Secondhand smoke is the smoke you breathe in when someone nearby is smoking. It includes the smoke given off by the burning tobacco. And it includes the smoke breathed out by the smoker.

In the U.S., cigarettes are the main source of secondhand smoke. Pipes, cigars, and other ways of smoking tobacco can also give off secondhand smoke. One cigar can create as much secondhand smoke as a whole pack of cigarettes.

If you're in an area where other people are smoking, you're being exposed to secondhand smoke. You may breathe in secondhand smoke in bars, restaurants, or other public places. And you may breathe it in at home, at your workplace, or in a car.

Tobacco smoke is a mixture of gases and particles. It is filled with many chemicals that can harm your health. Being exposed to the smoke increases your risk of lung diseases, some types of cancer, and other health problems. Even contact with a small amount of smoke can cause problems.

Millions of people in the U.S. are exposed to secondhand smoke each year. This includes many young children. Children are more at risk for the effects of secondhand smoke.

In recent years, contact with secondhand smoke has lessened. This is because smoking has been banned in many public places. Also, fewer people smoke. And more homes have smoke-free rules.

Thirdhand smoke (THS) is a term you may hear. THS is the smoke that stays on surfaces, such as furniture, toys, and carpets, and on clothing, skin, and hair. The American Academy of Pediatrics warns of the possible harmful effects of THS.

No level of secondhand smoke is safe. Even a small amount of contact can cause problems.

Secondhand smoke has thousands of chemicals. Many of these are known to be harmful, including:

• Nicotine.
• Ammonia.
• Arsenic.
• Benzene.
• Beryllium.
• Carbon monoxide.
• Formaldehyde.
• Hydrogen cyanide.
• Toluene.

The chemicals found in secondhand smoke depend on:

• The type and amount of tobacco.
• The chemicals added to it.
• The way it's smoked.

The chemicals have been found in the blood of nonsmokers who have been in contact with secondhand smoke. Many of these chemicals are known to cause cancer. Secondhand smoke is called a cancer-causing agent by several U.S. agencies.

These chemicals irritate your airways as soon as you're in contact with the smoke. The chemicals also have instant harmful effects on your heart and blood vessels. They may increase your blood pressure and lower your HDL ("good") cholesterol. The smoke may increase the clotting of your blood. This can make it more likely for you to form a blood clot that can lead to a stroke or heart attack.

You may be at a higher risk if you:

• Live with a smoker.
• Socialize with people who smoke.
• Work in a place that allows smoking.
• Live in states that don't have a state ban on public smoking.

The smoke can cause a number of symptoms right away, such as:

• Coughing.
• Sneezing.
• Feeling short of breath.
• Eye irritation.

Many people in contact with the smoke may not have any symptoms right away. But that doesn't mean the smoke hasn't caused harm. It can still lead to problems over time. These include chronic bronchitis and scarring of the air passages in the lungs.

Secondhand smoke exposure can be measured. This is done by testing indoor air for chemicals found in tobacco smoke, such as nicotine.

Your doctor can test your own level of exposure. This is done by testing the level of cotinine in your blood, saliva, or urine. Cotinine is a chemical that is created after nicotine enters the body. If you have high levels of cotinine, you likely have high levels of other chemicals from contact with smoke.

But this type of testing is not often needed. If you spend a lot of time in places where people smoke, you likely have high levels of chemicals in your body from the smoke. This is true even if you don't smoke. If you spend only a small amount of time around smoke, your levels are likely lower.

Contact with secondhand smoke greatly raises the risk of some health problems. It may make some health problems happen more often. It may cause them to be more severe. Because of this, the smoke causes thousands of deaths. Health problems linked to secondhand smoke include:

• Lung cancer.
• Breast cancer.
• Brain tumors.
• Leukemia and lymphoma.
• Cancers of the larynx, bladder, stomach, and more.
• Heart disease, which may lead to a heart attack.
• Peripheral artery disease (PAD).
• Stroke.
• Ear infections, especially in children.
• Asthma, especially in children.
• Severe lung disease, such as chronic obstructive pulmonary disease (COPD).
• Respiratory infections, such as bronchitis and pneumonia.
• Sudden infant death syndrome (SIDS).
• Miscarriage, stillborn birth, or babies with low birth weights.

It's important to protect all people from secondhand smoke. It's most important to protect children and people with health problems. Stay away from places where smoking is allowed. If you live with a smoker, ask them to smoke only outside. Don't let anyone smoke in your car.

Open windows, air filters, and air ventilation systems may lessen contact with secondhand smoke. But they don't stop contact. Preventing smoking indoors is the only way to protect people.

Many towns and cities have taken steps to reduce secondhand smoke. Many cities now ban smoking in public places. These bans include places such as restaurants, bars, hospitals, airports, and schools. Many states have banned smoking in the workplace. Many advocacy groups argue for more smoking bans to protect people from secondhand smoke.

• Secondhand smoke is the smoke you breathe in when someone nearby is smoking. You breathe in the chemicals in tobacco smoke, even if you don't smoke.
• The smoke causes instant harm to the lungs, heart, and blood vessels.
• The smoke can lead to many health problems. These include cancer, heart disease, stroke, lung infections, and asthma.
• No level of contact with secondhand smoke is safe. Your risk of health problems may increase with the amount of smoke you're exposed to.
• Millions of people in the U.S. are exposed to secondhand smoke each year.
• You can help prevent harm from secondhand smoke by staying away from places where smoking is allowed. Don't allow smoking in your home, car, or around your children.

Here are some tips to help you get the most from a visit to your doctor.

• Know the reason for your visit and what you want to happen.
• Before your visit, write down questions you want answered.
• Bring someone with you to help you ask questions and remember what your provider tells you.
• At the visit, write down the name of a new diagnosis and any new medicines, treatments, or tests. Also write down any new instructions your doctor gives you.
• Know why a new medicine or treatment is prescribed and how it will help you. Also know what the side effects are.
• Ask if your condition can be treated in other ways.
• Know why a test or procedure is recommended and what the results could mean.
• Know what to expect if you do not take the medicine or have the test or procedure.
• If you have a follow-up appointment, write down the date, time, and purpose for that visit.
• Know how you can contact your doctor if you have questions, especially after office hours or on weekends and holidays.