Introduction
Thirdhand smoke (THS) consists of residual tobacco smoke chemicals that persist in the environment (Matt et al., 2011). If non-smokers encounter THS, they can be exposed to tobacco-related chemicals that are still present even after the smoking has ceased (Matt et al., 2011). There is undeniable evidence that involuntary smoking is harmful to nonsmokers. Environmental tobacco smoke exposure is different from THS, which refers to exposure to tobacco smoke, while THS is exposure to cigarette smoke residues. Because of THS’s persistence and the known toxicity of its constituents, exposure to THS can potentially cause health risks.
There is a higher prevalence of asthma and other respiratory symptoms, such as coughing and mucus production, worse pulmonary capacity, and ear infections. The population most susceptible to THS exposure is children, especially infants, and toddlers, for three reasons. Children are more susceptible because of their behavior, as babies crawl on the ground where most of the dust settles, and they also tend to put everything in their mouths (Simon et al., 2015). They are active near the ground and can re-suspend fine dust, which can then be inhaled, ingested, or absorbed through the skin. Small children are more susceptible than adults because they are rapidly developing and may lack metabolic capacity (Simon et al., 2015).
THS pollutants can remain in settled dust or on the surface or remain in the gas phase. When these components react with environmental oxidants, they form secondary pollutants. Many researchers have shown that these pollutants remain for months after the last known cigarette was smoked (Matt et al., 2011). Chemicals in THS include both those in secondhand smoke (SHS) and novel compounds. Because THS is persistent, it contains many toxic and mutagenic compounds that have shown harmful effects on human cells using in vitro assays with a high level of inflammatory cytokines (Hang et al., 2013). One of the toxic compounds that are of concern found in thirdhand smoke is polycyclic aromatic hydrocarbons (PAHs) (Hoh et al., 2012).
Polycyclic aromatic hydrocarbons (PAHs) are hydrocarbons with more than one aromatic ring. They are carcinogenic environmental pollutants generated during the incomplete combustion of organic materials, mainly from anthropogenic activities such as the combustion of tobacco, burning of biomass, and coal. Benzo(a)pyrene has been classified as a Group 1 human carcinogen by the International Agency for Research on Cancer (IARC). Mainstream tobacco smoke is known to be contaminated with the 16 EPA PAHs, and the individual concentrations of PAHs range from 0.5 ng/cig to 40 ng/cig (Trommel et al., 2005).
Characteristics of PAHs
Formation of PAHs:
PAHs are formed through incomplete combustion of organic materials during human activities and industrial actions such as coal and biomass combustion, vehicular traffic, cooking, tobacco smoking, and garbage burning, as well as natural processes (IARC, 1985). They generally occur as complex mixtures (more than 100 different PAHs) existing as colorless, white, or pale yellow-green solids when in pure form (IARC, 1985). A few PAHs are used in medicines and to make dyes, plastics, and pesticides. Others are contained in asphalt used in road construction. They can also be found in substances such as crude oil, coal, coal tar pitch, creosote, and roofing tar. They are found throughout the environment in the air, water, and soil. PAH sources in the U.S. are mostly from consumer product use (35.1%), traffic oil combustion (23%), followed by waste incineration (9.5%), biofuel combustion (9.1%), petroleum refining (8.7%), wildfire (3.3%), gas oil distribution (3.0%), aerospace industrial (2.5%), and others (Y. Zang & Tao, 2009).
Physical and Chemical Characteristics of PAHs
PAHs are divided into two categories: low molecular weight compounds consisting of fewer than or equal to four rings, and high molecular weight compounds consisting of more than four rings. PAHs with low molecular weight are more volatile and are mainly found in the gas phase, while high molecular weight PAHs are less volatile and are mainly bound to particles (Atkinson & Arey, 1994; reviewed by Kameda, 2011; Kameda et al., 2005). One article measured 16 PAHs, 17 nitro-PAHs, and 9 oxy-PAHs in ambient air in the Marseille area of France. They found that approximately 50% of low molecular weight compounds were detected in the gas phase, while 90% of the heavy compounds were detected in the particulate phase (Albinet et al., 2007).
However, there were exceptions for some low molecular weight compounds such as 2-nitro fluorene (molecular weight 211 g/mol), which were mainly in the particle phase (about 80% at any sampling site) (Albinet et al., 2007). PAHs are generally semi-volatile organic compounds, but the low molecular weight compounds that have low boiling points (218°C) are relatively volatile. Polycyclic organic compounds that have boiling points between 240-260 and 380-400 Celsius are considered semi-volatile organic compounds (EPA, 1998). This shows that the physical and chemical properties of PAHs make them highly mobile in the environment, allowing them to distribute across the air, soil, and water.
Route of exposure
The main routes of exposure to PAHs are oral, dermal, and inhalational (ATSDR, 1995). In the US, inhalation of compounds in wood smoke, tobacco smoke, and consumption of PAHs in foods are the primary sources (ATSDR, 1995). PAHs are carcinogenic environmental pollutants generated during the incomplete combustion of organic materials, mainly from anthropogenic activities such as the combustion of tobacco, burning of biomass, and coal. When we breathe air into our lungs, PAHs can enter the body. Cigarette smoke, wood smoke, coal, and smoke from many industrial locations may contain PAHs. However, it is not known how rapidly or completely the lungs can absorb PAHs (ATSDR, 1995).
Another route of exposure is ingestion when people drink water or swallow food, soil, or dust particles that all contain PAHs that can enter the body, but absorption is comparatively slow. PAHs could also enter the body if the skin comes into contact with contaminated soil that contains high levels of PAHs. The rate at which PAHs enter the body by eating, drinking, or through the skin can be influenced by the presence of other compounds that people may be exposed to at the same time as PAHs (ATSDR, 1995). All tissues of the body that contain fat could be exposed to PAHs. After exposure and during metabolism, PAHs can form reactive epoxides that can covalently bind to DNA (ATSDR, 1995). These PAH-DNA adducts are established markers of cancer risk (ATSDR, 1995). PAH exposure has been associated with epigenetic alterations, including genomic cytosine methylation.
Metabolism of PAHs
Metabolism involves several possible pathways with varying degrees of enzyme activities. Benzo[a]pyrene is metabolized initially by microsomal cytochrome P-450 systems to several arene oxides. Once formed, these arene oxides may rearrange spontaneously to phenols, undergo hydration to the corresponding trans-dihydro diols in a reaction catalyzed by microsomal epoxide hydrolase, or react covalently with glutathione, either spontaneously or in a reaction catalyzed by cytosolic glutathione-S-transferases (IARC 1983). 6-Hydroxybenzo[a]pyrene is further oxidized either spontaneously or metabolically to the 1,6-, 3,6-, or 6,12-quinones. This phenol is also a presumed intermediate in the oxidation of benzo[a]pyrene to the three quinones catalyzed by prostaglandin endoperoxide synthase. Evidence exists for further oxidative metabolism to two additional phenols. 3-Hydroxybenzo[a]pyrene is metabolized to the 3,6-quinone, and 9-hydroxybenzo[a]pyrene is further oxidized to the K-region 4,5-oxide, which is hydrated to the corresponding 4,5-dihydrodiol (4,5,9-triol). The phenols, quinones, and dihydro diols can all be conjugated to glucuronides and sulfate esters; the quinones also form glutathione conjugates.
PAH Mechanisms and Genotoxicity
Toxicity of PAHs
According to the Agency for Toxic Substances and Disease, the toxicity of PAHs increases with an increase in exposure. For instance, occupational exposure to high levels of pollutant mixtures that contain PAHs is known to result in symptoms such as eye and skin irritation, inflammation, and long-term serious health problems such as skin, lung, intestinal, bladder, and gastrointestinal cancer. Scientists have found some non-parent PAHs such as oxy-PAHs and nitro-PAHs are electrophiles that form covalent bonds to nucleophile sites within protein and DNA. The transformation products of some non-parent PAHs are more toxic than parent PAHs and can lead to critical health effects. In evaluating human health risks, due to the absence of human data, animal and in vitro studies provide support and evidence of PAH toxicity.
PAHs convert to quinones, which supports the hypothesis that quinones cause DNA adducts, resulting in direct toxicity (Munoz & Albores, 2011; Szczeklik, 2005). Oxy-PAHs tend to be more active compounds, producing more DNA adducts on thin-layer plates in calf thymus DNA under reactive conditions (Umbuzeiro et al., 2008). Quinones are known as Michael acceptors, which means they have the potential to alkylate cellular nucleophiles, including DNA and RNA. Quinones also form reactive oxygen species (ROS) that can undergo non-enzymatic reactions causing two-electron reductions with cellular reducing equivalents (NADPH) and enzymatic reactions causing one-electron reduction.
In Vitro Assay-Non-Mammalian
The genotoxicity of PAHs was tested using the alkaline version of the Comet assay, employing V79 lung fibroblasts from Chinese hamsters as target cells. This assay detects DNA strand breaks generated either directly or indirectly through alkaline cleavage of abasic lesions formed after alkylation at the N-7 position of guanine. The research showed that out of 15 polycyclic aromatic hydrocarbons, benzo(a)pyrene, benz(s)anthracene, 7,12-dimethylbenz(a)anthracene, 3-methylcholanthrene, fluoranthene, anthanthrene, 11H-benzo(b)fluorene, dibenzo(a,h)anthracene, pyrene, benzo(ghi)perylene, and benzo(e)pyrene (11 PAHs) caused DNA strand breaks even without external metabolic activation, while anthracene, naphthalene, and phenanthrene were inactive. They also found that photoactivation was the cause of PAH-mediated DNA damage since the genotoxicity of PAHs disappeared when the Comet assay was performed in white light, which was then replaced by yellow fluorescent lamps. This concludes that the photoactivation property of PAHs may result in the risk of skin cancer. Another article tested 7 oxy-PAHs, 7 nitro-PAHs, and 6 PAHs using mouse embryonic fibroblast cells. They found that one oxy-PAH, 2 PAHs, and two nitro-PAHs exhibited particularly powerful tumor-promoting activity.
In Vitro Assay in Mammalian Cells
Ambient air genotoxicants can be generated from fuel combustion emissions, waste incineration, industrial processes, and atmospheric reactions. Thus, it is important to determine the sources of ambient genotoxicants. The Salmonella/microsome assay has been used in many studies to identify the presence of specific mutagens and classes of airborne mutagens. The mutagenicity level of PAHs and nitro-PAHs is higher with more tumor-promoting activity compared to oxy-PAHs. The article studied the mutagenic activity and DNA adduct-forming ability of fractionated UPM extractable organic matter. PAHs, nitro-PAHs, and oxy-PAHs were assayed for mutagenicity using Salmonella typhimurium strains TA98 and YG1041 with and without S9 metabolic activation. The results showed that the mutagenicity level of nitro-PAH and PAHs were higher in the presence of S9. However, the mutagenicity level of oxy-PAHs was only slightly increased in the presence of S9. Nitro-PAHs were found to have the highest mutagenic activity using YG1041 without metabolic activation.
In Vitro Assay in Human Cells
PAHs are pollutants found in urban air that pose increasing human health issues. In order to better assess the health risks associated with this class of compounds, a total of 67 PAHs were tested using the human B-lymphoblastic cell line h1A1v2 that expresses cytochrome P4501A1 (CYP1A1), which is known to be important in the activation of many PAHs. They found six PAHs, including dibenzo(a,l)pyrene, cyclopenta[c,d]pyrene, naphtho[2,1-a]pyrene, dibenzo[a,e]pyrene, and 1-methylbenzo[a]pyrene, to be more mutagenic than benzo[a]pyrene at 24-211, 6.9-4.2, 3.2-3.0, 2.9-2.9, and 1.6-1.4 times, respectively, and benzo[a,k]fluoranthene was approximately equally mutagenic to benzo[a]pyrene.
Additionally, 19 oxy-PAHs’ mutagenicity was tested, and they found phenomenon, 7H-benzo[d,e]anthracen-7-one, 3-nitro-6H-dibenzo[b,d]pyrene-6-one (BPK), cyclopenta[c,d]pyrene-3(4H)-one, 6H-benzo[c,d]pyrene (BPK), and anthanthrenequinone were 50 times more mutagenic than benzo[a]pyrene, except for BPK, which was only three times less active (Durant et al., 1996). Phenalenone is a major oxygenated polycyclic aromatic hydrocarbon (oxy-PAH) atmospheric pollutant formed from the combustion of fossil fuels. The mutagenicity of the phenomenon was measured in the study with Salmonella typhimurium TM677 and metabolically competent human B-lymphoblastoid cell lines (MCL-S and h1A1v2 cells), and its tumorigenicity was also assessed in a newborn mouse assay. They found that mutagenicity increased five-fold in B-lymphoblastic cells at the dose of 12 ng/ml in the presence of S9.
In Vivo Assay on Non-Human Mammals
The mutagenicity of PAHs and oxy-PAHs was compared in the study using zebrafish embryos. They were exposed for 6-72 hours with PAHs with a concentration of 0.1 mg/ml in soil content in the water tank. The results showed a statistical increase in the gene expression level of cytochrome P450, AHR gene, AHR isoform, and CYP family member 1 (CYP1B1, CYP1A, CYP1C1, CYP1C2). Embryonic zebrafish were exposed to different soil extracts from different contaminated sites, including the former gas worker site, former wood preservation site, and coke oven site, in another study. They found that coke PAC extracts increased three times more than the gas PAC extract, and wood PAC extract increased the gene expression by 80-85%.
In Vivo Humans
A study in California investigated the association between occupational and dietary PAH exposure and adducts among wildland firefighters (Rothman et al., 1993; Rothman et al., 1990). They measured PAH-DNA adducts in peripheral white blood cells (WBC) using an enzyme-linked immunosorbent assay with antiserum elicited against benzo(a)pyrene-modified DNA (Rothman et al., 1993; Rothman et al., 1990). They did not find a significant association between firefighter activities after spending hours extinguishing forest fires; however, they found a significantly elevated PAH-DNA adduct level after consuming charbroiled food within the previous week (Rothman et al., 1993; Rothman et al., 1990).
Thirdhand Smoke (THS)
Thirdhand smoke is the mixture of several chemical pollutants found in the indoor environment, such as on surfaces and in dust, due to tobacco smoke. When second-hand smoke is emitted into the indoor environment, toxic residues can build up, even after the cessation of smoking. This residue is then known as thirdhand smoke. Long-term exposure through inhalation, ingestion and dermal transfer of THS leads to potential health hazards, as they contain cancer-causing substances. Many studies have shown that THS persists in the homes of smokers even if the home is vacant for 2 months after cleaning.
PAHs in Tobacco
Many studies have documented PAHs in tobacco smoke, some in tobacco, and a few in tobacco substitutes (Azuer, 2017). Of the PAHs, the 16 EPA PAHs were the most commonly measured in mainstream smoke and sidestream smoke. Mainstream smoke is the smoke that smokers inhale and then exhale (Ding et al., 2005). Sidestream smoke refers to the smoke that wafts off the end of the lit cigarette, comprising about 85% of SHS (EPA, 1997). In the USA, the most abundant PAHs are fluorene, naphthalene, phenanthrene, and acenaphthene, respectively at 179-176 ng/cig, 240-135 ng/cig, 124-103 ng/cig, and 116-80 ng/cig (Ding et al., 2005).
PAHs in Dust
Maertens and others 2004 reviewed 18 publications concerning the concentration of PAHs in house dust. Benzo(b,k)fluoranthene, fluoranthene, and pyrene were reportedly higher in concentration, whereas acenaphthene, acenaphthylene, and cyclopentane (c,d)pyrene were the lowest (Maertens et al., 2004). They measured 13 PAHs in the house dust and found the total PAH ranging from 1.5 to 325 ng/g with the highest concentration of benzo(k) fluoranthene and the lowest of acenaphthene (Maertens et al., 2008). In an article, PAHs were measured from the dust samples among smokers and non-smokers, and the results showed a significantly positive correlation between nicotine and PAH levels (Matt et al., 2012).
In the Hunan Province of China, PAH concentration was measured in indoor dust and outdoor dust by collecting traffic road and window dust (Kang et al., 2014). The total PAHs in indoor dust ranged from 5007 to 24,236 ng/g, road dust ranged from 3644 to 12,875 ng/g, and window dust ranged from 803 to 12,590 ng/g (Kang et al., 2014). Naphthalene, benzo(b,k)fluoranthene, phenanthrene, and fluorene were the most abundant PAHs in the road and window dust (Kang et al., 2014).
PAHs in Thirdhand Smoke (THS)
Polycyclic aromatic hydrocarbons (PAHs) are known carcinogenic compounds found in settled house dust from tobacco smoke. Hoh and others reviewed that tobacco smoke is a source of PAHs in settled house dust. They collected dust samples from 132 houses in urban areas of Southern California and found that there was a significantly higher concentration of PAHs in smokers, with a concentration of 990 ng/g compared to non-smokers with a concentration of 756 ng/g. Among different PAHs, benzo(a)pyrene is a known human carcinogen, and others have been identified as class B2 probable carcinogens.
Health Effects of Thirdhand Smoke
Human exposure to thirdhand smoke pollutants still needs to be studied to date; hence, it is not possible to evaluate the health hazards associated with exposure to THS pollutants. Although it is possible to evaluate the detrimental effects of THS on human health of some of the known THS compounds. Nicotine plays an important role in carcinogenesis; it has a harmful effect on the vascular system and may promote inflammation through oxidative stress. Also, it can cause brain and lung damage in children. This compound reacts with other pollutants to form new compounds, such as TSNAs and formaldehyde, which are known as potential human carcinogens. Studies showed that even a low dose of Thirdhand Smoke may lead to long-term health hazards.
Other than nicotine, there are tobacco smoke residues that can cause harmful effects on human health, such as PAHs, particularly benzo[a]pyrene, which is carcinogenic. Oxidant gases promote damage and inflammation through the production of free radical species and can trigger allergic symptoms and asthma. Postnatal exposure to smoke plays a large role in lung function impairment seen in many studies. The direct effects of maternal smoking on lung development are mainly due to the smoke components that are transferred across the placenta. Nicotine is able to cross the human placenta and accumulates in amniotic fluid, several fetal tissues (including the lungs), and maternal milk. Therefore, the fetus is exposed to even higher levels than those of the smoking mother. There is no safe level of exposure to THS.
Conclusion
The purpose of this research paper was to determine the levels of PAHs associated with Thirdhand Smoke. THS contains many toxic substances, and one such substance is PAHs, which have many potential health effects. Smoking bans are essential for the indoor environment to prevent the additional accumulation of PAHs indoors caused by tobacco smoke.
