Second-hand vapour is the argument that decides most indoor vaping policies, and it is one of the few areas where the research has moved decisively in the last few years. The evidence now supports a clear answer: exposure is real, it is measurable, and it is a fraction of the equivalent exposure to cigarette smoke. Both parts of that sentence matter.
Here is what the studies actually measured, where the honest uncertainties still sit, and what it means for vaping in a shared space. If that shared space is your workplace, the rules your employer can set are the practical counterpart to this evidence.
The headline study
In July 2024 a UCL-led team published research in JAMA Network Open, funded by Cancer Research UK, using real-world data rather than a laboratory chamber. The study looked at blood tests and survey data for 1,777 children aged three to eleven in the United States, drawn from the National Health and Nutrition Examination Survey.
Because children are unlikely to have vaped themselves, their cotinine levels (a nicotine metabolite) are a clean measure of what they absorbed from the air around them. The findings:
| Children exposed to... | Nicotine absorbed compared with indoor smoking |
|---|---|
| Indoor smoking | Baseline (highest) |
| Indoor vaping | About 84% less |
| Neither | About 97% less |
Two conclusions followed. First, second-hand vaping exposure is much lower than second-hand smoking exposure. Second, it is not zero: children in vaping households still absorbed more than children in neither. The researchers also noted that exposure to the harmful non-nicotine substances in vapour would likely be substantially lower again, since e-cigarettes deliver similar nicotine levels to tobacco but only a fraction of the toxicants.
Why the gap is so large
The mechanism is straightforward. A cigarette burns continuously between puffs and produces sidestream smoke from the lit end, which is a major part of what a bystander breathes. A vape produces aerosol only when the user draws and exhales it. Studies measuring retention found that vapers keep roughly 99% of the nicotine they generate, which leaves relatively little in the exhaled cloud.
That single difference, a burning tip versus no burning tip, explains most of the measured gap.
Where the honest uncertainties are
"Much lower" is not "harmless," and the literature is careful about the difference.
- Respiratory irritation. A 2019 study in IJERPH collected the exhaled breath of 17 volunteers while vaping and found increased levels of nicotine, propylene glycol, glycerol, tobacco-specific nitrosamines and copper. The authors concluded bystanders may experience airway irritation from PG and glycerol, with nicotine effects possible in a confined space.
- Ventilation is the swing factor. The same paper modelled two scenarios: a non-ventilated car with two users, and a ventilated office with one. The estimated exposure differed enormously. A 2018 study of a large hotel event room found PM2.5 rising to as high as 819 µg/m³ when 59 to 86 people were vaping in a closed space.
- Indoor air standards. ASHRAE updated its indoor air quality standard in 2016 to fold e-cigarette emissions into its definition of environmental tobacco smoke, on the basis that it is incompatible with acceptable indoor air quality.
- Institutional positions differ. WHO has stated that the evidence on whether exhaled aerosol causes disease in bystanders is insufficient to conclude, while several medical bodies recommend restricting indoor vaping in line with smoking bans.
The pattern across all of it is consistent: the risk scales with concentration, enclosure and duration, not with the act of vaping in general.
What that means in practice
Three practical rules cover most situations:
- Ventilation first. Outdoors or with open airflow, exposure is negligible. A sealed car with the windows up is the opposite case and the one the modelling flagged.
- Children and people with respiratory conditions. Even though the measured exposure is far lower than smoke, the UCL authors' own recommendation was to avoid indoor vaping around children, because the alternative of no exposure is available.
- Follow the venue, not your own reading. Indoor vaping rules are set by premises, employers and transport operators, and they vary.
On the last point, the legal picture in Germany is worth knowing before you assume. Vaping indoors is generally permitted unless the venue prohibits it, while federal rules ban it on trains and in federal buildings. Our guide to where you can vape in Germany breaks that down state by state, and the car-specific position is covered in vaping in a car in Germany.
Does it leave a smell or residue?
This is the question people actually care about in practice, and it has its own answer. Exhaled aerosol disperses in seconds rather than the half hour or more that cigarette smoke lingers, and it does not deposit the tar that stains walls and fabric. Our comparison of whether vaping smells covers the exceptions, which are mainly confined spaces, strong dessert flavours and vehicles.
Frequently asked questions
Is second-hand vapour dangerous?
It carries measurable nicotine and some irritants, but at a small fraction of cigarette smoke exposure. In confined, unventilated spaces with heavy use, exposure rises meaningfully. Outdoors it is negligible.
Can second-hand vapour give someone nicotine?
Yes, at low levels. The 2024 UCL study found children in vaping households absorbed more nicotine than those exposed to neither, but about 84% less than children in smoking households.
Should I vape indoors around children?
The researchers behind the largest real-world study recommend avoiding it, mainly because the zero-exposure option is available. The evidence points to low rather than no risk.
How does it compare with second-hand smoke?
Much lower for nicotine and dramatically lower for the toxicants and carcinogens that make smoke dangerous, because there is no burning tip producing sidestream smoke. See also the wider cost comparison with smoking.
The short version
Second-hand vapour is not smoke, and the measured difference is large: about 84% less nicotine absorption than indoor smoking in the largest real-world study to date. But it is not nothing. Ventilation, enclosure and duration decide the exposure, which is why an open window and a closed car are two entirely different situations.
