To go with their gaunter-than-average frames, smokers may be surprised to learn the outer layer of their brain — the cortex — also gets thinner as they age. A new study from McGill University and several other institutions has found that among lifetime smokers and those who recently quit, the areas of the brain responsible for memory, language, and perception start to wither away.

Ever since the Surgeon General addressed the public on the physical consequences of tobacco smoking in 1964, scientists have been heavily invested in using hard data to run the habit into the ground. At least over the last decade, research has turned toward the neurological risks of smoking — in 2004, for example, a study was published that showed brain regions involved with balance and coordination were hit particularly hard. Since then, the evidence has only mounted.

“Cortical thinning seems to persist for many years after someone stops smoking,” said Dr. Sherif Karama, lead author of the new study and assistant professor of psychiatry at McGill. Karama and his team recruited 244 men and 260 women to carry out their study, which was five times larger than any related study prior. The average age of participants was 73.

Subjects underwent MRI scans and sat down for in-depth interviews about their current and former smoking habits. As predicted, non-smokers had the thickest cortical lining, followed by ex-smokers and current smokers. And while there may be good news in that the brain can heal itself over time, the recovery process is slow. Even 25 years after quitting, heavy smokers showed a thinner cortex than non-smokers.

Yellow/orange areas are regions where the thickness of the cortex at age 73 is associated with the amount of lifetime smoking; the greater the amount of lifetime smoking, the thinner the cortex. Molecular Psychiatry, S Karama, IJ Deary et al.

Cortical thinning isn’t confined to minor memory lapses or poor balance in cigarette smokers. In fact, it has been found associated with numerous other bodily dysfunctions, including psychopathy, Alzheimer’s disease, and, in some cases, schizophrenia. As the brain deteriorates, the neurons that once resided in each dying layer inevitably get subtracted from the overall total, impairing the organ’s function.

On the other hand, links have been found between smoking and nearly every major neurological disorder. The link between smoking and Parkinson’s and Alzheimer’s emerged in 2000, and then reappeared in 2010. Other research says smokers are more likely to display psychopathic tendencies, although the direction of causation is still foggy. It may be the case that people with the predilection to smoke already possess certain antisocial traits. Impulsivity, for instance, seems to crop up in both cases.

Whether all cognitive function returns with time is still unknown and up for future investigation, Karama and his co-authors conclude in their report. “Findings here suggest that cortical recovery could take as little as a few weeks to more than a theoretical 140 years depending on the amount smoked over one’s lifetime.”

Source: Karama S, Ducharme S, Corley J, et al. Cigarette smoking and thinning of the brain’s cortex. Molecular Psychiatry. 2015.