 9/11, smoking increased 2.3 percent among adults who had formerly smoked. This resulted in between 950,000 and 1.3 million adult former smokers becoming smokers again because of terrorism, HealthCanal reports. In contrast, they found no increase in smoking following the 1995 Oklahoma City bombing.

The results are published in the journal Contemporary Economic Policy.

“This study provides the first unbiased estimate of the effect of stress on smoking, and the finding that there was such a big increase in smoking nationwide, seemingly due to one event, is extraordinary, and surprising,” study author Dr. Michael F. Pesko said in a news release. “It sheds light on a hidden cost of terrorism.”

Dr. Pesko used data from the national Behavioral Risk Factor Surveillance System, which tracks yearly rates of risky behavior including smoking. He looked at self-reported days of stress, and whether former smokers began smoking again. “I was really surprised to find that former smokers across the nation resumed their old habit,” he said. “I was expecting to see impacts just in the New York City area — or, at most, the tri-state area.”

In the future, after stress-inducing events, public health officials might want to offer free nicotine replacement therapy, he suggested. Health professionals could be advised to conduct more substance abuse screening during regular medical appointments after events that are likely to cause widespread stress, he added.