Smoking does not only affect your heart and lungs, but it can also shrink your brain permanently, and the damage done is irreversible, according to a new study.

Although quitting smoking can prevent further loss of brain tissue, stopping smoking will not restore the brain to its original size, revealed the findings, published in the journal Biological Psychiatry: Global Open Science.

The study also explains why smokers are at high risk of age-related cognitive decline and Alzheimer's disease.

Since people's brains naturally lose volume with age, smoking effectively causes the brain to age prematurely, said researchers at the Washington University School of Medicine in St. Louis.

"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," said Laura J. Bierut, Professor of Psychiatry at the varsity.

"But as we've started looking at the brain more closely, it's become apparent that smoking is also really bad for your brain."

For the study, the team analysed de-identified data on brain volume, smoking history and genetic risk for smoking for 32,094 people.

They found a link between the history of smoking and brain volume; genetic risk for smoking and history of smoking; and genetic risk for smoking and brain volume.

Further, the association between smoking and brain volume depended on dose: The more packs a person smoked per day, the smaller his or her brain volume.

Using a statistical approach known as mediation analysis, the researchers determined the sequence of events: genetic predisposition leads to smoking, which leads to decreased brain volume.

"It sounds bad, and it is bad," Bierut said. "A reduction in brain volume is consistent with increased ageing. This is important as our population gets older because ageing and smoking are both risk factors for dementia."

And unfortunately, the shrinkage seems to be irreversible. By analysing data on people who had quit smoking years before, the researchers found that their brains remained permanently smaller than those of people who had never smoked.

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