Second-hand smoke: How dangerous is other people's smoke?

You’re probably familiar with the effects of secondhand smoke on non-smokers, such as stuffy air, unpleasant smells, or a scratchy throat. But what is often underestimated is this: it doesn’t stop at harmless discomfort. More than 1.6 million non-smokers worldwide die every year as a result of secondhand smoke¹. These are people who have never actively smoked themselves. But what exactly happens in the body? At what point does the exposure become dangerous, and why are children affected especially severely? How long can exposure to secondhand smoke still be detected? You’ll find the answers to these and other questions about secondhand smoke here.

What is secondhand smoke? Definition and how it occurs

Secondhand smoke refers to the involuntary inhalation of tobacco smoke released into the indoor air by other people, without actively smoking yourself. The tobacco smoke inhaled in this way contains thousands of different chemical substances, many of which are toxic or carcinogenic. Some harmful substances are even present in higher concentrations in secondhand smoke than in the smoke inhaled by the person smoking. Secondhand smoke consists of two components: mainstream smoke and sidestream smoke.

Is secondhand smoke worse than active smoking?

At first glance, it may seem obvious that active smokers inhale the smoke directly and that this must be more harmful than simply standing nearby. However, the composition of the smoke makes the issue more complex. That’s because the smoke inhaled through secondhand smoking is not just a diluted version of what the smoker inhales.

Mainstream smoke and sidestream smoke

Secondhand smoke is made up of mainstream smoke and sidestream smoke. Mainstream smoke is produced when drawing on a cigarette, is inhaled by the smoker, and then exhaled back into the surrounding air. Sidestream smoke, on the other hand, is produced between puffs by the smoldering cigarette. Mainstream smoke accounts for only about 15 percent of the smoke inhaled passively, while sidestream smoke makes up around 85 percent².

Mainstream smoke makes up only about 15% of secondhand smoke, while sidestream smoke accounts for around 85%.

Due to the different combustion temperatures, mainstream smoke and sidestream smoke differ in their chemical composition. While mainstream smoke is produced at high temperatures of around 950°C, sidestream smoke forms at approximately 500°C⁵. As a result, sidestream smoke has a pollutant concentration six to eight times higher than mainstream smoke⁴. The lower temperature leads to more incomplete combustion of the tobacco, which means the concentration of harmful substances such as carbon monoxide or benzene is particularly high⁸.

For children, exposure to secondhand smoke is particularly harmful to their health. If a child spends one hour in a heavily smoke-filled room, they can absorb an amount of harmful substances roughly equivalent to Equals inhaled when smoking one cigarette. The reason is that children have a higher breathing rate, lower body weight, and a detoxification system that is not yet fully developed³. You’ll read more about this later in the article.

Residual smoke and thirdhand smoke

Tobacco smoke does not simply disappear once you stub out the cigarette. Tiny particles and residues settle as residual smoke, also known as thirdhand smoke (derived from the English term “third-hand smoke”), on surfaces, furniture, textiles, walls, and hair. Air movement and drafts can stir these residues back up and release them into the indoor air again. They can then re-enter the body through the skin, mouth, and lungs.

So far, 26 substances harmful to health have been identified⁶ in residual smoke. There is also evidence of potentially harmful health effects, such as changes in cell function. However, how great the actual health risk is remains unclear at present. The pollutants from residual smoke can remain indoors for months or even years. Toddlers in particular may come into contact with them, as they often play on the floor and put toys or other objects in their mouths. This means they can absorb these residues even when no one is smoking in the room at that moment.

What harmful substances does tobacco smoke contain?

What actually makes tobacco smoke so dangerous? In fact, it contains more than 5,300 different substances, including numerous toxic and carcinogenic compounds². Known carcinogens, meaning cancer-causing substances, include arsenic and formaldehyde. Tobacco-specific nitrosamines are also particularly harmful to health. These are highly carcinogenic harmful substances found mainly in tobacco plants and tobacco products.

Burning tobacco also produces tar substances that settle in your lungs and contain carcinogenic hydrocarbons such as benzo[a]pyrene, among others. Carbon monoxide is also a component of cigarette smoke. This toxic gas binds to the red blood pigment hemoglobin and thereby impairs oxygen transport in the body. Other toxic gases in tobacco smoke include hydrogen cyanide, formaldehyde, and ammonia. They can irritate your airways and damage your cells.

Nicotine is the substance in tobacco primarily responsible for its strong addictive effect. At the same time, it puts strain on your cardiovascular system. In addition, tobacco smoke contains heavy metals such as cadmium, lead, nickel, and chromium, which can accumulate in the body and damage your organs.

Secondhand smoke also contains the same toxic and carcinogenic substances as cigarette smoke that is actively inhaled, although in different concentrations⁶ . There is therefore no such thing as "harmless" tobacco smoke. Particularly problematic are the respirable fine particulate matter particles. They can penetrate deep into the airways, overcome the lungs' natural cleaning mechanisms, and become deposited in lung tissue. For people with pre-existing respiratory conditions such as COPD (chronic obstructive pulmonary disease) or asthma, exposure to tobacco smoke can worsen symptoms even further.

Health effects: from what point is passive smoking harmful?

How exactly does passive smoking harm the body, and what health consequences can it have? There is no threshold below which exposure to tobacco smoke is considered harmless to health². Every exposure to tobacco smoke is associated with a health risk. The respiratory tract and the cardiovascular system are particularly affected.

Passive smoking poses health risks for adults, children, and pregnant women.

Symptoms and acute complaints

You may already be familiar with the body's immediate reactions to tobacco smoke: irritated eyes and mucous membranes, coughing, shortness of breath, a scratchy throat, as well as headaches and dizziness. Tiredness and difficulty concentrating can also occur. These passive smoking symptoms are caused by exposure to the harmful substances in cigarette smoke and can become noticeable during or shortly after spending time in smoky rooms.

Long-term effects in adults

Regular and long-term exposure to secondhand smoke, for example from a smoking partner at home or in a poorly ventilated workplace, has been proven to increase the risk of various illnesses. According to the DKFZ Tobacco Atlas 2025, passive smoking increases the risk of lung cancer and stroke by 20 to 30 percent⁶. According to the Swiss Federal Office of Public Health (FOPH), regular exposure to secondhand smoke increases the risk of heart attack and lung cancer by around 25 percent, and the risk of stroke by as much as 80 percent⁷.

Other health consequences can include asthma, a feeling of tightness in the chest, and the associated breathing difficulties. Passive smoking can also increase the risk of coronary heart disease, in which the blood vessels supplying the heart muscle with oxygen are narrowed by deposits. In people with existing COPD, secondhand smoke can also worsen the progression of the disease.

learn more about why smoking is harmful

Passive smoking in children: Special risks

Children are particularly vulnerable to passive smoking. But why are the effects more serious for them than for adults? Small children breathe in and out two to three times more often than adults in relation to their body weight⁷. As a result, they absorb more harmful substances relative to their body weight. At the same time, their detoxification system is not yet fully developed, so harmful substances are broken down more slowly⁶.

For infants, secondhand smoke is particularly dangerous because it increases the risk of sudden infant death syndrome (SIDS). In toddlers and older children, the airways are especially affected⁶. According to DAK, around one in ten schoolchildren is exposed to secondhand smoke¹⁰.

Airways, ears and development

Secondhand smoke irritates the mucous membranes and weakens the natural cleaning mechanisms of the airways. As a result, children are more frequently affected by respiratory infections and chronic coughing. The risk of asthma and pneumonia also increases. In addition, secondhand smoke can impair lung development.

Children from smoking households are also more likely to suffer from middle ear infections or otitis media with effusion, where fluid builds up behind the eardrum. This can affect hearing and, in turn, impact speech development. Developmental, concentration and behavioural issues, including ADHD, are also worsened in children by secondhand smoke.

The car as an underestimated risk area

Secondhand smoke in the car is especially problematic. In the small enclosed space, harmful substances from cigarette smoke can build up in high concentrations. Even open windows do not prevent harmful residues from settling inside the vehicle and on the upholstery.

In Germany, smoking in private vehicles in the presence of children, adolescents or pregnant women is currently not prohibited⁹. More than 750,000 children and adolescents are exposed to secondhand smoke in cars and the associated health risks¹⁰.

In the car, children are exposed particularly heavily to secondhand smoke.

Third-hand smoke: the invisible residual exposure

Can a home still be contaminated with harmful substances even though nobody has smoked there for weeks? And wouldn’t it be enough if parents consistently smoked only on the balcony or outdoors so that the home remains free of pollutants? That is exactly what a study by the University of Greifswald examined. In more than 50 percent of the children studied from 922 families with at least one smoker in the household, cotinine, the breakdown product of nicotine, was detected in the urine. Even among the 231 children whose parents stated that they never smoked indoors, exposure to tobacco smoke could still be measured¹¹.

How do harmful substances enter a child’s body if no one is smoking near them at all? This can be explained by what is known as third-hand smoke. Tobacco smoke residues settle on toys, carpets and clothing and can enter children’s bodies from there through the skin, mouth and airways.

Secondhand smoke during pregnancy

Why does smoke that only the mother inhales affect the unborn child at all? The reason is their shared bloodstream. Through the placenta, almost everything circulating in the mother's blood also reaches the child. This includes oxygen and nutrients, but also harmful substances. Active smoking during pregnancy is therefore the most intense form of tobacco smoke exposure for the unborn child. But even if you don't smoke yourself, regular exposure to secondhand smoke can affect your child's health.

The nicotine contained in tobacco smoke constricts the blood vessels in the placenta, while carbon monoxide simultaneously reduces the oxygen supply. As a result, less oxygen and fewer nutrients reach the unborn child¹³. What does this undersupply mean for the baby in concrete terms? Among other things, it leads to a lower birth weight and a smaller head circumference. In addition, tobacco smoke exposure increases the risk of premature birth and possibly also miscarriage⁶. Placental abruption occurs more frequently, and the development of the unborn child's lungs and brain can also be impaired⁸.

How strong the link between smoke exposure and these risks actually is was shown by a Belgian study from 2013. It found that after smoking bans were introduced in public areas, the preterm birth rate fell by six cases per 1,000 births¹².

You can find a detailed article on the risks & consequences of smoking during pregnancy here:

Smoking During Pregnancy

Detecting secondhand smoke in urine: What does the cotinine level indicate?

Cotinine is the main breakdown product of nicotine and is considered an important biomarker for tobacco smoke exposure. A biomarker is a measurable value in the body that provides information about a specific condition, for example whether someone has been exposed to tobacco smoke. When you absorb nicotine from tobacco smoke, it is converted into cotinine in your liver and then excreted in your urine. Since cotinine has a much longer half-life (the time it takes for the concentration in the body to be reduced by half) than nicotine itself, tobacco smoke exposure can still be detected several days later.

How long can secondhand smoke be detected in urine?

Why don't tests simply check directly for nicotine when you want to detect exposure to secondhand smoke? The reason is the speed at which the body breaks it down. Nicotine has a half-life of around 30 minutes¹⁷, which means that just half an hour after exposure, only half of it remains in the blood. After one to three days, nicotine itself can hardly be detected anymore¹⁶.

That is why cotinine, the breakdown product of nicotine, is used instead. With a half-life of around 19 hours, it remains in the body much longer¹⁷ — almost 40 times longer than the half-life of nicotine itself. But how long can secondhand smoke actually be detected then? In urine, cotinine can still be detected for several days after tobacco smoke exposure, and after about seven to ten days the level usually returns to the normal range^{15, 17} . Cotinine can be detected in the blood for up to ten days after exposure¹⁴. Testing blood is considered a particularly reliable method and is often used in non-smokers to determine tobacco exposure¹⁴.

In saliva, however, cotinine can only be detected for up to four days. In hair, it can even remain detectable for a significantly longer period, up to 90 days, as cotinine can accumulate in the hair follicles¹⁴.

>> You may also like to read: How long does nicotine stay in the body?

How can you protect yourself and others from passive smoking?

The most reliable protection against passive smoking is to avoid tobacco smoke completely. Measures such as airing out a room, an open window, a range hood, or an air purifier may reduce exposure to harmful substances, but they do not completely prevent the inhalation of tobacco smoke.

A general solution for indoor spaces is tobacco-free nicotine products, which do not produce combustion smoke when used. This means the people around you are not exposed to passive smoke, and smokers do not have to go without their nicotine. You can find an overview of these products and how they can be used as part of quitting smoking in the article smoke-free alternatives with Snus and Nicotine Pouches. You can also find a selection of our most popular Nicotine Pouches here:

view all Nicotine Pouches

At home

A completely smoke-free home is the most effective option. This also applies to the balcony and stairwell if tobacco smoke can enter the living areas from there. Smoking with the window open does not provide sufficient protection, as some of the smoke still enters the home and mixes with the indoor air that all residents breathe in². Even regular airing out does not remove the exposure completely, as harmful substances and tobacco smoke residue can settle on surfaces, furniture, and other places.

A smoke-free home protects everyone living there – especially the little ones.

So it is best to ask guests and the people you live with to smoke only outdoors and at a sufficient distance from doors and windows. Anyone who smokes outside can also use a separate jacket and then avoid wearing it inside the living areas afterwards.

After smoking, it makes sense to wash your hands before coming into contact with other people, especially babies and young children. If your home is already contaminated by tobacco smoke from previous residents, a thorough cleaning may be a good idea. Depending on the extent of the contamination, further remediation measures may also be necessary.

In the car

The best protection against passive smoking in the car is a completely smoke-free interior. Due to the small interior volume, harmful substances from smoking can quickly build up to high concentrations. Even an open window or the ventilation system cannot completely prevent exposure. Residues from cigarette smoke settle in the interior and on surfaces.

If you can’t go without smoking, especially on longer journeys, regular breaks outside the vehicle are a better option than smoking in the car. It’s important to keep your distance from other people during these breaks so they are not directly exposed to tobacco smoke.

Another option is to switch to smoke-free nicotine products such as tobacco-free Nicotine Pouches. These do not produce tobacco smoke, so there is no exposure of passengers to secondhand smoke. However, they are not harmless to health either and can continue nicotine dependence. In the long term, quitting smoking completely is the best way to protect both your own health and the health of others.

In restaurants and at the workplace

In Germany, there is generally a legal right to a smoke-free workplace¹⁹. However, there are differences between the federal states when it comes to non-smoker protection in the hospitality industry, as the specific regulations are defined at state level¹⁸. In most federal states, smoking is generally prohibited in enclosed spaces, although different exceptions may apply depending on state law.

If you want to avoid exposure to tobacco smoke, it makes sense to check before going out to eat whether it is a smoke-free restaurant. If you are already seated and smokers are sitting right at the next table, it can also help to politely ask for a little distance or move to a smoke-free area.

Quitting smoking as the most effective measure

Is it enough to smoke only on the balcony to protect your family? Not quite. As explained in the section on third-hand smoke, residues still make their way into the home. The most reliable way to protect the people around you from secondhand smoke in your own household is therefore to quit smoking completely. This is especially important if children or pregnant women live with you, as they are particularly sensitive to the harmful substances in tobacco smoke.

Why do so many people find quitting smoking difficult despite the clear health reasons? The reason is rarely a lack of willpower, but rather nicotine’s high addiction potential. It’s not for nothing that people speak of a nicotine addiction. Support and professional advice can help you overcome this hurdle and successfully lead a smoke-free life. For guidance, it’s best to read our guide to quitting smoking or our practical tips for quitting smoking and check out home remedies and self-help techniques.

More reasons to quit smoking

Besides protecting the people around you from secondhand smoke, there are more reasons why smoking is harmful and why it’s worth quitting:

• Smoker’s leg, smoker’s cough, and smoker’s lung are typical smoking-related illnesses, and the risk of these decreases with every smoke-free year.
• Smoking during pregnancy harms two bodies and comes with significant risks and consequences for the baby.
• Numerous benefits of quitting smoking for your body, mind, and wallet.