Thirdhand smoke (THS) consist 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 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 the exposure to cigarette smoke residues. Because of THS persistence and the known toxicity of its constituents, exposure to THS can potentially cause health risks. There a is higher prevalence of asthma and other respiratory symptoms as a cough and mucus, worse pulmonary capacity and ear infection. The population most susceptible to THS exposure is children, especially infants and toddlers, for three reasons. Children are more susceptible because of their behavior, 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 gas phase. When these components react with environmental oxidants forms 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 assay with a high level of inflammatory cytokines (Hang et al., 2013). One of the toxic compound that is 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 rings. They are carcinogenic environmental pollutants generated during incomplete combustion of organic materials, mainly from anthropogenic activities such as combustion of tobacco, burning of biomass and coal. Benzo(a)pyrene has been classified as a group 1 human carcinogen by International Agency for Research on Cancer (IARC). Mainstream tobacco smoke is known to be contaminated with the 16 EPA’s PAHs, and the individual concentrations of PAHs ranged from 0.5 ng/cig to 40 ng/cigs (Trommel et al., 2005).
Characteristics of PAH
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 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 characteristic of PAHs
Low molecular weight compounds consisting of fewer than or equal to four rings, and high molecular weight consisting of more than four rings are the two categories in which PAHs are divided. PAH with low molecular weight are more volatile and are mainly found in 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). The article measured 16 PAHs, 17 nitro-PAHs and 9 oxy- PAHs in ambient air in Marseilles area, France. They found 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 MW compounds such as 2-nitrofluorene (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 compounds that have low boiling points (218C) are relatively volatile. The polycyclic organic compounds that have boiling points between 240-260 to 380-400 Celsius are considered semi-volatile organic compounds (EPA, 1998). This shows that 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
Main routes of exposure for PAHs are oral, dermal and inhalational (ATDC 1995). In Us, inhalation of the compounds in wood smoke, tobacco smoke, and consumption of PAH in foods are the primary sources (ATDC 1995). They are carcinogenic environmental pollutants generated during incomplete combustion of organic materials, mainly from anthropogenic activities such as combustion of tobacco, burning of biomass and coal. When we breathe air from lungs, PAH can enter the body. Cigarette, wood smoke, coal and, smoke from many industrial locations may contain PAHs. However, it is not known how rapidly or completely can lungs absorb PAHs (ATDC 1995). Another route of exposure is ingestion when people drinking water and swallowing food, soil, or dust particles that all contain PAHs that can enter into the body, but absorption is comparatively slow into the body. PAHs could also enter the body if 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 with PAHs (ATDC 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 (ATDC 1995). These PAH–DNA adducts are established markers of cancer risk (ATDC 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. Benzopyrene 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-dihydrodiols 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 synthetase. Evidence exists for the further oxidative metabolism to two additional phenols. 3-Hydroxybenzopyrene is metabolized to the 3,6-quinone and 9-hydroxy-benzo[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 dihydrodiols can all be conjugated to glucuronides and sulfate esters; the quinones also form glutathione conjugates.
PAHs Mechanisms and Genotoxicity
Toxicity of PAHs
According to the Agency for toxic Substances and Disease, the toxicity of PAH increases with increase in the 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 which 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 PAHs toxicity.
PAHs convert to quinones, and this supports the hypothesis that quinones because DNA adducts resulting 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 with the alkaline vision of the Comet assay employing V79 lung fibroblasts of Chinese hamster 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 photo activation was also the cause of PAH- mediated DNA damage since the genotoxicity of PAHs disappeared when the comet assay was performed in the white light which was then replaced by yellow fluorescent lamps. This concluded that photo-activation property of PAHs may result in the risk of skin cancer. Another article showed tested 7 oxy-PAHs, 7 nitro-PAHs and 6 PAHs using mouse embryonic fibroblast cells. They found one oxy-PAH, 2 PAHs and two nitro-PAHs exhibits particularly powerful tumor promoting activity.
In Vitro Assay in Mammalian Cells
Ambient air genotoxicants can generate from the emission of fuel combustion, waste incineration and, industrial processes, and they are also formed by atmospheric reactions. Thus, it is important to determine the sources of ambient genotoxicants. The Salmonella/microsome assay has been used in many studies, for identifying the presence of specific mutagens and classes of airborne mutagens. Mutagenicity level of PAHs and nitro-PAHs are higher with more tumor-promoting activity as compare 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 nitro PAH and PAHs mutagenicity level were more in the presence of S9. However, the mutagenicity level of oxy-PAHs was only slightly increased in the presence of S9. The Nitro-PAHs found to have highest mutagenic activity using YG1041 without metabolic activation.
In Vitro Assay in Human Cells
PAHs are pollutants found in urban air with increasing human health issues and in order to better assess the health risks associated with this class of compounds, a total of 67 PAH were tested using te human B-lymphoblastic cell line h1A1v2 that expresses cytochrome P4501A19 (CYP1A1), which is known to be important in the activation of many PAHs. They found 6 PAHs that include dibenzo(a,l)pyrene, cyclopental(c,d)pyrene, naphtha(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, 1.6-1.4 times, respectively, and benzo(a,k)fluoranthene was approximately equally mutagenic to benzo(a)pyrene. As well, 19 oxy-PAHs mutagenicity were tested and they found phenalenone, 7H-benzo(d,e)anthracen-7-one, 3-nitro-6H-dibnzo(b,d)pyrene-6- one (BPK), cyclopental(c,d)pyrene-3(4H)-one, 6H-benzo(c,d)pyrene (BPK) and anthanthrenequinone were 50 fold more mutagenic than benzo(a)pyrene, except for BPK, which was only 3 fold 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. Mutagenicity of phenalenone was measured in the study with Salmonella typhimurium TM677 and metabolically competent human B-lymphoblastoid cell lines (MCL-S and hlAlv2 cells), and its tumorigenicity was also assessed in a newborn mouse assay. They found the mutagenicity increased five-fold in B-lymphoblastic cells at the dose 12 ng/ml in the presence of S9.
In Vivo Assay on Mammals Non-Human
The mutagenicity of PAHs and oxy-PAHs were 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 contented 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 in water to different soil extracts from different contaminated sites including former gas worker site, former wood preservation site, and coke oven site in another study. They found coke PAC extract 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 PAHs exposure and adducts among wild-land firefighters (Rothman et al., 1993; Rothman et al., 1990). They measured PAH-DNA adducts in peripheral white blood cells (WBC) using 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 fire; however, they found a significantly elevated PAHs-DNA adduct level after consuming charbroiled food within 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 like on the surface and, in the dust, due to tobacco smoke. When second-hand smoke is emitted in the indoor environment, toxic residues can build up, even after the stoppage of smoke. 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 showed that the THS persist in the homes of smokers even if the home is vacant for 2 months after cleaning.
PAHs in Tobacco
Many studies documented PAHs in tobacco smoke, some in tobacco and a few in tobacco substitutes (Azuer 2017). Of PAHs, the 16 EPA PAHs were the most commonly measured in mainstream smoke and sidestream smoke. The mainstream smoke is the smoke that smokers inhale and then exhale (Ding et al., 2005). Side-stream smoke refers to the smoke that wafts off the end of the lit cigarette, and it comprises about 85% of SHS (EPA, 1997). In the USA, the most abundant PAHs are fluorene, naphthalene, phenanthrene and acenaphthene receptively as179-176 ng/cig, 240-135 ng/cig, 124-103 ng/cig and 116-80 ng/cig (Ding et al., 2005).
PAHs in Dust
Maertens and other in 2004 reviewed18 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 cyclopenta(c,d)pyrene were lowest (Maertens et al., 2004). They measured 13 PAHs in the house dust and found the total PAH ranging from 1.5 and 325 ng/g with the highest concentration of benzo(k)fluranthene and the lowest acenaphthene (Maertens et al., 2008). In an article PAHs were measured from the dust samples among smokers and non-smokers and results showed significantly positive correlation between nicotine and PAHs level (Matt et al., 2012).
In the Hunan Province of China, PAHs 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 5007-24,236 ng/g, road dust ranged from 3644-12,875 ng/g, and window dust ranged from 803-12,590ng/g (Kang et al., 2014). Naphthalene, benzo(b,k)fluoranthene, phenanthrene and fluorene were the most abundant PAHs in road and window dust (Kang et al., 2014).
PAHs in Thirdhand smoke (THS)
Polycyclic aromatic hydrocarbons are known carcinogenic compounds of tobacco smoke found in settled house dust. 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 significantly higher concentration of PAHs in smokers with the concentration of 990 ng/g as compared to non-smokers with the 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 effect of Thirdhand smoke
Human exposure to thirdhand smoke pollutants still needs to be studied to date hence, it is not possible to evaluate what are the health hazards associated with the exposure of 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 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, known as potential human carcinogens. Studied showed that even low dose of Thirdhand Smoke may lead to long-term health hazardous. Other than nicotine there are tobacco smoke residues that can cause harmful effect on human health such as PAHs, particularly benzo[a]pyrene, are carcinogenic, oxidant gases promoting damage and inflammation through the production of free radical species and can trigger allergic symptoms and asthma. Postnatal exposure to smoke has a large role to play 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 in 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.
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 has many potential health effects. Smoking bans are essential for the indoor environment to prevent the additional accumulation of PAHs indoor caused by tobacco smoke.