(Photo: Marius Mellebye/Flickr)

Smoking harms pretty much every organ in the body. It's been linked to lung cancer, heart disease, and, perhaps most disconcerting, the atrophy of brain tissue in the cortex. (That's the wrinkled outer layer of tissue through which we sense our environment, form memories, and generate thoughts.) Fortunately, a new study published today in Molecular Psychiatry finds that, after quitting, the smoking-induced winnowing of brain tissue may be reversed over time.

“In the last 10 years or so there’s been accumulating evidence that the brain has much more plasticity than we thought,” says Sherif Karama, an assistant professor of psychiatry at McGill University in Canada and lead author of the study. “When you stop smoking there seems to be some form of recovery for other organs, so this made me wonder if we wouldn’t find the same kind of effect in the brain.”

In 1947, about 95 percent of 11-year-olds in Scotland were cognitively tested as part of a nationwide census. Those tests were never followed up on, according to Karama, but his co-author, Ian Deary, a psychology professor at Edinburgh, tracked down those testing subjects six decades later.

To find out, Karama and his colleagues at the University of Edinburgh turned to an old survey. In 1947, about 95 percent of 11-year-olds in Scotland were cognitively tested as part of a nationwide census. Those tests were never followed up on, according to Karama, but his co-author, Ian Deary, a psychology professor at Edinburgh, tracked down those testing subjects six decades later. During this second wave of testing, Karama and the researchers used magnetic resonance imaging to measure cortical thickness in 504 of the remaining Scottish cohort, now in their early 70s, that included smokers, ex-smokers, and those who had never smoked.

As expected, they found that current smokers had a thinner cortex than participants that had never picked up the habit. The thinning had occurred throughout the cortex, mostly sparing regions involved in movement and eyesight but hitting the prefrontal cortex—a region associated with personality, emotional stability, and socially appropriate behavior—fairly hard. Ex-smokers fell in between, with less cortical thickness than non-smokers but more than those who still smoked. And the longer it had been since an ex-smoker had given up the habit, the more it seemed cortical thickness had rebounded. While it is normal for brain tissue to wither along with the rest of our aging bodies, overall, the researchers found that the rate of smoking-induced thinning was roughly double the normal rate for aging adults.

“It’s a cross sectional study, and all we can do is infer from this kind of snap shot of one time point, that this this accelerated rate of thinning is due to smoking,” Karama says. But the team has already subjected the Scottish test group to a third wave of testing, three years after the initial follow up MRIs were taken, which will allow the researchers to follow the rates of thinning as the years go on.

This is just another bullet point in a long, long list of reasons to quit smoking.