A Critical Perspective on the Role of Thirdhand Smoke in Tumorigenesis: Initiator or Promoter

Abstract

Thirdhand smoke (THS), lingering tobacco residues on surfaces and in dust after smoking, has emerged as a growing public health concern. While the carcinogenic risks of first- and secondhand smoke are well established, the tumorigenic potential of THS remains inadequately understood. In this perspective, we systematically review the known THS-related compounds together with the existing biological findings to provide evidence on the carcinogenic potential of THS. We assess the role of THS in the context of the classic multistage model of tumorigenesis to address whether THS acts primarily as a tumor initiator, or as a promoter, or both. We also discuss methodological challenges in isolating the effects of THS on human health and the importance of animal models and outline emerging strategies for risk assessment, biomarker discovery, and precision prevention. This Perspective is the first to integrate chemical, toxicological, and mechanistic evidence to evaluate the carcinogenic potential of THS within the framework of the multistage model of tumorigenesis, offering a new conceptual foundation for understanding the role of THS in human cancer etiology, which is crucial for informing public health policies aimed at mitigating its impact.

1. Introduction

Cancer is a major cause of death, with 9.7 million global deaths in 2022 and an estimated 618,120 in the United States in 2025, representing nearly 1 in 9 men and 1 in 12 women globally. Cancer arises from a complex interplay of genetic susceptibility and environmental exposures. , It is estimated that up to 40% of cancers worldwide are attributable to preventable environmental and lifestyle factors. − While classical carcinogens like tobacco smoke and asbestos are well recognized, − newer threats, such as thirdhand smoke (THS), − are gaining attention. Unlike firsthand smoke inhaled directly by the smoker, or secondhand smoke (SHS) exhaled or emitted from a burning cigarette, THS refers to the residual chemicals from tobacco smoke that persist in the environment long after the cigarette has been extinguished. − These residues can adhere to surfaces such as walls, furniture, clothing, carpets, and even dust particles, and create a long-lasting and often invisible source of toxic exposure. − Moreover, the evidence shows that these chemical residues can undergo chemical changes over time through interaction with environmental factors, including heat, humidity, and light, which results in the formation of additional carcinogenic compounds. − Furthermore, studies demonstrate the widespread presence of THS in indoor environments. −

As research and awareness of THS continue to grow, concerns about its potential health effects have increased, especially for vulnerable populations such as infants, young children, and nonsmoking adults who may come into contact with contaminated surfaces. , While public health efforts have traditionally focused on minimizing active smoke and SHS exposure, emerging evidence demonstrates that THS may also play a significant role in disease development. , For example, in human keratinocyte (skin cell) cultures, THS exposure caused dose-dependent cytotoxicity, reduced intracellular glutathione and SOD (antioxidant defenses), increased expression of matrix-degrading enzyme MMP-1, elevated IL-6 (inflammatory cytokine), impaired wound healing, and increased pro-inflammatory responses. A clinical study in healthy nonsmokers who underwent 3-h dermal exposure to THS (via clothing impregnated with THS) observed increased urinary biomarkers of oxidative damage (to DNA, lipids, proteins) and changes in plasma proteome consistent with activation of inflammatory pathways, skin-disease–related pathways (e.g., epidermal keratinization, psoriasis, dermatitis). Of particular interest is its possible contribution to cancer risk. − This concern is not unfounded, as THS contains a complex mixture of carcinogenic compounds, including tobacco-specific nitrosamines (TSNAs), polycyclic aromatic hydrocarbons (PAHs), heavy metals, and volatile organic compounds (VOCs), many of which are known to damage DNA or interfere with cellular processes. In addition, animal studies show THS exposure in early life stages increases cancer risk. − For example, our recent study showed that short-term THS exposure increased the incidence of gastric tumors in a mouse model of spontaneous gastric cancer; and in human gastric cancer cell lines, THS exposure promoted proliferation, invasion, migration, and activated epithelial–mesenchymal transition (EMT) pathways, a hallmark of cancer progression. These findings collectively suggest that THS represents a systemic environmental carcinogen, with the potential to impact diverse organs through persistent chemical residues and their bioactive metabolites, which is beyond the respiratory system.

Experimental studies have also shown that genetic background significantly influences THS-induced tumor risk. Despite this growing body of knowledge, important questions remain among them: What role does THS play in the multistage process of tumorigenesis? In cancer biology, carcinogens are often categorized based on their ability to initiate or promote tumors. , Tumor initiators are agents that cause genetic mutations or other permanent cellular alterations that can lead to cancer, whereas tumor promoters act by stimulating the proliferation of already altered cells, thereby accelerating cancer progression. Determining whether THS acts as a tumor initiator, a promoter, or both is crucial for understanding the role of THS in human cancer etiology, its prevention, as well as informing public health policies aimed at mitigating its impact.

In this Perspective, we aim to explore the current evidence regarding THS and its potential role in cancer development. We will examine the chemical constituents of THS and how these interact with biological systems, focusing on their genotoxic, mutagenic, and epigenetic effects. We will also discuss the experimental models and epidemiological studies used to evaluate the carcinogenic potential of THS and assess whether current findings support its classification as an initiator, a promoter, or a dual contributor to tumorigenesis and outline emerging strategies for risk assessment, biomarker discovery, and precision prevention. This Perspective uniquely synthesizes the catalog of THS-associated compounds with emerging biological data to assess their relevance to cancer development. By positioning THS within the classic initiation-promotion-progression model, it provides a novel lens for evaluating how THS may contribute to tumorigenesis and highlights innovative approaches, including animal models and biomarker discovery, needed to accurately assess THS-related cancer risk.

2. Thirdhand Smoke as a Tumor Initiator

Tumorigenesis is a multistage process consisting of initiation, promotion, and progression. Studies involving chemical carcinogens have helped establish the distinction between initiation and promotion. , These carcinogens are commonly classified according to their tumor-initiating or promoting capabilities. There is growing evidence that chemical compounds found in THS possess this mutagenic capability, positioning THS as a plausible and concerning contributor to cancer initiation (Figure , top panel).

1.

Potential roles of third-hand smoke in multistage tumorigenesis. Top panel: Tumor initiation role. Chemical compounds in thirdhand smoke (THS) form DNA adducts, leading to gene mutations that initiate tumor development. Bottom panel: Tumor promotion role. THS exposure contributes to tumor progression by inducing immune dysregulation, altering the microbiome, promoting angiogenesis, causing epigenetic modifications and oxidative stress, and triggering chronic inflammatory responses.

Chemical analyses have revealed that THS is composed of a complex mixture of toxic substances, many of which persist on surfaces and in dust for extended periods. , A recent study confirmed and extended the list of chemical constituents in THS. To date, over 100 chemical constituents have been identified in THS. − Among these, nicotine and TSNAs have emerged as particularly dangerous. TSNAs are formed when nicotine interacts with ambient nitrous acid (HONO), a common indoor pollutant, leading to the formation of potent carcinogens, such as NNK (4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone), NNN (N′-nitrosonornicotine), and NNA (1-(N-methyl-N-nitrosamino)-1-(3-pyridinyl)-4-butanal). , NNA, NNK, and NNN have been extensively studied in the context of tobacco-related carcinogenesis and are known to exert their effects by forming DNA adducts, in which chemical agents covalently bind to DNA, distorting its structure and potentially causing mutations during replication. ,,−

TSNAs are metabolized in the body into reactive intermediates that readily attack DNA. , These DNA adducts, if not adequately repaired by cellular mechanisms, can result in point mutations, chromosomal instability, and activation of oncogenes or inactivation of tumor suppressor genes, all of which are critical early events in tumor development. The respiratory tract, especially the lungs, is particularly susceptible due to its direct exposure to inhaled particulates and its high metabolic activity. Human and animal cells exposed to THS-contaminated materials exhibit elevated levels of DNA strand breaks, oxidative stress, and alterations in cell cycle progression. ,,,, These responses reflect the classical markers of genotoxic stress and early cellular transformation. Additionally, some studies have reported increased expression of genes involved in DNA repair and apoptosis following THS exposure, suggesting that cells recognize and attempt to respond to THS-induced genetic damage. ,,, Moreover, the risk associated with THS as a tumor initiator is particularly concerning for populations that may experience chronic, low-level exposure, such as infants and young children, who are more likely to come into direct contact with contaminated surfaces and ingest dust particles. Their developing tissues are more sensitive to environmental insults, and their detoxification systems are not fully matured, making them especially vulnerable to the mutagenic effects of THS.

Collectively, these findings indicate the potential of THS to function as a tumor initiator through mechanisms involving direct DNA damage and interference with normal cellular repair processes. While more research is needed to fully map the pathways involved and quantify risk across different exposure scenarios, the existing evidence warrants serious attention to THS as a significant carcinogenic threat in indoor environments. Moreover, the initiation step in carcinogenesis is considered irreversible as it involves permanent genetic alterations such as DNA mutations or epigenetic changes that can predispose a cell to malignant transformation. Once these alterations occur, they cannot be reverted, making early exposure to carcinogens particularly critical. Chronic contact with THS, especially in infants, children, and nonsmoking adults, may contribute to the initiation of tumorigenesis by introducing DNA-damaging compounds even in the absence of direct smoking, which underscores the need to recognize THS as a significant environmental risk. Therefore, policies and interventions aimed at minimizing THS exposure, such as smoke-free homes, cleaning contaminated surfaces, and public education, are critical to preventing irreversible genetic damage and reducing long-term cancer risk.

3. Thirdhand Smoke as a Tumor Promoter

While THS is increasingly recognized for its potential to initiate cancer through direct DNA damage, increasing evidence also suggests that it may play a crucial role in the promotion of the phase of tumorigenesis. Tumor promotion refers to the stage in cancer development in which previously initiated cells, those carrying early genetic mutations, are promoted to survive, expand, and acquire further genetic alterations. This phase does not necessarily involve direct mutagenesis but instead creates a microenvironment that favors the expansion of altered cells and the progression toward malignancy. THS, even at low levels of exposure, may significantly contribute to this process (Figure , bottom panel).

A central mechanism by which THS may promote tumorigenesis is oxidative stress. , Many chemical constituents in THS, including VOCs, aldehydes, and residual nicotine byproducts, can trigger inflammatory responses in exposed tissues. , This inflammation, in turn, leads to the production of reactive oxygen species (ROS) and reactive nitrogen species (RNS), unstable molecules that can inflict secondary damage to cellular components, such as DNA, proteins, and lipid membranes. Chronic exposure to these reactive molecules can compromise the integrity of the genome, interfere with cell signaling pathways, and suppress normal apoptotic processes, thereby enabling the survival and clonal expansion of precancerous cells. For example, ROS can modulate signaling pathways crucial for tumor development, such as the PI3K/Akt and MAPK pathways, which promote cell proliferation and survival. Importantly, oxidative stress not only inflicts damage but also fosters a proliferative and antiapoptotic cellular environment, tipping the balance away from cell death and toward uncontrolled growth. , This environment provides a selective advantage to mutated cells, which might otherwise be eliminated by normal physiological processes. In this way, THS does not need to be directly mutagenic to contribute to carcinogenesis; it simply needs to promote conditions in which genetically unstable cells can thrive.

Beyond oxidative stress, epigenetic modifications represent another pathway through which THS may act as a tumor promoter. ,, Epigenetics refers to heritable changes in gene expression that do not involve alterations in the DNA sequence itself, such as DNA methylation, histone modification, and noncoding RNA regulation. Studies have shown that exposure to THS can lead to aberrant epigenetic alterations that silence tumor suppressor genes or activate proto-oncogenes. For example, hypermethylation of promoters associated with cell cycle control or apoptosis can prevent these genes from being expressed, allowing damaged cells to bypass growth checkpoints and evade programmed cell death.

Furthermore, some research indicates that THS exposure may lead to chronic low-grade inflammation, , a known contributor to cancer promotion. Persistent inflammatory signaling can activate transcription factors like NF-κB and STAT3, which are commonly associated with tumor growth, immune evasion, angiogenesis, and metastasis. These pathways are frequently hijacked in the tumor microenvironment to support the progression of cancer. In addition, studies have shown that THS exposure can disrupt angiogenesis and can lead to immune suppression, ,,, which significantly influence tumor development and progression.

It is also worth noting that tumor promotion is often dose- and time-dependent; therefore, even low-level, repeated exposure to THS in indoor environments may be sufficient to exert these effects over time. This raises significant concerns for individuals who live in homes or environments contaminated with residual tobacco smoke, particularly children, the elderly, and individuals with preexisting health conditions.

Taken together, these findings suggest that THS may not only initiate carcinogenic processes through direct genotoxicity but also promote the progression of early stage mutations into full-blown malignancies. An important consideration in evaluating the tumor-promoting role of THS is the extent to which these promotional effects may be reversible. Unlike irreversible genotoxic events associated with tumor initiation, the inflammatory, oxidative, and epigenetic alterations induced by THS exposure are, in principle, modifiable following the removal of the environmental source. Emerging experimental data suggest that cessation of exposure can lead to partial normalization of oxidative stress markers, inflammatory cytokines, and certain epigenetic modifications, indicating that THS may operate as a potentially reversible promoter. This reversibility underscores the value of THS as a preventable risk factor for carcinogenesis. From a public health perspective, recognizing THS as a modifiable promoter highlights opportunities for intervention, including enhanced cleaning strategies, source elimination, ventilation improvements, and targeted exposure-reduction policies. Incorporating reversibility into risk assessment frameworks may also improve estimates of attributable cancer risk and inform precision prevention by elucidating the molecular basis of the THS promotional activity. Recognizing this dual role of THS, as both initiator and promoter, further emphasizes the need for comprehensive policies to reduce tobacco smoke residue in living and working spaces and protect populations from this insidious and persistent form of environmental contamination.

4. Challenges and Future Research

Isolating the effects of THS presents several methodological challenges that complicate efforts to assess its specific health impacts. Unlike active smoking or SHS, THS consists of residual tobacco compounds that persist on surfaces and in dust, interacting with environmental pollutants over time. This makes it difficult to establish standardized exposure metrics, especially in real-world settings where individuals are often simultaneously exposed to multiple sources of tobacco-related toxins, particularly those from SHS. Furthermore, the delayed and low-dose nature of THS exposure complicates causal inference, as health outcomes may emerge long after the initial contact. Current animal and in vitro studies offer insights into potential biological mechanisms, but their relevance to human health remains unvalidated due to differences in exposure duration, concentration, and environmental complexity. Future research should focus on:

• Biomarker discovery and validation: Develop a robust translational pipeline for biomarker discovery and validation, which is critical for risk assessments of THS. Key steps include:

  (i) defining exposure phenotypes to capture variability in THS-contaminated environments; (ii) collecting accessible biospecimens such as urine, saliva, hair, or nasal/oral brushings; (iii) integrating multimodal data sets, including multiomics, exposure metrics, and clinical data; (iv) controlling for confounders such as SHS or vaping; (v) deriving a parsimonious biomarker panel or score that reliably reflects THS exposure; and (vi) advancing clinical qualification, moving from analytical validity to clinical validity and ultimately to clinical utility by demonstrating decision-making impact. Importantly, this framework must prioritize the development of robust THS exposure biomarkers, which quantify internal exposure, and early biological effect biomarkers, which assess both short- and long-term health impacts.

• Risk assessment: Use the classic four steps: hazard identification → exposure assessment → dose–response → risk characterization, adapted to THS’ unique pathways: (i) dermal uptake from contaminated surfaces/fabrics, (ii) ingestion of dust (critical for infants/toddlers), and (iii) inhalation of re-emitted volatile and particle-bound residues.

• Prospective cohort: Relocate participants from high-THS to remediated homes, and track environmental measures, biomarkers, and symptoms.

• Quasi-experimental remediation trials: Compare repaint plus HVAC vs full carpet/furniture replacement to establish the cost-effective end points.

• Animal models: Use a two-stage tumorigenesis model (or initiation and promotion model) to experimentally assess THS as an initiator or promoter; and genetically diverse mouse models, such as Collaborative Cross (CC) mouse model, ,− to identify genetic susceptibility and fully characterize the molecular pathways through which THS contributes to tumorigenesis.

• Artificial intelligence (AI) methodologies: Develop AI-driven methodologies within THS research. AI offers transformative opportunities to advance THS research by accelerating biomarker discovery, risk assessment, and precision prevention. For example, AI plus wearable sensors for real-time exposure monitoring; cross-species comparative AI models (e.g., mouse-human) to validate mechanisms; and integration with cancer prevention initiatives.

• Precision prevention: Develop predictive modeling and digital health tools to personalize interventions for high-risk populations.

Overall, interdisciplinary approaches integrating toxicology, environmental science, and public health are essential for advancing our understanding of THS and for informing regulatory policies.

5. Conclusion

The growing body of evidence underscores the potential carcinogenic effects of THS, although its precise role in tumorigenesis is still largely unknown. While THS undoubtedly contains a variety of harmful chemicals capable of inducing genetic mutations, its role as a tumor promoter is equally significant. THS likely acts in a dual capacity, both as an initiator and a promoter of cancer through mechanisms involving DNA damage, oxidative stress, and epigenetic alterations. Given the persistence of THS in environments long after smoking has ceased, it is imperative that public health initiatives address this hidden source of tobacco-related harm. In the meantime, awareness campaigns should continue to highlight the dangers of smoking and the lasting effects of tobacco exposure, not only for active smokers but also for those who may unknowingly encounter THS in their daily environments.

Glossary

Abbreviations

DNA

DNA

NNA

1-(N-methyl-N-nitrosamino)-1-(3-pyridinyl)-4-butanal

NNK

4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

NNN

N′-nitrosonornicotine

PAH

polycyclic aromatic hydrocarbon

ROS

reactive oxygen species

RNs

reactive nitrogen species

SHS

secondhand smoke

THS

thirdhand smoke

TSNA

tobacco-specific nitrosamine

VOC

volatile organic compound

AI

artificial intelligence

Biographies

Dr. Hang graduated from Nanjing Medical University in 1982 and Fudan University in 1988 with an MS degree in Experimental Pathology. He received his PhD in 1994 from Rutgers University, NJ. He joined the Lawrence Berkeley National Laboratory as a postdoc in 1994 and worked as a Scientist and Staff Scientist until 2022. Dr. Hang’s primary research had been focused on combining chemical, biochemical, and biological approaches to understand key questions in environmental mutagenesis and carcinogenesis, with one of his research interests being the genotoxicity and carcinogenic potential of thirdhand smoke (THS). His lab and collaborators have shown, for the first time, that THS as well as its unique constituent NNA causes DNA strand breaks and oxidative damage in human genes. Later, his studies indicated that short-term early life exposure to THS can induce lung cancer in A/J mice as well as various tumors in different organs in CC mice. These findings were highlighted by prominent coverage in mainstream media and were also used by the California State Assembly and New York State to approve legislation targeting THS in home-based daycare centers. Through collaboration, Dr. Hang is continuing his efforts on THS exposure-caused biological alterations and on developing technology to eliminate environmental contaminations of THS.

Dr. Jian-Hua Mao is a Senior Scientist in the Biological Systems and Engineering Division at Lawrence Berkeley National Laboratory and an Adjunct Professor in Comparative Biochemistry at the University of California, Berkeley. His research integrates systems genetics, high-throughput genomics, and artificial intelligence (AI) to elucidate cancer susceptibility, tumor evolution, and therapeutic response. Dr. Mao’s work spans mouse systems genetics, cancer genomics, functional studies using engineered mouse models, microbiome-cancer interactions, and AI-driven biomarker discovery, advancing precision medicine for cancer prevention and treatment.

P.W.: Writingoriginal draft, review and editing; Conceptualization; X.L.: Writingreview and editing; Y.X.: Writingreview and editing; H.C.: Writingreview and editing; B.H.: Writingoriginal draft, review and editing; Conceptualization. J.-H.M.: Writingoriginal draft, review and editing, Funding acquisition, Conceptualization.

The authors declare no competing financial interest.