Thirdhand smoke refers to the residual constituents of tobacco smoke that settle and persist in the indoor environment after smoking has occurred. The constituents of tobacco smoke can remain on surfaces and in dust and react with other compounds in the indoor environment to form harmful byproducts, both of which can then be re-emitted back to the gas phase over the course of hours, weeks, and months. Particulate matter from tobacco smoke that remains on surfaces and in dust can also be re-suspended in the air of the indoor environment.^1,2

4.3.1 What is in thirdhand smoke?

As described in Section 12.4, there are more than 7,000 constituents in tobacco smoke, including over 60 carcinogens. While some of these constituents are removed from the indoor environment through ventilation, a proportion are deposited on surfaces and in dust where they can remain and react with other compounds.

Analyses of house dust samples have been conducted to further understand what constituents are present in thirdhand smoke.^3-6 One analysis identified 140 constituents that were specific to the homes of people who smoke, or present at higher levels compared to the homes of people who do not smoke.³ Of these constituents, 10 were identified as potential markers for assessing the presence of thirdhand smoke, including nornicotyrine, nicotyrine, 3-ethenylpyridine, myosmine, 2,3’-dipyridyl, nicotine (already used as a thirdhand smoke marker), tris phosphate, benzyl methyl ketone, scopoletin, and propanoic acid.³

Nicotine is an important constituent of thirdhand smoke that reacts with ambient nitrous oxide to form tobacco specific nitrosamines, most of which are potent carcinogens. These nitrosamines include NNK and NNN, which are found in thirdhand smoke as well as in mainstream and sidestream smoke. Another nitrosamine known as NNA is rarely found in mainstream or sidestream smoke but can be detected in thirdhand smoke.⁷ NNA causes damage to DNA and may be contributing to the possible health effects thirdhand smoke exposure.⁸ Other known carcinogens present in thirdhand smoke include benzene, benzopyrene, cadmium, and formaldehyde. There are also several other constituents in thirdhand smoke that are classified as probable or possible carcinogens by the International Agency for Research on Cancer.⁹

4.3.2 Exposure to thirdhand smoke

The constituents of thirdhand smoke can accumulate in the homes of people who smoke and once they are present, they can persist for months and years, even if smoking no longer occurs. Nicotine and NNK have been found at higher levels in the homes of people who smoke.^10-12 One study found that nicotine levels were approximately 35–98 times higher in the air, 12–21 times higher in dust, and 30–150 times higher on surfaces in the homes of people who smoke compared to the homes of people who do not smoke.¹⁰ Another study explored the impact of smoking cessation on thirdhand smoke presence in the home, and found that six months after cessation, nicotine levels declined on surfaces and on the fingers of residents who do not smoke. However, nicotine and NNK levels in dust remained mostly unchanged.¹³

Exposure to thirdhand smoke can occur via inhalation, ingestion and absorption through the skin. Infants and young children are particularly vulnerable to thirdhand smoke exposure because they spend more time indoors and have frequent contact with contaminated surfaces. They tend to be active near the floor, close to contaminated dust, carpets, and blankets, put objects and toys in their mouths, and are also in close physical contact with their smoking parents, whose clothes, hair and skin tend to be coated with thirdhand smoke.¹¹ Infants and young children who reside in disadvantaged households are even more vulnerable to thirdhand smoke exposure. This may be due to their increased likelihood of residing in a home where smoking previously occurred, or because they have a parent who smokes inside the home.^14,15 See Section 9.1.6 for more information about disadvantage and secondhand smoke exposure.

Levels of nicotine in the home have been demonstrated to decrease after a change in occupancy from occupants who smoke to occupants who do not smoke. However, nicotine can persist in the indoor environment, and has been found in the air, on surfaces, and on the fingers of new occupants two months after moving into a home where smoking previously occurred.¹⁰ One study found that nicotine levels on the fingers of new occupants who moved into a home where smoking previously occurred were 7–8 times greater compared to occupants who moved into a home were smoking did not occur.¹⁰

Even with indoor smoking bans, thirdhand smoke presence and exposure among children remain higher than in completely smoke-free households.^10,11,16,17 This may be partly due to smoke drift from outdoor areas through open windows and doors, and the residual smoke coming from the lungs of a person who smokes. However, indoor contamination is also likely due to past indoor smoking as well as introducing smoke constituents through contaminated clothing, skin and dust carried into the home.¹¹ The introduction of nicotine to environments where smoking has never occurred has been demonstrated in a study of nicotine contamination in a neonatal intensive care unit.¹⁸ In this study, nicotine was tested for on hospital furniture and in the urine of newborn babies who had a parent who smoked. Nicotine was found on cots and other hospital furniture as well as in the urine of the newborn babies participating in the study.

4.3.3 What are the health effects of thirdhand smoke exposure?

Evidence on the health effects of thirdhand smoke exposure is emerging. Despite thirdhand smoke containing known carcinogens and other toxic chemicals, the extent to which it contributes to human disease is not yet understood.

There is some evidence from laboratory studies that suggests thirdhand smoke exposure can have genotoxic effects and impair cellular processes.^8,19-21 Evidence from animal studies suggests that thirdhand smoke exposure may be associated with gastric cancer,²² thrombosis,^23-25 delayed wound healing,²⁶ and changes to the immune system.^27,28

A small clinical study demonstrated that acute inhalation of thirdhand smoke caused changes in nasal epithelial cells, specifically, the activation of DNA repair pathways, increased mitochondrial activity, and increased cell proliferation.²⁹ Another found that acute dermal exposure to thirdhand smoke activated the immune system and elevated biomarkers that are associated with inflammatory skin diseases such as dermatitis and psoriasis.³⁰

For information on public attitudes to thirdhand smoke and its possible health effects, see Section 4.19.

4.3.4 Policy responses to thirdhand smoke

Smokefree policies were originally introduced to protect people from the harms of secondhand smoke—see Chapter 15 for more information. However, these policies may not protect people from being exposed to the constituents of tobacco smoke that linger in indoor environments after smoking has stopped. As described below, several policy recommendations have been made to protect people against the emerging harms of thirdhand smoke.⁹

1. Redefine ‘smokefree’ as ‘free of tobacco smoke pollutants’
   Expand the goal of smokefree policies to achieve indoor environments that are free of tobacco smoke pollutants in the air, on surfaces, in dust and embedded in materials.

2. Identify indoor environments with thirdhand smoke reservoirs
   Update smokefree policies to require the identification of indoor environments that are contaminated with thirdhand smoke. This could be achieved through the disclosure of any smoking history in the context of real estate and vehicle transactions, or through testing for the presence of thirdhand smoke.

3. Eliminate exceptions to comprehensive indoor smoking bans
   Close loopholes in existing smokefree policies that exempt certain indoor environments. For instance, some indoor environments may only have smokefree policies during the day and not at night, or only when children are present.

4. Remediate thirdhand smoke pollution
   Undertake efforts to remove thirdhand smoke contamination in indoor environments. This may involve carpet cleaning, repainting walls, and wiping surfaces, and in severe cases, the replacement of drywall, floors, insulation, built-in furniture, and HVAC systems.

In October 2025, California introduced legislation addressing thirdhand smoke in the context of real estate transactions. Under Assembly Bill 455, sellers of single-family homes are required to disclose any known history of smoking or vaping. The Bill also required the Homeowner’s Guide to Environmental Hazards to be updated with information on thirdhand smoke.³¹

Related reading

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References

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