Definition and Dangers of Passive Smoking

Passive smoking refers to the involuntary inhalation of smoke from cigarettes or other tobacco products smoked by other people. The definition includes exposure to both secondhand and thirdhand smoke, as well as in utero exposure of a fetus due to the presence of tobacco toxins in the mother's blood. While many people link passive smoking with lung cancer, it is actually much more likely to cause heart disease or strokes. And while the connection with respiratory infections, sudden infant death syndrome, and miscarriage has been known for some time, it is now thought to be an important risk factor in conditions ranging from multiple sclerosis to depression. Learn about the types of passive smoking, effects and dangers, and how to avoid secondhand smoke in your daily life.

Passive Smoking Exposures

Exposure to environmental tobacco smoke (ETS) has gained considerable attention in recent years, and exposure is now broken down into two types of secondhand as well as thirdhand smoke.

It's important to note that passive smoking includes exposure to not only cigarette smoke, but smoke and vapors from cigars, hookah, marijuana, and even e-cigarettes.

Types of Secondhand Smoke

There are two different types of secondhand smoke. While these have been grouped together in the past, they may affect people who are exposed ("passively smoke" them) in different ways. In addition, you may be more likely to be exposed to one type than the other depending on the setting.

• Mainstream Smoke (MSM): The term mainstream smoke refers to the smoke exhaled by a smoker.
• Sidestream Smoke (SSM): The term sidestream smoke refers to the smoke that is released from the end of a cigarette, cigar, pipe, hookah pipe, or joint, and accounts for roughly 85 percent of secondhand smoke exposure. SSM can be a greater danger than MSM not only in that it contains greater amounts of carcinogens and toxins, but because it persists for a longer period of time—often lasting even after a cigarette has been extinguished.

Passive smoking can result in different levels of exposure based on several variables. These include:

• Heat
• Humidity
• Ventilation in a room, car, or other space
• How many smokers are present, and how much they smoke

Thirdhand Smoke

Thirdhand smoke, the gases, and particles left over after a cigarette or another form of tobacco are extinguished, may also be inhaled via passive smoking. Through a process called "off-gassing," substances that have been deposited on surfaces as a result of smoking are released back into the air as gasses. Though this is likely a minor portion of the secondhand smoke inhaled as a result of passive smoking, thirdhand smoke can remain a problem for a long period of time after smoking has occurred.

Thirdhand smoke is of particular danger to young children who may be crawling around on the surfaces where thirdhand smoke accumulates. In addition, children are more likely to ingest these particles than adults.

Exposure to this kind of smoke may increase the risk of asthma development in children, and may also cause exacerbations in children who already have asthma.

Dangers of Passive Smoking

Just as smokers are exposed to known carcinogens and other toxic substances, passive smokers are exposed as well. Secondhand smoke is now considered a class A carcinogen (the worst kind) and there is no level of exposure that is considered safe. A few of the medical conditions that have been linked to passive smoking include:

Lung Cancer

Certainly, lung cancer is the first consequence of passive smoking that most people may think of, but the concerns don’t stop here. People are actually 15 times more likely to die from heart disease due to passive smoking than lung cancer. Roughly 7,300 people die from lung cancer as a result of secondhand smoke exposure each year and living with someone who smokes increases the risk of developing lung cancer by 20 to 30 percent.

Other Cancers

Just as smoking is associated with a number of different cancers, passive smoking is as well. A few cancers that are more common in people exposed to secondhand smoke include head and neck cancers, bladder cancer, kidney cancer, esophageal cancer, pancreatic cancer, and acute myelogenous leukemia (AML). A 2018 study found that passive smoking is associated with an increased risk of cervical cancer. Passive smoking is also being evaluated as a potential cause for a recent outbreak of gallbladder cancer (cholangiocarcinoma) in China.

Heart Disease and Strokes 

According to the CDC, secondhand smoke is thought to cause 34,000 deaths from heart disease and 8,000 deaths from strokes in non-smokers in the United States each year, with numbers even higher being reported by the World Health Organization. Passive smoking raises the risk of heart disease by 25 to 30 percent and the risk of stroke by 20 to 30 percent. The risk of peripheral arterial disease is increased as well.

Even exposures less than 30 minutes in duration cause detectable changes in blood vessels that are associated with heart disease, so again, no level of exposure is safe.

Lung Disease

Passive smoking is associated with diseases including asthma and chronic obstructive pulmonary disease and can result in a worsening of asthma in children and adults who already have the condition.

Lung Infections

Roughly 50,000 to 300,000 cases of lower respiratory infections such as pneumonia occur each year in the U.S. in children under 18 months due to secondhand smoke. Children who live with a smoker and develop these infections are also more likely to need intensive care and ventilator support. Passive smoking in children is also associated with an increased risk of middle ear infections as well as meningococcal disease (meningitis and meningococcemia).

Sudden Infant Death Syndrome

Young children exposed to secondhand smoke have a higher risk of sudden infant death syndrome.

Pregnancy Complications and Birth Defects

Passive smoking while pregnant increases the risk of miscarriage, stillbirth, and low-birth-weight babies. According to a 2019 review, maternal smoking, maternal passive smoking, and paternal smoking were all linked to an increased incidence of congenital heart defects.

Less Well-Known Associations

In addition to the above risks, research is also finding that passive smoking may be linked with a number of diseases not previously associated with secondhand smoke.

• Multiple Sclerosis: A 2019 review notes that passive smoking is considered a key environmental risk factor for multiple sclerosis.
• Depression: A 2019 study found that depressive symptoms are 57 percent more common among people exposed to secondhand smoke than those not exposed.
• Other Conditions: Listing all recent associations with passive is beyond the scope of this article, but includes conditions ranging from metabolic syndrome to concerns over reduced levels of antioxidant enzymes in the bloodstream of those exposed to secondhand smoke.

Passive Smoking Can be Additive

Most diseases associated with smoking (or secondhand smoke) are multifactorial, meaning that more than one factor is often involved.

With heart disease, the combination of passive smoking and a family history of coronary artery disease or high blood pressure increases risk beyond that of a single risk factor alone.

With lung cancer, passive smoking plus exposure to household radon (the leading cause of lung cancer in never smokers) raises risk beyond the risk inherent in either factor alone. In some cases, combining two risk factors is more than additive. For example, the combination of smoking plus asbestos exposure is riskier than would be expected from simply adding the health risks of the two together.

The addition of passive smoking can likewise further increase the risk of a wide range of medical conditions, ranging from miscarriage to pneumonia.

Preventing Passive Smoking

Fortunately, there are laws in place that now limit passive smoking in public places, but outside of places where smoking is regulated, there are still many things you can do to lower your exposure.

• Do not allow others to smoke in your home or in your car.
• Teach your children to avoid secondhand smoke and be a good role model yourself by not smoking.
• Many public places in the United States are now smoke-free, but this isn't always the case when you travel abroad. Avoid establishments that allow smoking indoors or out-of-doors.

A Word From Verywell

It's much easier than in the past to avoid "passive smoking" although there are a number of situations in which people could still be at risk. There are no laws that prevent smoking in homes or cars, places where children as adults are often present. Though you may not always be popular for your choice, choose to always refuse to allow yourself to be a passive smoker. In some cases, standing up for your own health may even be the encouragement that someone who smokes needs to consider quitting.