People who smoked heavily in the past before quitting carry a long-term risk for stroke, warned doctors at the American Stroke Association’s International Stroke Conference.

“How much you smoked matters more than how long ago you quit,” Dr. Sachin Agarwal told Reuters Health. “Smokers should quit as soon as possible, and new smokers should be aware that there will be a lifelong risk. Cessation can reduce risk but it can’t erase it,” he said.

Agarwal and colleagues at Johns Hopkins Hospital in Baltimore, Maryland, used magnetic resonance imaging to compare aortic wall volume, as well as carotid (neck) artery thickness, in 27 former smokers and 15 people who had never smoked.

Many strokes occur when one or both of the neck arteries narrow due to plaque build up and a clot plugs the artery and shuts off blood flow to the brain. Thickening of the neck arteries precedes the build up of plaque and contributes to increased stroke risk.

The former smokers had quit an average of 30 years previously and had an average exposure of 20 pack-years - derived from the number of years smoked and the number of packs of cigarettes smoked daily.

They were divided into four groups: group 1 had smoked 20 pack-years or less and had quit more than 15 years previously, on average; group 2 had smoked 20 pack-years or less and had quit fewer than 15 years previously; group 3 had smoked 20 pack-years or more and had quit more than 15 years or more; and group 4 had smoked 20 pack-years or more and had quit within the past 15 years.

Comparing all of the former smokers with the never smokers, Agarwal’s team found that the aortic walls of former smokers were thicker than the aortic walls of never smokers. Wall dimensions increased progressively with the length and intensity of smoking history.

Similarly, carotid artery wall volume was higher in former smokers than never smokers, with differences correlated with smoking history.

“We saw there was a strong and direct relationship between arterial wall thickness and increasing levels of risk of former smoking,” Agarwal said in a statement.

The associations between smoking and arterial wall thickness remained significant after accounting for age, gender, history of high blood pressure, diabetes, alcohol intake, and cholesterol levels.

Agarwal said the message is clear: “Smokers should quit as soon as possible.”