But butting out for good can reduce and even eliminate that risk over time, they say.
Sudden cardiac death (SCD) results from the abrupt loss of heart function, usually within minutes after the heart stops. One major cause is ventricular arrhythmia, an abnormal rhythm that develops in the lower chambers of the heart.
SCD is not the same as a heart attack — which occurs when one or more of the arteries to the heart are blocked — but it can occur during a heart attack. An estimated 300,000-plus North Americans experience sudden cardiac death each year.
"Cigarette smoking is a known risk factor for sudden cardiac death, but until now we didn't know how the quantity and duration of smoking affected the risk," said Dr. Roopinder Sandhu, lead author of a newly published study.
To determine that relationship, researchers looked at the incidence of sudden cardiac death among more than 101,000 women in the U.S. Nurses' Health Study, which has collected health questionnaires every two years from female nurses since their enrolment in 1976.
At the start of the study, the nurses were aged 30 to 55 and had no heart disease. On average, those who smoked reported that they had started in their late teens.
During the study, 351 of the women died of sudden cardiac death. Of those, 223 were either current or past smokers.
"What we found was that there was a significant dose-response relationship between the quantity of cigarettes smoked daily and sudden cardiac death for women," said Sandhu, a cardiac electrophysiologist at the Mazankowski Heart Institute in Edmonton.
"And even small to moderate quantities of daily smoking — the one to 14 cigarettes a day — were associated with a two-fold elevated risk (of sudden cardiac death)," she said Tuesday from Edmonton. "And also the longer women smoked, the higher the cardiac death risk."
When it came to duration, she said that for every five years of continued smoking, there was an eight per cent increased risk of suddenly dying.
"But importantly, women who quit smoking had a lower sudden cardiac death risk. And it continued to decrease over time, and it was equivalent to that of a never-smoker at 20 years," said Sandhu, who is also a visiting scientist at Brigham and Women's Hospital in Boston.
Among women with heart disease, within 15 to 20 years of quitting, their risk of sudden cardiac death dropped to that of someone who had never smoked. For women without heart disease, the risk of sudden cardiac death began immediately dropping.
Dr. Beth Abramson, a cardiologist at St. Michael's Hospital in Toronto, said the study is yet another piece of evidence that smoking is bad for health.
"What was concerning was that small to moderate amounts of cigarette consumption — so half a pack per day or less — was associated with an almost two-fold increase in sudden cardiac death risk," said Abramson, a spokeswoman for the Heart and Stroke Foundation.
"And women who smoked 25 cigarettes or more had an almost three-fold increase in sudden cardiac death risk compared to never-smokers."
Abramson said smoking has a number of deleterious effects on the heart and coronary arteries, in both women and men. It can lead to arterosclerosis, or a build-up of plaque in the arteries; change the balance of chemicals in heart cells; and make blood platelets sticky and more prone to stroke- and heart attack-causing clots.
"The study shows us that although it can take some time, quitting smoking is beneficial."
Sandhu said sudden cardiac death, which may occur in the absence of symptoms or often within an hour of the onset of symptoms, often strikes women who didn't realize they had heart disease.
"So lifestyle changes that reduce that risk are important," she said.
"Cigarette smoking's an important modifiable risk factor for sudden cardiac death among all women, and attempts at smoking cessation should not wait until somebody develops heart disease."
The study is published in the journal Circulation: Arrhythmia & Electrophysiology.