The short answer
Secondhand smoke is the smoke that nonsmokers breathe in from other people's tobacco. It contains many of the same cancer-causing chemicals as directly smoked tobacco and can cause lung cancer in people who do not smoke. Experts say there is no safe level of exposure.
Secondhand smoke is the tobacco smoke that nonsmokers breathe in from others.
It contains many of the same cancer-causing chemicals as smoke breathed directly by smokers.
Secondhand smoke can cause lung cancer in people who have never smoked.
There is no safe level of secondhand smoke exposure.
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The full explanation.
The simple version
Secondhand smoke is the tobacco smoke that people breathe in when they are near someone who is smoking. You do not have to smoke to be exposed to it.
This smoke is not harmless. It contains many of the same cancer-causing chemicals as the smoke a smoker breathes in. Because of this, secondhand smoke can cause lung cancer in people who have never smoked.
Experts agree there is no safe level of secondhand smoke.
What is secondhand smoke?
Secondhand smoke comes from two sources:
- The smoke that drifts off the burning end of a cigarette, cigar, or pipe
- The smoke that a smoker breathes out into the air
When these mix, they fill the air around a smoker. Anyone nearby breathes them in. This is why secondhand smoke is sometimes called "passive smoking" or "involuntary smoking."
Why is it harmful?
Tobacco smoke contains thousands of chemicals. Many of them are known to cause cancer. These same chemicals are present in secondhand smoke.
When a nonsmoker breathes secondhand smoke, those chemicals enter their body. Over time, they can damage cells in the lungs and lead to cancer.
Secondhand smoke is known to cause lung cancer in adults who do not smoke. It is also linked to other serious health problems, such as heart disease and breathing problems.
Breathing someone else's smoke carries real risk.
There is no safe level
You might think a little smoke here and there is harmless. But that is not what the science shows.
Experts have concluded that there is no safe level of secondhand smoke exposure. Even brief exposure can harm the body.
Common myths do not hold up:
- Opening a window does not clear the harmful chemicals.
- A fan does not make the air safe.
- Smoking in another room still lets smoke travel through the home.
The only way to fully protect nonsmokers is to keep spaces completely smoke-free.
Who is most at risk?
Secondhand smoke can harm anyone who breathes it. Some groups are especially vulnerable.
Children are at higher risk because their bodies are still growing and they breathe faster than adults. Secondhand smoke can cause serious health problems in children, including breathing illnesses.
People with existing heart or lung conditions may also feel the effects more strongly. And unborn babies can be affected when a pregnant person is exposed to smoke.
How to protect yourself and others
The best protection is to keep smoke out of the places you spend time.
Helpful steps include:
- Make your home completely smoke-free.
- Keep your car smoke-free, even with windows down.
- Avoid indoor places where smoking is allowed.
- Support smoke-free rules in shared spaces.
If someone you live with smokes, encouraging and supporting them to quit helps everyone. Quitting protects the smoker and clears the air for the whole household.
Your healthcare team can share resources and help connect a loved one with quit support.
What about smoke that lingers?
Sometimes you can still smell smoke long after someone has finished a cigarette. This lingering residue settles onto surfaces like walls, furniture, carpets, and clothing.
Researchers are still studying this residue and its effects. But the presence of a stale smoke smell is a reminder that smoke leaves traces behind. The most reliable way to keep these traces out of your living space is to keep smoking out of it entirely.
This is another reason a fully smoke-free home is the goal, rather than trying to smoke in just one room or near a window.
A shared benefit
Keeping spaces smoke-free is not only about avoiding harm. It also creates a healthier environment for everyone.
When a home or car is smoke-free:
- Children breathe cleaner air.
- People with asthma or heart conditions are better protected.
- The person who smokes may find it easier to cut back or quit.
Setting a smoke-free rule can feel like a big change, but it is one of the clearest ways to protect the people you care about. Your healthcare team can offer tips for making the change and can help a household member who wants to quit.
Words to know
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Common questions
▸What exactly is secondhand smoke?
Secondhand smoke is a mix of the smoke that comes off a burning tobacco product and the smoke that a smoker breathes out. Nonsmokers nearby breathe it in without smoking themselves.
▸Can secondhand smoke really cause cancer?
Yes. Secondhand smoke contains many of the same cancer-causing chemicals found in smoke that smokers breathe. It is known to cause lung cancer in nonsmokers.
▸Is there a safe amount of secondhand smoke?
No. Experts have concluded that there is no safe level of exposure to secondhand smoke. Even short-term exposure can be harmful.
▸Does opening a window make secondhand smoke safe?
No. Opening windows, using fans, or moving to another room does not remove the harmful chemicals. The only way to fully protect nonsmokers is to keep spaces completely smoke-free.
▸Who is most affected by secondhand smoke?
Everyone can be harmed, but children are especially vulnerable. Secondhand smoke can also affect people who already have breathing or heart problems.
▸How can I lower exposure at home?
Making your home and car completely smoke-free is the best way. Avoiding indoor public places where smoking is allowed also helps.
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