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For decades, public health campaigns have drilled one core truth about cigarette smoking into public consciousness: it causes lung cancer. But a landmark new study published in BMJ Open has laid bare a troubling, life-threatening gap in global public understanding: the vast majority of smokers remain unaware that smoking is equally, if not more deadly to the heart and vascular system, causing thousands of preventable heart attacks and strokes every year. Even less known are the severe cardiovascular risks posed by secondhand smoke exposure to non-smokers.

To unpack the study’s findings and answer pressing questions about these underrecognized risks, CNN correspondent spoke with Dr. Leana Wen, emergency physician, clinical associate professor at George Washington University, and former Baltimore health commissioner. Dr. Wen walked through the study’s methodology, its alarming conclusions, and what it means for smokers and their loved ones.

The research team drew on data from two of the largest international tobacco surveys ever conducted: the International Tobacco Control Project, which collected responses across 26 countries between 2002 and 2022, and the Global Adult Tobacco Survey, which surveyed participants in 32 countries from 2008 to 2020. With overlapping country participation, the combined dataset included responses from adult smokers across 46 nations, all asked to confirm their knowledge of smoking’s links to lung cancer, heart disease, heart attack, and stroke, as well as secondhand smoke’s risks for non-smokers.

Across both datasets, the results painted a consistent picture of widespread awareness for lung cancer, and steep declines in recognition for cardiovascular harms. Roughly 92% of participating smokers correctly identified smoking as a cause of lung cancer. By comparison, just 85% and 80% of respondents (across the two surveys) recognized the link between smoking and heart disease and heart attack, respectively. Awareness dropped even further for stroke, with only 75% and 66% of respondents in each survey acknowledging the connection.

The gap was even more stark for secondhand smoke. Between 77% and 82% of smokers knew secondhand smoke causes lung cancer in non-smokers, but only 62% to 73% recognized it also causes heart disease and heart attacks. Most surprisingly of all, the study found almost no progress in closing this awareness gap over the past two decades. Across all five measured health risks, the average annual increase in public knowledge was just 0.14 percentage points.

When asked why this gap is so concerning, Dr. Wen emphasized that cardiovascular disease is the leading preventable cause of death linked to smoking worldwide. In 2019 alone, smoking-related heart disease and stroke claimed roughly 2.6 million lives globally — twice the 1.3 million deaths attributed to lung, trachea, and bronchus cancers from smoking. Smokers who only recognize lung cancer as a primary risk are missing half of the life-threatening picture, she explained.

“Knowledge directly shapes behavior,” Wen noted. “Smokers who understand the full scope of smoking’s harms are far more motivated to quit. This is even more critical for secondhand smoke: if a smoker doesn’t know their habit raises cardiovascular risk for the people around them, they’re far less likely to take steps to protect their family, friends, and colleagues from exposure.”

Dr. Wen went on to explain the biological mechanisms that make cigarette smoke so damaging to the heart and blood vessels. Cigarette smoke contains thousands of toxic chemicals, many of which directly damage the inner lining of blood vessels, trigger chronic inflammation, and spur plaque buildup inside artery walls. Smoking also makes blood far more prone to clotting — the primary trigger for both heart attacks (which occur when a clot blocks blood flow to the heart) and the most common type of stroke, which occurs when a clot cuts off blood flow to the brain.

Additional harms come from nicotine, which raises resting heart rate and blood pressure, and carbon monoxide, which reduces the blood’s ability to carry oxygen to vital organs. Over time, these effects stiffen and narrow blood vessels, accelerating the progression of cardiovascular disease. Crucially, Wen added, these risks are not limited to heavy, long-term smokers: cardiovascular damage begins early, and even low levels of cigarette consumption carry substantial risk.

Secondhand smoke carries nearly identical risks, Wen explained: non-smokers exposed to secondhand smoke inhale the same toxic chemicals, which cause the same vascular damage and inflammation that raise coronary heart disease risk. The public health toll is staggering: the study estimates secondhand smoke caused 1.3 million premature deaths globally in 2021 among non-smokers, yet fewer than half of respondents in several high-income countries including the U.S., Canada, the U.K., Australia, and Germany were aware of this cardiovascular risk.

When asked why lung cancer awareness is so much higher than cardiovascular awareness, Wen noted that the link between smoking and lung cancer has been the centerpiece of public health messaging for decades. It features prominently on tobacco warning labels and national education campaigns, so the connection has become deeply ingrained in public knowledge. Cardiovascular harms have never received the same level of focus in public outreach, leading to the major awareness gaps seen across countries, with gaps correlating to a lack of targeted mass education campaigns.

A common misconception holds that cutting back on cigarette consumption substantially reduces cardiovascular risk, but Wen pushed back on this idea. Research confirms that even a few cigarettes a day carry nearly the same level of cardiovascular risk as a full pack a day. The only effective way to reduce risk is to quit entirely. The good news, she added, is that cardiovascular benefits begin almost immediately after quitting: heart rate and blood pressure drop within hours, carbon monoxide levels in the blood return to normal quickly, and over time the risk of heart attack, stroke, and other smoking-related diseases falls dramatically. Quitting offers meaningful health benefits at any age, and stopping earlier delivers larger gains, so it is never too late for a smoker to quit.

For smokers looking to quit, Wen emphasized that the most impactful step is complete cessation, and that proven treatments exist to improve success rates. Nicotine replacement therapy, prescription medications, and combined treatment that pairs medication with behavioral counseling all significantly increase the chance of long-term success. Smokers should work with their healthcare provider to find the right treatment plan for them, and should not be discouraged by failed attempts: most people require multiple attempts to quit for good, and each attempt can help identify triggers and needed support for future success.

In the meantime, smokers must take steps to protect others from secondhand smoke: opening a window or smoking in a separate room does not eliminate the risk of indoor exposure. Smokers should never smoke inside homes, cars, or other enclosed spaces, and should always avoid smoking near children, pregnant people, older adults, and people with pre-existing cardiovascular or respiratory disease.

“Every smoker should know that their habit does not just threaten their lungs,” Wen concluded. “Cigarette smoke damages blood vessels throughout the entire body, directly increasing the risk of fatal heart attacks and strokes. Recognizing this full scope of harm is one more powerful motivation to quit — for your own health, and for the health of everyone you love.”