The short answer
There is no safe level of secondhand smoke exposure. Cigarette smoke contains thousands of chemicals, including chemicals that damage the lungs and blood vessels and chemicals known to cause cancer. In apartment buildings, secondhand smoke can move from one unit to another through ventilation systems and other openings. After the visible smoke is gone, tobacco chemicals can stay on walls, carpet, furniture, dust, and clothing as thirdhand smoke.
What is in cigarette smoke?
Cigarette smoke is a mix of gases, tiny particles, nicotine, and many other chemicals. The Centers for Disease Control and Prevention says commercial tobacco smoke contains more than 7,000 chemicals. Hundreds are toxic, and about 70 can cause cancer.
The National Cancer Institute lists harmful chemicals in tobacco smoke such as carbon monoxide, formaldehyde, benzene, arsenic, cadmium, hydrogen cyanide, tobacco-specific nitrosamines, and radioactive polonium-210. A person does not have to be the smoker to breathe many of these chemicals.
Smoke you cannot see can still matter
EPA warns that not seeing or smelling smoke does not mean smoking-related chemicals are gone. How long pollutants stay in a room depends on the size of the room, ventilation, the amount smoked, and the surfaces that can hold the chemicals.
How cigarette smoke affects the human body
Smoking harms nearly every organ in the body. Secondhand smoke can also damage the body of a person who does not smoke. The harm depends on how much smoke reaches you, how often it happens, and how long it continues.
Lungs and airways
Smoke irritates the nose, throat, and lungs. It can cause coughing, wheezing, chest irritation, and more mucus. Long-term cigarette smoking is a major cause of COPD and lung cancer. Secondhand smoke can trigger asthma attacks and raises the risk of lung cancer in adults who do not smoke.
Heart and blood vessels
Secondhand smoke can quickly affect the lining of blood vessels and make blood platelets stickier. That raises the chance of a heart attack or stroke. CDC estimates secondhand smoke raises the risk of coronary heart disease by about 25% to 30% and stroke by about 20% to 30% in adults who do not smoke.
Blood and oxygen
Carbon monoxide in tobacco smoke reduces the blood's ability to carry oxygen. Nicotine also puts extra stress on the heart and blood vessels. This is one reason even short periods of smoke exposure can be a problem for people with heart or breathing conditions.
Cancer risk
Tobacco smoke contains many cancer-causing chemicals. Secondhand smoke is a known human carcinogen. Adults who do not smoke but are regularly exposed to secondhand smoke have a higher risk of lung cancer.
Babies and children
Children are especially vulnerable. Secondhand smoke is linked to more asthma attacks, respiratory infections, ear infections, slower lung growth, and sudden infant death syndrome (SIDS).
Pregnancy
Secondhand smoke exposure during pregnancy is linked with lower birth weight. Pregnant tenants should take smoke entering the home seriously and discuss ongoing exposure with a health care professional.
Secondhand smoke in apartments and shared ventilation
Secondhand smoke is smoke from a burning cigarette plus smoke breathed out by the person smoking. CDC and EPA both warn that secondhand smoke can move between apartment units.
Smoke can travel through shared ventilation systems, hallways, stairwells, gaps around pipes, light fixtures, electrical outlets, ceiling spaces, doorways, and cracks in the building. This means a tenant can have a strict no-smoking rule inside their own apartment and still be exposed to cigarette smoke from another unit.
Fans and air cleaners are not a complete fix
Ventilation, filters, air purifiers, and open windows may reduce some pollution, but EPA and CDC state that they do not eliminate secondhand smoke. The most effective solution is stopping smoking inside the building or near places where smoke can be pulled back indoors, such as windows, doors, and outdoor air intakes.
What is thirdhand smoke?
Thirdhand smoke is the tobacco-related chemical residue that remains after the cigarette is out and the visible smoke is gone. These chemicals can stick to walls, floors, furniture, carpet, curtains, bedding, clothing, skin, and household dust.
EPA explains that some of these chemicals can later come back into the air. People can be exposed by breathing contaminated air, touching contaminated surfaces, or getting contaminated dust on their hands and then into their mouth. This is especially important for babies and young children who crawl on floors and put their hands and objects in their mouths.
Research funded by NIH has found nicotine and tobacco-specific chemicals on surfaces in homes where smokers live. Laboratory research has also shown that nicotine left on surfaces can react with other indoor chemicals and form tobacco-specific nitrosamines, some of which are cancer-causing.
What we know — and what is still being studied
The dangers of active smoking and secondhand smoke are firmly established. Scientists are still working out the exact size of the long-term human health risk from thirdhand smoke. What is clear is that tobacco chemicals can remain on surfaces and in dust long after smoking stops, and some can return to the air or change into other harmful chemicals.
How to use an air quality meter when cigarette smoke enters your apartment
A consumer air quality meter can help you build a time-stamped record of changes in your indoor air. It can be useful when smoke comes through shared vents or from a neighboring unit. But a meter is a screening and documentation tool, not a laboratory test.
Important: CO2 is not carbon monoxide
CO2 means carbon dioxide. CO means carbon monoxide. They are different gases. A CO2 reading does not replace a certified household carbon monoxide alarm.
Read the manual first
Find out what the meter measures, what units it uses, how long it needs to warm up, and whether the manufacturer requires a fresh-air calibration.
Create a normal baseline
Before a smoke event, record CO2, HCHO, TVOC, and PM2.5 if your meter has it. Take readings at the same location for several days. A baseline is more useful than one isolated number.
Put the meter in your breathing zone
EPA suggests placing low-cost monitors where people breathe, generally about 3 to 6 feet above the floor, with open airflow around the device. Do not put it right next to your face, a window, a candle, a stove, or cleaning products.
Keep the meter in the same spot
If smoke normally enters near a vent, start with a repeatable location a few feet from that vent rather than holding the meter directly in the air stream. Consistent placement makes before-and-after comparisons more meaningful.
Record what is happening
When you smell cigarette smoke, note the date, time, room, HVAC status, whether windows are open, and any cooking or cleaning activity. Take a photo or screenshot of the meter display.
Look for repeatable patterns
One spike may be caused by cooking, cleaners, perfume, new furniture, or another source. A stronger record is a pattern that repeats when cigarette odor enters your unit and falls when the event ends.
What CO2, HCHO, and TVOC readings mean around cigarette smoke
The most important rule is this: none of these three readings is a cigarette-smoke fingerprint. They can help show that indoor air conditions changed, but other household sources can cause the same readings.
| Meter reading | What it tells you | How it relates to cigarette smoke | How to read it |
|---|---|---|---|
| CO2 Carbon dioxide Usually shown in ppm |
CO2 is mainly useful as a ventilation clue. People breathing in a closed room are usually the main indoor source. | Cigarette smoke is not diagnosed by CO2. A high CO2 level can tell you the room has weak fresh-air exchange, which can allow smoke pollutants to build up and clear more slowly. | Below about 1,000 ppm: does not prove the air is clean. Around or above 1,000 ppm: EPA guidance says this can point to inadequate outdoor-air ventilation for the number of people present. Watch the trend, not just one number. |
| HCHO Formaldehyde Often mg/m³ or ppm |
HCHO is the chemical formula for formaldehyde, an irritating gas and known human carcinogen. | Tobacco smoke is one source of formaldehyde. A jump in HCHO that repeatedly happens when cigarette smoke enters may support your record, but furniture, pressed wood, glues, cooking, and other combustion can also raise it. | The World Health Organization indoor-air guideline is 0.1 mg/m³ averaged over 30 minutes. EPA notes that levels above about 0.1 ppm can cause eye, nose, and throat irritation in some people. Always check your meter's units before comparing numbers. |
| TVOC Total volatile organic compounds Often mg/m³ or ppb |
TVOC is a combined signal from many gases that evaporate into indoor air. | Cigarette smoke contains many VOCs, so TVOC may rise during a smoke event. But cleaners, air fresheners, perfume, paint, fuel, new flooring, furniture, and cooking can do the same thing. | There is no single federal indoor "safe TVOC" number. EPA warns that the total number does not tell you which VOCs are present or how toxic they are. Compare your normal baseline with repeated event spikes. |
| PM2.5 Fine particles Usually µg/m³ |
PM2.5 measures very small airborne particles that can get deep into the lungs. | Secondhand cigarette smoke contains PM2.5. If you are choosing a monitor specifically for smoke problems, PM2.5 is one of the most useful measurements to have. Cooking, candles, fireplaces, and outdoor pollution can also raise it. | Use your normal indoor level as a baseline and watch for sharp, repeatable increases that line up with smoke odor. A PM2.5 spike is evidence of a particle pollution event, not proof by itself that cigarettes caused it. |
Meter caution: EPA says low-cost indoor monitors vary in accuracy. Placement, age, humidity, temperature, and other chemicals can change sensor performance. Manufacturer color zones such as “good,” “fair,” or “poor” are not universal health standards.
What an air quality meter can — and cannot — prove
It can help document
Dates and times of pollution events, changes from your normal baseline, possible poor ventilation, particle spikes, and repeated HCHO or TVOC increases that happen when you smell cigarette smoke or when shared ventilation turns on.
It cannot prove by itself
That a specific neighbor smoked, that nicotine is present, that a TVOC reading is dangerous, or that thirdhand smoke chemicals are on a particular surface. Confirming specific chemicals normally requires professional sampling or laboratory testing.
If your goal is to document secondhand smoke coming through a shared vent, the strongest practical record combines meter data + smoke odor log + dates/times + photos or video + written complaints + any witness statements.
What tenants can do when cigarette smoke enters the home
- Keep a written smoke log. Record date, time, room, duration, odor strength, and where the smoke seems to enter.
- Save air-meter screenshots. Include the time and keep the meter in the same location when possible.
- Put repair or smoke complaints in writing. Email or use the landlord's written maintenance system so there is a record.
- Ask management to inspect smoke pathways. Shared vents, pipe openings, wall outlets, doors, and pressure differences between units can move smoke.
- Do not rely on fragrance or air freshener. Covering the odor does not remove cigarette smoke chemicals and can raise TVOC readings.
- Use filtration as a backup, not the main solution. A properly sized HEPA air cleaner can reduce particles. Gas-phase filtration such as activated carbon may reduce some gases, but EPA says air cleaning does not eliminate secondhand smoke exposure.
- Get tenant help if the problem continues. RentalFacts.online maintains tenant assistance resources that can help renters find legal-aid programs, advocacy groups, fair-housing resources, and other organizations.
When to get medical help
If smoke exposure causes repeated coughing, wheezing, headaches, burning eyes, shortness of breath, or asthma symptoms, talk with a health care professional. Keep notes about when symptoms happen and whether they improve when you are away from the apartment.
Emergency warning
Call 911 for severe trouble breathing, chest pain, fainting, blue lips or face, or another life-threatening emergency. An indoor air quality meter is not an emergency medical device.
Why environmental exposure matters
Environmental health science looks at the chemicals people actually meet in everyday life, not just one chemical in a laboratory. Dr. Linda S. Birnbaum, a board-certified toxicologist and former director of the National Institute of Environmental Health Sciences and National Toxicology Program, spent decades studying how real-world chemical exposures can affect health.
RentalFacts.online cites Dr. Birnbaum's NIEHS biography as background on environmental toxicology. This article is not written by, reviewed by, or endorsed by Dr. Birnbaum.
Frequently asked questions about cigarette smoke in apartments
Can secondhand smoke travel through apartment air vents?
Yes. CDC says secondhand smoke can drift between units through ventilation systems, hallways, and stairwells. EPA also identifies openings such as light fixtures, electrical outlets, ceiling spaces, and doorways as possible pathways.
Is smelling cigarette smoke enough to know I am exposed?
A cigarette odor is a useful warning sign that tobacco-related chemicals may be entering the space, but smell is not a dose meter. Some chemicals may remain after the odor fades, and some people can smell smoke at lower levels than others.
Does a high CO2 reading mean cigarette smoke is present?
No. CO2 is mainly a ventilation indicator. High CO2 can mean the room needs more outdoor air, but it does not identify cigarette smoke.
Does a high HCHO reading mean my neighbor is smoking?
No. Cigarette smoke can release formaldehyde, but many other indoor sources can raise HCHO, including pressed-wood products, furniture, glues, cooking, and combustion. Look for repeatable changes from your baseline and consider professional testing if formaldehyde remains elevated.
Does a high TVOC reading prove secondhand smoke?
No. TVOC responds to many kinds of gases. A smoke event can cause TVOC to rise, but so can cleaners, perfume, paint, new furniture, fuel, and cooking. TVOC is best used as a trend indicator.
What is the best air quality measurement for cigarette smoke?
If you are choosing a monitor for a smoke problem, look for a unit with a true PM2.5 particle sensor and a true NDIR CO2 sensor. HCHO and TVOC can add useful information, but they are less specific. No consumer monitor can identify every harmful chemical in cigarette smoke.
Can thirdhand smoke come back into the air?
Yes. EPA says chemicals left behind after smoking can stay on indoor surfaces and soft materials, and some can later be released back into the air. Research also shows tobacco residues can react with other indoor chemicals.
Authoritative sources and references
- Centers for Disease Control and Prevention (CDC): About Secondhand Smoke and Health Problems Caused by Secondhand Smoke.
- CDC: Preventing Exposure to Secondhand Smoke in the Home.
- U.S. Environmental Protection Agency (EPA): Secondhand Tobacco Smoke and Indoor Air Quality.
- EPA: Secondhand Smoke and Electronic-Cigarette Aerosols — includes information on chemicals remaining indoors and thirdhand exposure.
- EPA: Secondhand Smoke Exposure Coming From a Neighbor's Apartment.
- EPA: Low-Cost Air Pollution Monitors and Indoor Air Quality.
- EPA: Using Indoor CO2 to Get Information on Ventilation.
- EPA: Formaldehyde and Indoor Air Quality.
- EPA: Volatile Organic Compounds' Impact on Indoor Air Quality and EPA explanation of TVOC limitations.
- National Cancer Institute (NCI): Harms of Cigarette Smoking and Health Benefits of Quitting.
- National Institute of Environmental Health Sciences (NIEHS): Linda S. Birnbaum, Ph.D., D.A.B.T., A.T.S..
- National Institutes of Health / PubMed: Formation of carcinogens indoors by surface-mediated reactions of nicotine with nitrous acid.
- National Institutes of Health / PubMed: Contamination of surfaces in children's homes with nicotine and the tobacco-specific nitrosamine NNK.
- National Institutes of Health / PubMed: Human transport of thirdhand tobacco smoke into nonsmoking environments.
- World Health Organization (WHO): Air quality, energy and health — pollutant guideline values.