The Science

The science behind harm reduction

What happens when adults who smoke switch to reduced-risk nicotine products? Here is what peer-reviewed research shows, in plain language, along with the limits of each study.

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Smoking is the leading cause of preventable death largely because burning tobacco produces thousands of harmful chemicals. Harm reduction asks a practical question: for an adult who is not going to quit nicotine today, are there options that expose them to far less of that harm?

The studies below tackle that question in different ways, from laboratory emissions tests to randomized trials to large surveys. We summarize each one, link to the original, and note its limits, because a small survey and a randomized trial do not carry the same weight.

Three things the research is consistent about. First, reduced-risk does not mean risk-free, and nicotine is addictive. Second, the benefits are seen in people who switch completely; studies of dual use, vaping while still smoking, show little to no benefit. Third, most evidence is short- to medium-term, and long-term data are still being collected.

1Exposure to carcinogens and toxicants

Most of this work measures what people are exposed to, not whether they develop cancer. Cancer takes decades to appear, so researchers rely on emissions testing, biomarkers in urine and blood, and risk modeling.

Systematic review and meta-analysis

Exposure to tobacco-specific nitrosamines among people who vape, smoke, or do neither

Taylor et al., Nicotine & Tobacco Research, 2024

Pooled 22 studies, including 8 randomized trials. Urinary NNAL, a marker of a major tobacco-specific lung carcinogen, was 79% lower in vapers than smokers across three randomized trials and 96% lower across three cross-sectional studies. In studies that followed smokers who switched, tobacco-specific nitrosamine levels fell by 53% to 97%.

Keep in mind: People who neither smoke nor vape still had about 80% lower NNAL than vapers, so vaping is not exposure-free. Some included studies had industry funding.

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Cross-sectional biomarker study

Nicotine, carcinogen, and toxin exposure in long-term e-cigarette and nicotine replacement therapy users

Shahab et al., Annals of Internal Medicine, 2017 (funded by Cancer Research UK)

Compared 181 people in the UK across five groups. Long-term e-cigarette-only users had significantly lower levels of tobacco-specific nitrosamine and volatile organic compound metabolites than people who only smoked and than dual users, and had the lowest NNAL of any group. Nicotine levels did not differ clearly between groups.

Keep in mind: Cross-sectional design and a self-selected sample, so it shows association, not cause.

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Systematic review of emission studies

Tobacco-specific nitrosamine exposure from e-cigarettes versus combustible cigarettes

Miguel, Ali and El Joumaa, Frontiers in Oncology, 2025

Looked at 13 laboratory studies that directly compared nitrosamines in e-cigarette aerosol and cigarette smoke. Levels in e-cigarette aerosol were undetectable or lower than in smoke, typically by more than 99%. Most modern devices showed none detectable.

Keep in mind: Only 13 eligible studies with varied methods, so no meta-analysis was possible. Eleven of the 13 were industry-funded.

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Laboratory measurement and risk modeling

Measurements of electronic cigarette-generated particles for the evaluation of lung cancer risk of active and passive users

Scungio, Stabile and Buonanno, Journal of Aerosol Science, 2018

Measured the particles in e-cigarette aerosol and combined them with published data on toxic compounds to estimate lifetime cancer risk. The modeled risk for users was about 6 to 7 in a million, roughly five orders of magnitude below that modeled for cigarette smoke, and far lower again for bystanders.

Keep in mind: A modeled estimate from lab conditions and literature values, not a measurement in people. The paper also found particle counts higher than in cigarette smoke.

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Modeling study

Comparing the cancer potencies of emissions from vapourised nicotine products including e-cigarettes with those of tobacco smoke

Stephens, Tobacco Control, 2018

Used published chemical analyses to estimate the cancer potency of emissions. Most e-cigarette products came in below 1% of the potency of tobacco smoke, in the same range as a medicinal nicotine inhaler.

Keep in mind: A small minority of products had much higher potency, linked to high levels of carbonyls produced when too much power is sent to the coil. The author notes cancer risk from e-cigarettes can escalate in those circumstances.

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Cross-sectional biomarker study of youth

Nicotine and tobacco-specific nitrosamine exposure among youth in England who smoke cigarettes and/or vape

Taylor et al., Tobacco Control, December 2025

Analyzed urine from 201 young people aged 16 to 19. Youth who only vaped had significantly lower NNAL than smokers and dual users, at levels indistinguishable from youth who did neither. However, their nicotine intake was similar to that of smokers.

Keep in mind: More than 77% of the vapers used disposable devices, and only nine participants smoked exclusively. The nicotine finding is a reminder of why youth access must be prevented.

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Systematic review

Systematic review of e-cigarette use and head and neck cancer risk

Review of studies published through August 2020

Reviewed studies from five databases and found the risk of head and neck squamous cell carcinoma remains lower in e-cigarette users than in tobacco smokers.

Keep in mind: The authors say more long-term studies are needed to establish the safety of e-cigarettes.

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2Heart and blood vessels

Researchers track blood pressure, blood vessel function, and inflammation markers, and a few studies follow people long enough to count heart attacks and strokes.

Randomized controlled trial

Cardiovascular effects of switching from tobacco cigarettes to electronic cigarettes (VESUVIUS)

George et al., Journal of the American College of Cardiology, 2019

Chronic smokers were randomized to e-cigarettes with or without nicotine for one month, with a comparison group that kept smoking. Within a month of switching, blood vessel function (flow-mediated dilation) improved by 1.49 percentage points, and vascular stiffness improved. Benefits were larger in women and in people who used the product consistently.

Keep in mind: Single center and only one month long. The authors say it does not show e-cigarettes are safe in themselves.

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Randomized controlled trial

Effects of electronic cigarette on platelet and vascular function after four months of use

Ikonomidis et al., Food and Chemical Toxicology, 2020

Forty smokers without heart disease either kept smoking or switched to e-cigarettes for four months. Arterial stiffness (pulse wave velocity) fell by 0.8 m/s in switchers and rose by 0.8 m/s in continuing smokers. Oxidative stress markers were also lower in switchers.

Keep in mind: A small trial of 40 people over four months.

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Prospective cohort study

Impact of smoking cessation on cardiovascular outcomes in patients with myocardial infarction with nonobstructive coronary arteries

Gao et al., American Journal of Preventive Cardiology, 2025

Followed 1,179 heart attack patients for a median of about 3.5 years. Smokers who switched to e-cigarettes had a lower risk of major adverse cardiovascular events than those who kept smoking. The authors stress that no level of smoking is safe and that quitting entirely remains the best strategy.

Keep in mind: Single center, self-reported smoking status, and too few events to analyze individual outcomes reliably.

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Cross-sectional study (PATH)

Association of cigarette and electronic cigarette use patterns with levels of inflammatory and oxidative stress biomarkers among US adults

Circulation (American Heart Association), 2021

Used blood and urine samples from 7,130 US adults. People who only vaped had biomarker levels similar to non-users and significantly lower than smokers on almost all measures. Dual users looked like exclusive smokers.

Keep in mind: Cross-sectional, so it cannot show cause, and it does not follow people who switched.

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Longitudinal cohort study (PATH)

E-cigarette use, dual use, and cardiovascular disease, 2013 to 2019

Circulation (American Heart Association), as summarized in the AHA press release

Followed more than 24,000 US adults. Dual users showed no significant difference from smokers on any cardiovascular outcome, meaning no benefit from adding vaping while still smoking. Exclusive e-cigarette users reported 30% to 40% fewer cardiovascular events than smokers.

Keep in mind: Based on self-reported data and few events among exclusive vapers (41 versus 569 among smokers), so the authors say it cannot support firm conclusions about exclusive vaping.

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Systematic review and meta-analysis

Are electronic cigarettes associated with the risk of myocardial infarction and stroke?

Singh et al., BMC Public Health, 2025

Pooled 12 studies. Among exclusive e-cigarette users who were not smokers, there was no statistically significant association with heart attack or stroke (pooled odds ratios of about 0.96 and 0.97, with wide confidence intervals).

Keep in mind: All but one study were cross-sectional, use frequency and duration were rarely recorded, and results varied widely between studies. Wide intervals mean modest effects cannot be ruled out.

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3Lung health

These studies look at symptoms like cough and wheeze, lung function, and flare-ups in people with asthma or COPD who switched from cigarettes.

Randomized controlled trial

A randomized trial of e-cigarettes versus nicotine-replacement therapy

Hajek et al., New England Journal of Medicine, 2019 (funded by NIHR and Cancer Research UK)

Randomized 886 people at UK stop-smoking services. At one year, 18.0% of the e-cigarette group were smoke-free versus 9.9% of the nicotine-replacement group. Cough and phlegm declined more in the e-cigarette group.

Keep in mind: Most successful e-cigarette quitters (80%) were still using the product at one year, versus 9% in the nicotine-replacement group.

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Longitudinal cohort study (PATH)

Functionally important respiratory symptoms and continued cigarette use versus e-cigarette switching

Sargent et al., eClinicalMedicine, 2025

Followed 5,653 US adults over 1 to 2 years. Among people with many symptoms at the start, 45.8% of those who switched completely to e-cigarettes improved, versus 27.7% of those who kept smoking, a result similar to quitting entirely. Switching also lowered the chance of symptoms getting worse among people with few symptoms.

Keep in mind: Observational and short-term, with some loss to follow-up. Unmeasured factors could affect the results.

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Cross-sectional population study

Differences in respiratory wheezing between exclusive e-cigarette users, exclusive cigarette smokers, and never users

Adebisi et al., Harm Reduction Journal, 2025

Looked at about 9,000 Scottish adults. Exclusive smokers had 1.8 times the odds of wheezing compared with exclusive e-cigarette users. People who never used either had somewhat lower odds than vapers, though not by a statistically significant margin.

Keep in mind: Cross-sectional with self-reported wheezing and no clinical confirmation.

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Prospective cohort study

COPD smokers who switched to e-cigarettes: health outcomes at 5-year follow-up

Polosa et al., Therapeutic Advances in Chronic Disease, 2020

Compared 20 COPD patients who switched with 19 matched patients who kept smoking. Yearly exacerbations in the switchers fell from 2.3 to 1.1, lung function improved while it declined in the comparison group, and the distance walked in six minutes improved more.

Keep in mind: Very small and self-selected, and about half of participants did not complete the walk test. The authors call for larger studies.

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Retrospective case review

Effect of smoking abstinence and reduction in asthmatic smokers switching to electronic cigarettes

Polosa et al., International Journal of Environmental Research and Public Health, 2014

Reviewed records of 18 asthmatic smokers. After 12 months, lung function and asthma control improved, airway hyperresponsiveness dropped, and average cigarette use fell from about 22 to under 2 a day.

Keep in mind: A very small retrospective review with a self-selected group and self-reported smoking.

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Online survey

Characteristics, perceived side effects and benefits of electronic cigarette use: a worldwide survey of more than 19,000 consumers

Farsalinos et al., International Journal of Environmental Research and Public Health, 2014

Surveyed 19,414 e-cigarette users. Among those with asthma, 65.4% reported improvement, and among those with COPD, 75.7%.

Keep in mind: Self-selected, mostly recruited from vaping forums, so it describes the experience of dedicated users rather than all smokers who try e-cigarettes.

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Online survey

Changes in the frequency of airway infections in smokers who switched to vaping

Miler, Mayer and Hajek, 2016

Surveyed 941 former or near-former smokers on German vaping forums. 66% reported fewer airway infections after switching, 29% saw no change, and 5% reported more.

Keep in mind: Subjective reports from a self-selected group. People who had a bad experience may have stopped vaping and not responded. The authors note possible bias toward favorable answers.

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Umbrella review

Health effects of electronic cigarettes: an umbrella review and methodological considerations

Travis et al., International Journal of Environmental Research and Public Health, 2022

Reviewed 17 systematic reviews. It concluded that long-term e-cigarette use may carry respiratory benefits for people who switch from chronic smoking, particularly those with asthma or COPD.

Keep in mind: Evidence on intermediate and long-term cancer effects is lacking. The authors also point to poor reporting of devices, e-liquids, and participants' smoking history across the underlying studies.

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4People who never smoked, and what we don't know yet

The strongest case for e-cigarettes is for adults who already smoke. These studies look at people who vape without a smoking history, and the answers are less certain.

Prospective observational study

Health impact of e-cigarettes: a prospective 3.5-year study of regular daily users who have never smoked

Polosa et al., Scientific Reports, 2017

Followed nine never-smoking daily vapers and 12 never-smoking controls for 3.5 years. There were no significant changes in lung function, blood pressure, or symptoms, and no pathological findings on lung CT scans.

Keep in mind: Only nine users, giving very low statistical power, with a young group and modest vaping. The authors acknowledge it cannot rule out small effects.

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Cross-sectional population study (NHANES)

Smoking is associated with elevated blood level of volatile organic compounds

Wu et al., Archives of Public Health, 2023

Compared 1,117 US adults, including 41 e-cigarette users. Blood levels of nine volatile organic compounds in e-cigarette users were similar to those in never-smokers, while combustible smokers and dual users were higher.

Keep in mind: Small e-cigarette group and self-reported use. One compound (2,5-dimethylfuran) was elevated in e-cigarette users in the regression analysis.

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What the research does not show

Dual use is not harm reduction. Adults who keep smoking while vaping show biomarker and cardiovascular profiles like those of smokers.
Youth should not use nicotine. Even where toxin exposure is low, young vapers can take in as much nicotine as smokers, which is why we support strict age verification and enforcement.
Long-term outcomes are still unknown. Cancer and chronic disease take decades to develop, and most of the studies above are short- or medium-term.
Quality varies. Several studies are small, self-selected, or industry-funded, which is why we note their limits.

These summaries are provided for general information and are not medical advice. Adults who smoke should talk with a health professional about the best way to quit, including FDA-approved cessation medications.

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