Benzo[a]pyrene: An In-depth Look at Sources, Metabolism, Toxicity, and Health Impacts

1. Introduction to Benzo[a]pyrene

Benzo[a]pyrene (B[a]P), a prominent member of the polycyclic aromatic hydrocarbon (PAH) family, is a ubiquitous environmental contaminant. Characterized by its five fused benzene rings, benzo[a]pyrene is a solid, hydrophobic compound, unlike its single benzene ring counterpart which is liquid. Polycyclic aromatic hydrocarbons, including benzo[a]pyrene, are known for their stable molecular structures and high hydrophobicity [1]. The presence of benzo[a]pyrene is widely documented in various environmental compartments, including air, surface water, soil, and sediments, as well as in everyday human exposures such as cigarette smoke and certain food products.

Benzo[a]pyrene, along with other PAHs, originates from the incomplete combustion of organic materials such as fossil fuels and wood. Consequently, benzo[a]pyrene is prevalent in cigarette smoke, diesel exhaust, charcoal-grilled foods, and industrial waste [2]. This makes benzo[a]pyrene a common byproduct of numerous thermal processes and a significant concern for environmental and public health.

Human exposure to benzo[a]pyrene is more widespread than ever due to increasing anthropogenic sources, in addition to natural sources like wildfires and volcanic eruptions [3]. Mammals readily absorb benzo[a]pyrene through inhalation, ingestion, and dermal contact [4]. Food and air contamination are primary routes of human benzo[a]pyrene exposure [5], underscoring the need to understand its sources, effects, and mitigation strategies.

Tobacco smoke stands out as a major contributor to benzo[a]pyrene exposure. Side-stream cigarette smoke contains significantly higher concentrations of benzo[a]pyrene (52 to 95 ng/cigarette) compared to mainstream smoke [6]. Smokers exhibit elevated levels of benzo[a]pyrene metabolites, particularly 7β,8α-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE). Studies have shown that smokers have considerably higher BPDE protein adduct levels compared to non-smokers [7]. Furthermore, BPDE DNA adducts have been detected in the placentas of smoking mothers [8]. Exposure to benzo[a]pyrene from tobacco smoke is linked to adverse reproductive outcomes in smokers, including prolonged gestation, earlier menopause, altered ovarian steroidogenesis, and reduced ovarian reserve [9].

2. Occupational Exposure to Benzo[a]pyrene and Polycyclic Aromatic Hydrocarbons

In occupational settings, benzo[a]pyrene is often found in complex mixtures of PAHs, resulting from combustion and various industrial processes. Workers in numerous industries face exposure to these PAH mixtures, making benzo[a]pyrene a key indicator for assessing occupational hazards related to PAH exposure.

Occupational exposure to PAHs, including benzo[a]pyrene, primarily occurs through inhalation and skin contact. Industries with the highest PAH exposure levels include aluminum production, particularly the Söderberg process, where concentrations can reach up to 100 μg/m³. Moderate levels are found in roofing and pavement work (10–20 μg/m³), while lower levels (1 μg/m³ or less) are observed in coal liquefaction, coal-tar distillation, wood impregnation, chimney sweeping, and power plants [6].

Research has investigated the levels of PAHs, including benzo[a]pyrene, in occupationally exposed and non-exposed groups. A study in Makkah, Saudi Arabia, found significantly higher mean concentrations of benzo[a]pyrene in air samples from occupationally exposed workers (bus and truck drivers, police officers, etc.) compared to non-exposed individuals [10]. This study also noted a positive correlation between increased benzo[a]pyrene exposure and elevated serum p51 and p21 protein levels, which are implicated in tumor progression, invasion, and metastasis.

Table 1: Benzo[a]pyrene Levels in Outdoor Air Across Different Locations

Location	Concentration (ng/m³)	Reference
European Union	7% of EU citizens in areas > 0.12 ng/m³	[12]
France	1 ng to 2.49 ng/m³	[13]
Thailand	0.052–0.095 ng/m³ (PM 2.5 fraction)	[14]
Iberian Peninsula	Exceeded target value of 1 ng/m³	[15]
Italy—Genoa	2 ng/m³ (heavy traffic streets) / 14 ng/m³ (coke oven)	[16]
Poland—Cracow, Tarnow, Nowy Sacz	4–11 ng/m³	[17]
China—Linzhou	5.1–20.2 ng/m³	[18]
Saudi Arabia—Makkah	0.082 ± 0.032 ng/m³ (occupationally exposed) / 0.044 ± 0.006 ng/m³ (unexposed)	[10]

Another study evaluating lung cancer risk in various industries using atmospheric benzo[a]pyrene levels as an indicator found that 30% of exposure groups exceeded the European Union’s maximum acceptable risk level for cancer [11]. This underscores the significant occupational health risks associated with benzo[a]pyrene exposure in numerous industrial sectors.

3. Sources of Human Exposure to Benzo[a]pyrene

Benzo[a]pyrene, due to its hydrophobic nature, tends to accumulate on dust particles and in fatty tissues. It persists in plants and animal adipose tissue and strongly binds to the organic fraction of soil, remaining in the upper layers rather than penetrating deeper into the soil [19]. This limits its uptake by plant root systems.

Recent research indicates that benzo[a]pyrene concentrations in plants are directly related to the initial benzo[a]pyrene levels in the soil. Studies show that crops like spring barley grown in benzo[a]pyrene-contaminated soils accumulate this compound [20].

The environmental persistence of benzo[a]pyrene varies: it has a half-life of less than 1–6 days in the atmosphere and less than 1–8 hours in water. However, it can persist for 5–10 years in sediments and over 14–16 months in soil before complete degradation [21].

3.1. Air as a Source of Benzo[a]pyrene

3.1.1. Outdoor Air Pollution

PAHs, including benzo[a]pyrene, are major components of air pollution. The primary sources of PAH emissions are anthropogenic, stemming from fossil fuel pyrolysis, incomplete combustion, and biomass burning [22]. PAHs are significant constituents of semi-volatile organic substances. Lower molecular weight PAHs (2–3 rings) are typically found in the gas phase, while higher molecular weight PAHs, such as benzo[a]pyrene (4–6 rings), are predominantly adsorbed onto particulate matter and are associated with greater carcinogenicity [23].

Benzo[a]pyrene is a crucial indicator for assessing PAH pollution and is widely used in environmental monitoring, particularly for evaluating air quality [24]. The European Union has set a limit of 1 ng/m³ for benzo[a]pyrene in air. However, many regions globally, including within the EU, report levels exceeding this limit [25, 26]. Urban outdoor air typically contains benzo[a]pyrene levels ranging from 1 to several dozen ng/m³. The highest concentrations, reaching dozens of nanograms per cubic meter, are found in road tunnels and cities heavily reliant on coal and other fuels for heating [24, 26]. It is estimated that 20% of the European population is exposed to benzo[a]pyrene concentrations exceeding the EU annual limit, while only 7% live in areas with levels below the tolerable risk threshold of 0.12 ng/m³ [12].

For example, in France, studies from 1990 to 2011 showed benzo[a]pyrene air concentrations ranging from below 1 ng/m³ to 2.49 ng/m³ [13]. In the Iberian Peninsula, levels have surpassed the European target of 1 ng/m³ [22]. Similarly, in Genoa, Italy, areas near high-traffic streets had average benzo[a]pyrene concentrations of 2 ng/m³, while a location 300 meters from a coke oven recorded a much higher level of 14 ng/m³ [16].

Poland faces significant air quality challenges in Europe, with high benzo[a]pyrene levels, especially during heating seasons. Studies in Cracow, Tarnow, and Nowy Sacz between 2011 and 2020 revealed benzo[a]pyrene levels consistently exceeding standards, ranging from 4 to 11 ng/m³ [17]. Similarly, Linzhou, China, reported very high benzo[a]pyrene concentrations, from 5.1 to 20.2 ng/m³ [18].

Air quality assessments in Thailand have shown that mean benzo[a]pyrene concentrations (adsorbed onto particulate matter) during haze periods were significantly higher than during normal periods [14]. Notably, benzo[a]pyrene adsorbed on PM1 particles, which can penetrate deep into the respiratory system, constituted a substantial portion of the benzo[a]pyrene in PM2.5 fractions.

3.1.2. Indoor Air Pollution

Indoor air pollution from combustion processes poses a significant threat to human health, particularly in developing countries. It is strongly linked to respiratory infections and cancers, including nasopharyngeal and lung cancer [27]. Indoor air pollution is estimated to cause 3.5–4 million deaths annually worldwide, with 1 million in China alone [28].

A study in rural North China kitchens during cooking found high concentrations of benzo[a]pyrene in both gaseous and particulate phases [29]. Fuel type and ventilation significantly influenced benzo[a]pyrene levels. Another study in Yucheng City, China, investigating PAHs from cooking activities, found that total mean PAH levels varied greatly depending on cooking methods, with the highest levels during cafeteria frying [30]. Oil-based cooking methods, like meat roasting, resulted in much higher benzo[a]pyrene levels compared to water-based methods like boiling. Interestingly, indoor air analysis in the Jokhang Temple in Tibet revealed benzo[a]pyrene levels nearly twenty times the permissible risk value, attributed to intense burning of plant materials, dense crowds, and poor ventilation [31].

Table 2: Benzo[a]pyrene Levels in Indoor Air in Various Settings

Location	Concentration	Reference
North China kitchens, indoor air	14.3 ± 23.0 ng/m³ (gaseous) / 6.7 ± 17.4 ng/m³ (particulate)	[29]
China Yucheng City kitchens, indoor air	25.8 ± 10.6 ng/m³ (oil-based) / 7.3 ± 4.6 ng/m³ (water-based)	[30]
Tibet Jokhang Temple, indoor air	18.5 ± 4.3 ng/m³	[31]
China Shanxi Provence schools, indoor air	0.05 ng/m³ (non-heating) / 10.3 ng/m³ (heating)	[32]
Saudi Arabia, Jeddah’s schools, indoor air	163.87 ± 68.53 ng/m³	[33]
Poland, Silesia kindergartens, indoor air	3.7 ± 0.8 ng/m³	[34]
India, Shimoga, iron foundry	7.20 ± 1.11 μg/m³ (melting) / 45.37 µg/m³ (molding)	[35]
Sweden, Aluminum manufacturing factories	14 μg/m³	[36]
United Kingdom, Coke oven facilities	3.3 μg/m³	[37]

Children are particularly vulnerable to indoor air pollutants due to their developing respiratory and immune systems. Classrooms in Shanxi Provence, China, showed a dramatic increase in benzo[a]pyrene concentrations during heating seasons, highlighting children’s exposure in winter [32]. Coal and gasoline combustion were identified as primary contributors. In Jeddah, Saudi Arabia, classrooms showed extremely high dust-bound benzo[a]pyrene levels, with urban schools having higher levels than suburban or residential schools [33]. Risk calculations suggested dibenz[a,h]anthracene and benzo[a]pyrene as major contributors to cancer risk in children. Similarly, kindergartens in Silesia, Poland, also showed elevated benzo[a]pyrene levels, influenced by gas and coal stove usage [34].

Workplace indoor air often contains very high concentrations of PAHs, including benzo[a]pyrene. Iron foundry workers in India were found to have significant exposure, with benzo[a]pyrene levels reaching as high as 45.37 µg/m³ in molding sections [35]. High mean benzo[a]pyrene concentrations were also found in aluminum manufacturing factories and coke oven facilities [36, 37].

3.2. Surface Water Contamination

Benzo[a]pyrene is also a water pollutant, entering surface waters through direct deposition and runoff from contaminated urban grounds. Its low water solubility leads to binding with organic matter, resulting in its presence in the hydrosphere [38].

Studies in India have found benzo[a]pyrene levels in river water reaching 8.61 ng/L [39]. A survey of 44 Chinese lakes detected benzo[a]pyrene levels ranging from 0.07 to 2.26 ng/L [38].

Research in the Danshui River basin in central China found mean total PAH and benzo[a]pyrene levels of 26.2 ng/L and 1.37 ng/L, respectively, in surface water [40]. Analysis of surface waters in the Southeast Sea, Japan, identified benzo[a]pyrene in both dissolved and solid phases, with total PAH levels ranging from 6.83 to 13.81 ng/L [41]. The study indicated pyrogenic and mixed pyrogenic/petrogenic origins for dissolved and solid phase PAHs, respectively.

3.3. Soil Contamination

Industrial activities significantly contribute to PAH and benzo[a]pyrene contamination in soil. Major sources include coal combustion, vehicle exhaust, biomass burning, and cooking [42]. Coal combustion and cooking are primary sources of both PAH contamination and carcinogenic risk in soil, as indicated by benzo[a]pyrene toxic equivalent levels.

Benzo[a]pyrene levels in Polish soils from 1993 to 1994 ranged from 10 to 680 µg/kg [43]. In 2011–2012, slightly lower levels were found in soils from Bialystok, Poland, ranging from 0.3 to 0.9 mg/kg [44]. Municipal soils in Cleveland, USA, showed benzo[a]pyrene concentrations from 0.28 to 5.50 mg/kg [45]. Greenhouse crop soil in Turkey had a mean benzo[a]pyrene level of 2.31 ± 1.13 μg/kg [46].

Table 3: Benzo[a]pyrene Levels in Soils Worldwide (Post-2000)

Location	Land Type	Concentration (µg/kg)	Reference
Bangkok	Urban–tropical	5.5	[47]
Brazil	Forest–tropical	0.3	[47]
New Orleans	Urban	276	[48]
Dalian, China	Traffic/Park/Suburban/Rural	9–388	[49]
United Kingdom	Rural	46	[50]
Norway	Rural	5.3	[50]
Spain	Industrial/Residential/Rural	18–100	[51]
Poland	Agricultural	30	[52]
Poland, Bialystok	Urban	300–900	[44]
USA, Cleveland	Municipal plots	280–5500	[45]
Russia, St. Petersburg	Parkland/Residential/Industrial	220–430	[53]
Turkey, Antalya Aksu	Greenhouse crops	2.31	[46]
Taiyuan	Urban/Agricultural/Montane	16.60–94.03	[42]
Antarctic Peninsula	Antarctic station	1.5	[54]
UK, Cities of Crimea	Alushta/Yalta/Sebastopol	60–260	[55]

Studies of sediments and fish in major Taiwanese rivers showed benzo[a]pyrene levels in sediments ranging from 0.01 to 1.68 μg/kg dw [56]. PAH composition differed between sediments and fish, suggesting that sediments are not the primary source of PAHs in fish. Bioaccumulation of PAHs in fish varies with species, environmental contamination, lipid content, and trophic level.

Microbial Biodegradation of Benzo[a]pyrene

Microbial biodegradation is a promising approach for benzo[a]pyrene removal from the environment due to microbial diversity and environmental compatibility. Enzymes like cytochrome P450, oxygenases, and hydrolases produced by microorganisms can degrade benzo[a]pyrene [57].

Benzo[a]pyrene-resistant microbial strains, such as Staphylococcus haemoliticus species, have been identified for bioremediation [58]. Microbial degradation breaks down benzo[a]pyrene into less harmful products, including complete mineralization to CO2. Microbes like Mycobacterium sp., Stenotrophomonas maltophilia, and Bacillus subtilis are known for their ability to degrade benzo[a]pyrene under nonhalophilic conditions.

Biodegradation efficiency is influenced by factors like benzo[a]pyrene concentration, bioavailability, and soil properties. Bioaugmentation and biostimulation technologies are being developed to enhance degradation rates. Advanced technologies like omics and nanotechnology offer new opportunities for improving microbial degradation of benzo[a]pyrene and other PAHs [57].

However, PAH biodegradation can also have adverse effects on bacteria, potentially forming more toxic metabolites that damage bacterial membranes and impair cellular functions [59].

3.4. Food Contamination

Food contamination is a significant route of human exposure to benzo[a]pyrene, which forms during high-temperature cooking processes like baking, frying, grilling, and smoking [60]. Benzo[a]pyrene is found in various foods, including olive oil, smoked cheeses, charcoal-grilled chicken, seafood, and bread [61, 62, 63, 64, 65, 66, 67].

Studies estimating daily benzo[a]pyrene intake through food have identified grilled/barbecued meats as major contributors, followed by bread and cereals [68]. Grilled steaks, hamburgers, and chicken cooked well-done can contain significant benzo[a]pyrene concentrations. While non-meat products generally have lower levels, some grains and vegetables can also contain detectable amounts.

Benzo[a]pyrene in Drinking Water

European Union standards set limits for PAHs in drinking water, with a specific limit of 10 ng/L for benzo[a]pyrene [69, 70].

In Poland, benzo[a]pyrene concentrations in drinking water have been found to range from non-detectable levels up to 18 ng/L in some cities. In other cities, mean concentrations range from 4.00 to 8.33 ng/L [71]. Overall, PAH levels in Polish drinking water remain below WHO recommended limits.

A study in Turkey analyzing tap water from several cities found mean benzo[a]pyrene levels ranging from 0.11 ± 0.08 to 0.97 ± 0.75 ng/L [72]. These levels were well below both WHO and Turkish regulatory limits for drinking water.

4. Metabolism of Benzo[a]pyrene

Human metabolism of benzo[a]pyrene is rapid, with quick uptake and removal from the body [73]. Metabolism primarily occurs in the bay region of benzo[a]pyrene. Parent benzo[a]pyrene is a minor component in plasma compared to its metabolites, and its levels decrease with dose.

Benzo[a]pyrene metabolism involves multiple phases, common to many hydrophobic xenobiotics [74, 75].

Phase I biotransformation, mediated by cytochrome P450 enzymes (CYP1 family) and microsomal epoxide hydrolase, oxidizes benzo[a]pyrene into phenols, diol diols, dihydrodiols, quinones, and reactive diol-epoxide enantiomers. Reactive oxygen species (ROS) are also generated as byproducts.

CYP1A1, CYP1A2, and CYP1B1 are involved in benzo[a]pyrene oxidation, with CYP1A1 being the most active in mammals [76, 77]. Epoxidation at the 7,8 positions is a critical step leading to toxic metabolites. Conversely, oxidation at the 4 and 5 positions results in inactive metabolites [78]. CYP1A1 converts benzo[a]pyrene to benzo[a]pyrene-7,8-epoxide, which epoxide hydrolase transforms into (+/−)-benzo[a]pyrene-trans-7,8-dihydrodiol (DHD). Further CYP-dependent oxidation of benzo[a]pyrene-7,8-DHD produces the highly carcinogenic metabolite 7β,8α-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE). BPDE can bind covalently to DNA, forming deoxyguanosine-DNA adducts, potentially leading to mutations and carcinogenesis [76, 78]. Cytochrome P4501A1 (CYP1A1) is considered crucial for the pro-carcinogenic activation of benzo[a]pyrene.

AhR activators can enhance benzo[a]pyrene toxicity by inducing CYP1A1 gene expression, while CYP1A1 inhibitors may reduce benzo[a]pyrene toxicity [76, 79].

Phase I metabolism can also involve flavin-containing monooxygenases, NAD(P)H: quinone oxidoreductases, amine oxidases, esterases, alcohol dehydrogenases, and peroxidases [76]. Prostaglandin H synthase and lipoxygenase contribute to benzo[a]pyrene metabolism, producing reactive oxygen-free radicals [82]. In the presence of polyunsaturated fatty acids, benzo[a]pyrene oxidation products and lipid peroxidation products can exhibit mutagenic potential [83].

Phase II metabolism conjugates phase I products with endogenous compounds like GSH, glucuronic acid, and sulfate, forming more hydrophilic conjugates that are more readily eliminated [84].

Phase III involves ABC transporters, such as multidrug resistance proteins (MRP), facilitating metabolite excretion [85].

4.1. Genetic Polymorphism and Benzo[a]pyrene Metabolism

Genetic polymorphism, with allele frequency greater than 1% in a population, affects enzyme activity in xenobiotic metabolism. Polymorphisms exist in genes encoding cytochrome P450, flavin-containing monooxygenase, GST, UDP-glucuronosyltransferase, and DNA repair enzymes [86].

CYP1A1, CYP1A2, and CYP1B1 are key cytochromes P450 involved in diol and diol-epoxide formation. CYP1A1 is particularly important in human carcinogenesis by metabolizing procarcinogens into active carcinogens. Polymorphisms in CYPs and phase II enzymes can modulate cancer susceptibility [88]. Aldo-keto reductase enzymes also metabolize benzo[a]pyrene-7,8-dihydrodiol to benzo[a]pyrene-7,8-quinone, which can induce ROS formation [6].

Polymorphisms in CYP2C9 (CYP2C92 and CYP2C93 alleles) lead to interindividual differences in drug and xenobiotic responses, reducing enzyme activity and benzo[a]pyrene metabolism. Studies in Pakistan have shown that a significant portion of the population carries at least one low-activity CYP2C9 allele [89].

Polymorphisms in metabolic genes related to benzo[a]pyrene toxicity have been studied in diseases like oral squamous cell carcinoma (OSCC) [90]. CYP2C9 polymorphisms are more frequent in OSCC patients, suggesting a slightly increased risk associated with poor metabolizing variants.

Polymorphisms in DNA repair genes, such as nucleotide excision repair (NER) genes like ERCC2/XPD, also influence individual susceptibility to benzo[a]pyrene toxicity [91]. Variations in ERCC2/XPD (Lys751Gln polymorphism) affect DNA repair capacity after carcinogen exposure, potentially acting as a biomarker for cancer risk [92]. Wild-type genotypes show superior DNA repair compared to polymorphic genotypes.

4.2. Benzo[a]pyrene Carcinogenesis Mechanisms

Benzo[a]pyrene-induced carcinogenesis involves metabolic pathways, particularly dioxides and radical cations [6].

The diol-epoxide mechanism proceeds through a series of metabolic steps: benzo[a]pyrene → benzo[a]pyrene-7,8-oxide → benzo[a]pyrene-7,8-diol → benzo[a]pyrene-7,8-diol-9,10-epoxides [93]. Diol-epoxides react with DNA, primarily purine bases, forming DNA adducts. The most common adduct is the N2-deoxyguanosine adduct, formed by the metabolite BPDE. BPDE can produce both unstable and stable DNA adducts [94].

In vivo, anti-benzo[a]pyrene-7,8-diol-9,10-oxide forms stable adducts mainly with guanines. Studies in mice exposed to benzo[a]pyrene showed lung tumors with G → T and G → A mutations in the Ki-Ras gene, and skin tumors with G → T mutations in the Ha-Ras gene [94, 95].

The radical-cationic mechanism involves single-electron oxidation of benzo[a]pyrene to a radical cation, forming adducts with guanine and adenine. These adducts are unstable and may lead to apurinic sites [94].

The mesoregion mechanism involves biomethylation of benzo[a]pyrene and subsequent oxidation, forming DNA adducts. These adducts have been observed in rat liver [96].

Aldo-keto reductase enzymes contribute by forming ortho-quinone/ROS. They oxidize benzo[a]pyrene-7,8-diol to benzo[a]pyrene-7,8-quinone [97], which can react with DNA and generate ROS, causing DNA oxidation and strand breaks.

Benzo[a]pyrene and other PAHs can activate the AhR nuclear complex, leading to CYP gene transcription changes. AhR knockout mice are resistant to benzo[a]pyrene-induced oncogenesis [98].

4.2.1. Oxidative Stress and Apoptosis

CYP and peroxidase activity can lead to radical cation formation and superoxide anion production, contributing to oxidative stress [99].

Benzo[a]pyrene influences ROS formation through the ROS/HIF-1α/HO-1 signaling pathway [100]. Studies on cancer cells have shown that benzo[a]pyrene increases ROS levels and alters HIF-1α and HO-1.

Benzo[a]pyrene induces oxidative stress and apoptosis as mechanisms of toxicity. It causes morphological and physiological abnormalities in zebrafish skeletons due to oxidative stress-induced apoptosis [101].

Benzo[a]pyrene decreases antioxidant enzyme activities (CAT, GSH-Px, SOD, GST, GR), reduces GSH, vitamin C, and vitamin E levels, and increases lipid peroxidation, protein carbonylation, and CYP1A1 and proinflammatory cytokine expression [102, 103, 104, 105, 106]. Benzo[a]pyrene metabolites, particularly BPDE, and ROS are major contributors to these oxidative effects [107, 108, 109, 110].

Benzo[a]pyrene increases AhR translocation, leading to elevated CYP450 and BPDA-DNA adduct levels, as well as inflammation and oxidative stress [103]. It also depletes antioxidant enzyme activities and non-enzymatic antioxidant levels [102, 103, 104, 105].

Maternal benzo[a]pyrene exposure is a risk factor for neural tube defects (NTD) [105]. Studies in mice showed that benzo[a]pyrene exposure during pregnancy induced NTD, increased oxidative stress gene expression (CYP1A1, SOD1, SOD2), suppressed GPX1, and enhanced apoptosis in neural epithelium.

Benzo[a]pyrene also represses DNA repair genes (MSH2, MSH6, EXO1, RAD51), downregulating mismatch repair (MMR) and homologous recombination repair (HR). This DNA repair repression contributes to genotoxic stress-induced aging and the accumulation of irreparable oxidative DNA damage [111].

4.2.2. Oxidative Stress and Neurotoxicity

Benzo[a]pyrene exhibits neurotoxic potential, particularly harmful to the brain due to its lipophilicity and bioaccumulation in lipid-rich tissues [112, 113]. It can cross the blood-brain barrier [114] and elevate PAH levels in the brain [113].

Benzo[a]pyrene exposure leads to declines in spatial learning, memory, and exploratory behavior in mice [115]. It exacerbates neurodegeneration induced by aging Aβ peptide by enhancing NADPH oxidase-derived oxidative stress, potentially contributing to Alzheimer’s disease progression.

Studies in mice showed that benzo[a]pyrene exposure caused behavioral abnormalities, such as anxiety, short-term memory loss, and motor activity deficits, correlating with oxidative stress and altered expression of NMDA receptor subunit mRNA in the hippocampus and cortex [116, 117].

Repeated benzo[a]pyrene exposure in mice induced cytochrome overexpression in the brain, indicating brain metabolism of benzo[a]pyrene [118]. Anxiety reduction and dose-dependent changes in NMDA subunit expression were observed, along with detectable benzo[a]pyrene metabolites in serum at concentrations associated with cognitive damage.

In clams, benzo[a]pyrene showed neurotoxic effects by inhibiting acetylcholinesterase and choline acetyltransferase activity and inducing DNA hypomethylation [108].

4.2.3. AhR Receptor Role in Benzo[a]pyrene Toxicity

AhR is a ligand-activated transcription factor responding to environmental chemicals, including carcinogens. It induces genes involved in immune regulation, detoxification, and proliferation, acting as an environmental sensor [119].

Benzo[a]pyrene activates AhR, causing its translocation to the nucleus and dimerization with ARNT, activating genes like CYP1A1, CYP1A2, and CYP1B1 [120]. The benzo[a]pyrene-AhR-CYP1A1 axis also generates ROS and activates proinflammatory cytokines [120].

Benzo[a]pyrene increases mRNA expression of both AhR and CYP1A1 genes in vivo [121], suggesting a role in carcinogenesis. AhR is expressed in various tissues, including liver, adipose tissue, and bronchial epithelial cells, and plays a role in lung diseases and cancer [123]. It influences lung cancer development by modulating CYP 450 expression and interacting with transcription factors like NF-κB, Nrf2, and ER [123].

AhR activation has diverse downstream effects, impacting carcinogenesis, inflammation, DNA adduct formation, and cell proliferation [124]. AhR expression is altered in tumors compared to normal tissue, influencing pro- or antitumor cell actions. AhR is also a susceptibility factor for skin squamous cell carcinoma and a prognostic factor for melanoma [125]. It modulates carcinogenic effects of UV radiation and chemical substances, regulating inflammation, apoptosis, DNA repair, and metabolism of carcinogens [120]. AhR activation also affects melanoma therapy effectiveness by triggering resistance to B-Raf and serine-threonine kinase inhibitors.

Benzo[a]pyrene-induced metabolic reprogramming, mediated by AhR, contributes to malignant transformation [126]. Metabolomic studies in lung cells exposed to benzo[a]pyrene revealed significant changes in metabolite levels and metabolic pathways, particularly amino acid and fatty acid metabolism, linked to AhR signaling.

Benzo[a]pyrene regulation of lipid metabolism via AhR has been demonstrated in mice [127]. Benzo[a]pyrene treatment led to weight loss, lipid depletion, and glucose/insulin resistance, associated with AhR activation, altered expression of lipid metabolism genes (C/EBPα, PPARγ, FABP4, PGC-1α, PPARα), and increased inflammatory markers (NF-κB, MCP-1, TNF-α). Benzo[a]pyrene inhibits fat synthesis and oxidation while promoting inflammation, leading to white adipose tissue dysfunction and metabolic complications.

5. Adverse Effects of Benzo[a]pyrene in Studies

5.1. Genotoxicity and Carcinogenicity

Benzo[a]pyrene promotes metabolic reprogramming characteristic of cancer cells, including the Warburg effect (glycolytic shift) and mitochondrial dysfunction [128]. This metabolic shift is crucial for benzo[a]pyrene-exposed cell survival.

Synergistic carcinogenic effects between estrogens and benzo[a]pyrene have been suggested, potentially increasing lung cancer aggressiveness [129].

Benzo[a]pyrene can also act independently of the AhR pathway, inducing fibrotic changes and inhibiting differentiation in lung stem cells, increasing lung dysfunction and cancer risk [130].

Genotoxicity of benzo[a]pyrene has been observed in Rainbow Trout, showing increased ethoxyresorufin-O-deethylase activity and micronuclei formation after exposure [131].

5.2. Epigenetic Effects

Benzo[a]pyrene is an epigenetic modifier, disrupting DNA methylation patterns [132, 133, 134, 135], histone expression [137, 138], and miRNA expression [139].

Benzo[a]pyrene epigenotoxicity is linked to CpG-BPDE adduct formation, altering 5-methylcytosine levels [133, 134, 135]. Benzo[a]pyrene can inhibit DNA methyltransferase activities and increase histone deacetylase (HDAC) activities [137, 140, 141].

Ancestral benzo[a]pyrene exposure can cause intergenerational osteotoxicity, mediated by epigenetic deregulation of bone miRNA/genes [142]. Offspring of benzo[a]pyrene-exposed sea medaka showed circadian rhythm disturbances and DNA damage, with paternal exposure causing more severe effects [143].

5.3. Epidemiological Evidence of Benzo[a]pyrene’s Epigenetic and Carcinogenic Effects

The IARC classifies benzo[a]pyrene as “carcinogenic to humans” (Group 1) [6]. Epidemiological studies link PAH and benzo[a]pyrene exposure to increased risks of lung, skin, bladder, larynx, kidney, prostate, breast, blood (leukemia), brain, and colon cancers [11, 13, 144, 145, 146, 147, 148, 149].

Benzo[a]pyrene metabolites induce DNA adducts and mutations, including TP53 gene mutations common in smokers’ lung cancer [150]. Specific G → T transversions in TP53 are associated with benzo[a]pyrene exposure from smoking and coal combustion [151].

Geographic studies in Poland linked benzo[a]pyrene emissions from domestic heating to lung cancer risk, especially outside urban areas [152]. Benzo[a]pyrene exposure may account for up to 31% of lung cancer cases in certain regions.

Studies on breast cancer risk and benzo[a]pyrene exposure showed increased risk associated with cumulative benzo[a]pyrene exposure, varying with menopausal and hormone receptor status [13].

Epidemiological studies indicate correlations between PAH exposure and disturbed DNA methylation in offspring [133, 134, 135]. Prenatal PAH exposure is linked to lower global DNA methylation in umbilical cord blood [133], and maternal smoking during pregnancy alters DNA methylation in newborns [134, 135].

5.4. Reproductive Effects

5.4.1. Effects on Male Reproduction

Benzo[a]pyrene exposure damages DNA at all spermatogenesis stages, reducing sperm count [154]. Subchronic inhalation exposure in male rats depleted testicular and epididymal functions [155]. Benzo[a]pyrene can also cause sperm hyperactivity due to premature capacitation [156].

Chronic exposure to sublethal benzo[a]pyrene doses in male rats altered steroidogenesis and spermatogenesis, reducing fertility [157]. Benzo[a]pyrene decreased testicular and epididymal weights, reduced sperm production, motility, and viability, and decreased steroidogenic enzyme activities and serum testosterone levels. In silico studies suggest benzo[a]pyrene interaction with StAR protein, affecting cholesterol transport and androgen synthesis.

Peripubertal benzo[a]pyrene exposure in male rats caused reproductive disorders in offspring (F1 generation) [158]. Paternal benzo[a]pyrene exposure reduced offspring body weight, altered anorectal distance, accelerated puberty in females, delayed puberty in males, reduced sperm production, and caused sperm morphology changes. Histological changes were observed in testes and ovaries of F1 generation, indicating transgenerational reproductive effects.

5.4.2. Effects on Female Reproduction

Benzo[a]pyrene exposure can compromise ovarian function, impairing oocyte maturation and follicle integrity [159, 160]. Follicle damage can disrupt steroid hormone production, leading to hormonal imbalances and ovarian failure [161]. Developing ovaries are more vulnerable to prenatal benzo[a]pyrene exposure than testes [162].

Benzo[a]pyrene metabolite BPDE DNA adducts have been found in oocytes and luteal cells of women exposed to cigarette smoke [163]. Benzo[a]pyrene crosses the placenta, and maternal exposure can reduce live births and fetal weight, and increase fetal mortality [164, 165, 166, 167].

Studies on scallops exposed to benzo[a]pyrene showed toxic effects on gonadosomatic index, endocrine-related gene expression, hormone levels, and ovarian histology [168]. Benzo[a]pyrene decreased sex hormone levels and steroidogenic enzyme expression, and downregulated estrogen receptor and vitellogenin levels.

5.4.3. Effects on Fetal Development

Benzo[a]pyrene exposure in Japanese Medaka embryos caused teratogenic and developmental effects, including cardiovascular abnormalities and spinal curvature [169]. Gene expression analysis revealed upregulation of genes related to cardiovascular disease and neuronal development, indicating teratogenic potential.

Epidemiological studies link PAH and benzo[a]pyrene exposure to adverse pregnancy outcomes, including lower progesterone levels, reduced fetal growth [170], and increased miscarriage risk [171]. Mouse studies showed fetal weight reduction, preterm delivery, birth defects, and growth reduction after benzo[a]pyrene exposure [172]. Benzo[a]pyrene and BPDE induce apoptosis in human trophoblasts and disrupt chorion explant migration, potentially contributing to miscarriage [171].

6. Impact of Benzo[a]pyrene on Virus Development

Benzo[a]pyrene enhances HIV-1 replication by increasing CYP1A1 expression and activity, leading to oxidative stress and NF-κB pathway activation [173]. Inhibition of CYP1A1 or NF-κB reduces HIV-1 replication, suggesting a CYP-mediated oxidative stress pathway.

Benzo[a]pyrene promotes HPV-related cancer development [174]. It enhances HPV16 E7 oncogene expression and cotransformation potential in rat renal cells. Long-term benzo[a]pyrene exposure increases HPV-positive cancer cell growth and alters cell mobility and invasiveness, indicating a role in HPV-related carcinogenesis at initiation, promotion, and progression stages.

7. Other Health Effects of Benzo[a]pyrene

Benzo[a]pyrene is implicated in accelerated mucus secretion and increased mucin 5AC (MUC5AC) expression in airways [175]. It induces proinflammatory responses in allergic airway inflammation through AhR activation, potentially mediated by increased IL-33 expression [176]. Benzo[a]pyrene is also associated with nonatopic asthma [177].

8. Summary and Conclusion

Benzo[a]pyrene is a prevalent environmental contaminant found in air, water, soil, food, dust, and cigarette smoke. Its hydrophobic nature leads to high concentrations in sediments and soil, and concerning levels in outdoor and indoor air, especially in specific occupational and residential settings.

Benzo[a]pyrene is a highly toxic Group 1 carcinogen with genotoxic, mutagenic, epigenotoxic, teratogenic, and neurotoxic properties, and it impairs fertility. Its toxicity mechanisms involve DNA adduct formation, ROS generation, AhR activation, and epigenetic modifications.

Widespread benzo[a]pyrene air pollution is a significant environmental health threat. Governments need to implement stricter measures to reduce benzo[a]pyrene emissions. Furthermore, bioremediation strategies using benzo[a]pyrene-degrading bacteria are crucial for eliminating existing benzo[a]pyrene contamination, particularly in heavily polluted soils.

Author Contributions:

B.B.: Conceptualization, Writing—original draft, Writing—review & editing. K.M.: Visualization, Writing—review & editing. J.M.: Writing—original draft, Writing—review & editing. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: Not applicable.

Conflicts of Interest: The authors declare no conflicts of interest.

Funding Statement: This work was funded by Research grant (B2011000000191.01) from the Department of Biophysics of Environmental Pollution, Faculty of Biology and Environmental Protection, University of Lodz.

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References: (References are identical to the original article and are therefore omitted here for brevity, but would be included in a full markdown output.)

Associated Data:
Data Availability Statement: Not applicable.