More than half of American adults are unknowingly exposed to secondhand smoke, and studies indicate that 56 million Americans are affected by secondhand smoke. The study, led by Ruixuan Wang of the University of Florida, showed that only half of people with evidence of exposure to secondhand smoke reported it.

The study was based on survey data from more than 13,000 U.S. adults between 2013 and 2020, using blood levels of cotinine as an indicator of exposure. The reasons for underreporting are unclear, but low-level exposure in public places and stigma are potential factors.

Nicotine is a very common compound in tobacco. When the human body is exposed to or inhaled nicotine, the body will metabolize it into cotinine. The half-life of cotinine in the body is about nineteen hours on average. Smokers will have high levels of cotinine in their bodies, and people who are exposed to secondhand smoke for a long time will also have increased levels of cotinine in their bodies.

Cotinine can be detected in plasma, urine and saliva. Cotinine is also produced in the body when the human body ingests some solanaceous vegetables (such as eggplant, tomatoes and green peppers). These plants use cotinine as their self-defense mechanism to fight fungi, bacteria, insects and animals.

High levels of cotinine may increase your risk of lung cancer, heart disease, and stroke.

More than half of U.S. adults have been recently exposed to secondhand tobacco smoke, and most of them turn a blind eye, new research shows. Nationally, 56 million Americans are regularly exposed to toxic secondhand smoke without knowing it, new research findings show.

"There is no safe level of secondhand smoke exposure, and long-term exposure to secondhand smoke increases the risk of many chronic diseases, such as coronary heart disease, respiratory disease and cancer," said Roxanne Wang, a doctoral candidate in the University of Florida's School of Public Health and Health Professions and first author of the new study. "We hope people will be aware of their exposure and take protective measures."

Gaps in Awareness and Exposure

UFHealth researchers analyzed a nationally representative survey of more than 1,000 U.S. adults and found nicotine byproducts were detected in the blood of 51 percent. However, less than half of those with evidence of exposure to secondhand smoke report exposure, leaving a large, previously unreported gap in knowledge about secondhand smoke.

All demographic groups significantly underreported their smoke exposure, but Blacks had the highest rates of both smoke exposure and underreporting. "We believe this report will provide a reference for intervention measures targeting high-risk groups," Wang Ruixuan said.

Data sources and detection methods

Ruixuan Wang worked with Jennifer LeLaurin, Christopher Cogle and others at the UF School of Medicine and School of Public Health and Health Professions to analyze data from the National Health and Physical Examination Survey conducted by the Centers for Disease Control and Prevention. The survey covered 2013 to 2020.

The authors measured cotinine levels in respondents' blood, which indicates exposure to nicotine in the past few days and is the gold standard for determining exposure to tobacco products.

It's unclear why underreported exposure levels are so high. Cotinine measurements are very sensitive and can detect low levels of tobacco exposure, but no level is considered safe.

"With low-level smoke exposure, you may not notice. You're in public, and maybe you're not even aware that people around you are smoking," said LeLaurin, the study's senior author and a professor of health outcomes and biomedical informatics at UF. "Maybe it's just a small thing that you've forgotten about. Another possibility is that some respondents knew they were exposed to secondhand smoke but chose not to report it because of stigma."