Smoking shrinks the human brain, and when that brain mass is lost, it's gone for good, according to a new study.

Brain scans from more than 32,000 people strongly link a history of smoking with a gradual loss of brain volume. In fact, the more packs a person smoked per day, the smaller their brain volume, researchers found.

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The study also establishes the potential series of events that leads to smoking-related brain loss, with a genetic predisposition to smoking eventually causing decreased brain volume.

"It sounds bad, and it is bad," said senior study author Laura Bierut, a professor of psychiatry at Washington University School of Medicine in St. Louis.

"A reduction in brain volume is consistent with increased aging. This is important as our population gets older, because aging and smoking are both risk factors for dementia."

The study, published recently in the journal Biological Psychiatry: Global Open Science, helps explain previous studies that have found smokers at higher risk for age-related brain decline and Alzheimer's disease.

"Up until recently, scientists have overlooked the effects of smoking on the brain, in part because we were focused on all the terrible effects of smoking on the lungs and the heart," Bierut said. "But as we've started looking at the brain more closely, it's become apparent that smoking is also really bad for your brain."

Scientists have long known that smoking and smaller brain volume are linked, but they haven't been able to figure out which causes the other.

There's also a third factor — genetics. Brain size and smoking behavior are influenced by genetics; in fact, about half a person's risk of smoking can be attributed to their genes.

To untangle the relationship, Bierut and her colleagues analyzed smoking history, genetic data and brain scans on more than 32,000 people gathered as part of a large United Kingdom database that contains info on half a million people.

Analysis revealed that a person's genetic predisposition leads to smoking, and smoking then causes a decline in brain volume.

The shrinkage seems to be irreversible. The brains of people who had quit smoking years before remained permanently smaller than those of people who never smoked, data show.

"You can't undo the damage that has already been done, but you can avoid causing further damage," said lead researcher Yoonhoo Chang, a graduate student at Washington University. "Smoking is a modifiable risk factor. There's one thing you can change to stop aging your brain and putting yourself at increased risk of dementia, and that's to quit smoking."