Source identification and risk assessment of polycyclic aromatic hydrocarbons (PAHs) in air and dust samples of Lahore City

During two consecutive summer and winter seasons in Lahore, the health risk of air and dust-borne polycyclic aromatic hydrocarbons (PAHs) was evaluated. Gas chromatography/mass spectrometry (GS/MS) was used to determine air and dust samples from various functional areas across the city. The mean ∑16PAHs were higher in air 1035.8 ± 310.7 (pg m−3) and dust 963.4 ± 289.0 (ng g−1 d.w.) during winter seasons as compared to summer seasons in air 1010.9 ± 303.3 (pg m−3) and dust matrices 945.2 ± 283.6 (ng g−1 d.w.), respectively. PAHs ring profile recognized 3 and 4 rings PAHs as most dominant in air and dust samples. Estimated results of incremental lifetime cancer risk (ILCR) highlighted high carcinogenic risk among the residents of Lahore via ingestion and dermal contact on exposure to atmospheric PAHs. The total ILCR values in air among children (summer: 9.61E − 02, winter: 2.09E − 02) and adults (summer: 1.45E − 01, winter: 3.14E − 02) and in dust, children (summer: 9.16E − 03, winter: 8.80E − 03) and adults (summer: 1.38E − 02, winter: 1.33E − 02) during the study period. The isomeric ratios in the study area revealed mixed PAH sources, including vehicular emission, petroleum, diesel and biomass combustion. As a result, it is advised that atmospheric PAHs should be monitored throughout the year and the ecologically friendly fuels be used to prevent PAHs pollution and health concerns in the city. The findings of this study are beneficial to the local regulating bodies in terms of controlling the exposure and promoting steps to reduce PAHs pollution and manage health in Lahore.


Materials and methods
Study area. Lahore is Pakistan's second most populous city 35 . According to the 2017 census, Lahore's total population is 11.13 million, with a land area of about 1772.43 km 2 36 . It is situated in the Punjab province between latitudes 31° 20′ and 31° 50′ N and longitudes 74° 05′ and 74° 37′ E 43,44 . Around 82% of the population resides in the city, and the remaining 18% lives in the rural areas around the city 45 . Lahore is situated at 63.0936 m in height above sea level. It has hot and semi-arid climate and is classified by the Köppen classification as a desert climate 46 . The average temperature is 24.1 °C, and 75.28 °F and 607-23.9 mm is annual rainfall recorded per year in Lahore 47 . The city has expanded due to a population movement and grown through a population shift for better sociocultural and economic reasons 31 . Figure 1 represents the map of the study area showing sampling sites of Lahore.
Dust sampling. Approximately 5 g of dust samples from each air sampling location were also collected after midday between 4 and 5 p.m. over the same time period as air samples. Each dust sample was made up of five separate subsamples collected from each sampling site and then combined to produce a composite sample 52 . To assemble the tiny particles, samples were gathered in stainless steel dustpans by using plastic brushes in a gentle sweeping movement. Each time new disposable dustpan and brush were used and covered with aluminum foil to www.nature.com/scientificreports/ minimize cross-contamination in samples 53 . Grits, hairs and organic materials were removed from the samples by subsequent sieving through 2 mm mesh (AASHTO classification). The samples were stored at − 20 °C until analysis 54 .
Sample preparation and extraction. Air and dust samples were spiked with 50 µL of deuterated PAHs as recovery standards (Nap-d8, Phe-d10, Chry-d12, 2, 4, 5, 6-T-m-x) and separately extracted with DCM for 24 h by using Soxhlet. The samples were then extracted (in triplicate) by 30 min ultra-sonication with dichloromethane and hexane solution (1:1 v/v), followed by 1 min of vortex agitation and centrifuge for 30 min at 3500-5000 rpm at room temperature 55 . Rotary evaporation (DIAHAN Scientific WEV-1001L) was performed for volume reduction before clean-up the samples. Alumina/silica column was used to purify the samples with 8 mm internal diameter, tightly packed with 3 cm neutral alumina (3% deactivated), 50% sulfuric acid-silica, 3 cm neutral silica gel (3% deactivated) and 1 cm anhydrous Na 2 SO 4 . Later, purified sample was extracted with 1:1 DCM and hexane (by volume), blown down to a final volume of 1 mL under a moderate nitrogen flow (0.2 mL) 51 . The samples were then placed in septa vials for further examination using gas chromatography-mass spectrometer (GC-MS).
Instrumental analysis. The samples were analyzed using GC/MS (QP2010, Shimadzu) for 16 priority PAHs (2-6 rings) in the Split Injection Mode (SIM). The injector and ion sources were both 200 °C. As the carrier gas, helium was used. The Column Flow was set to 1.6 mL/min. The oven temperature was held for 4 min at 50 °C, raised to 320 °C (held for 3 min) 54, 55 . Quality assurance/quality control. Throughout sampling and analysis, strict quality assurance and control procedures were followed. All the chemicals and solvents used in the current study were analytical researchgrade, acquired from Sigma-Aldrich now Merck KGaA (Germany), and checked for impurities prior to use. Na 2 SO 4 was baked for 12 h at 450 °C and stored at 120 °C till use to eliminate any organic debris. The internal and recovery standards were purchased from Chem Service, USA. All the chemicals utilized in the laboratory procedures, for example, acetone (Ace), hexane (Hex), and dichloromethane (DCM) were of the GC analytical grade. Glassware used in sample preparation was heated at 400 °C overnight and stored at 100 °C before use. A set of PAHs standards was performed daily to maintain the instrument's stability, and the instrument's fluctuation was less than 10%. Method detection limits (MDLs) of target compounds were estimated as three times the standard deviation of the mean procedural blank concentrations. Recoveries of the native analytes tested for the reference material were greater than 72% for all PAHs samples. QA/QC was performed to identify any possible laboratory contamination by conducting method blanks, standard reference material recoveries, standard spiked recoveries, and GC/MS detection limits 22 . The dilutions for standards were ranged from 0.001 to 200 μg g −1 .
Human health risk assessment. Cancer risk assessment. The carcinogenic potential of many PAHs, particularly High Molecular Weight (HMW) PAHs, is extensively documented in the literature 59 . The benzo(a) pyrene toxic equivalency factors (TEFs) were used to estimate benzo(a)pyrene equivalent (BaPeq) or benzo(a) pyrene toxicity equivalent (BaP-TEQ) concentrations to evaluate the incremental lifetime cancer risk (ILCR) of PAHs in air and dust samples. Equation (1) was used to calculate ILCR.
BaP − TEQs was calculated by multiplying individual PAH concentration (Ci) by the WHO-recommended TEFs (toxic equivalency factors) values such as 0.001 (Nap, Ace, Acy, Fla, Phe, Flu and Pyr), 0.01 (Ant, Chr and B(ghi)P), 0.1 (B(a)a, B(b)F, B(k)F, and I(cd)P) and 1 (B(a)P and D(ah)A) established by Nisbet and LaGoy 60 ( Table 1). The computed BaP as TEQ values indicated a significant toxicity hazard linked with PAHs in air and dust samples 61 . Cancer risk from inhalation was estimated using WHO (2000) methods, and the unit risk (UR) of 8.7 × 10 −5 (ng m −3 ) was used for a lifetime of 70 years exposure as one individual exposed to one unit BaP (1 ng m −3 ) on average. The potential cancer risk of human exposure via inhalation, ingestion, and dermal contact to air and dust-related PAHs was assessed in different age groups by an Incremental lifetime Cancer Risk (ILCR) model 53,61 .
(1) www.nature.com/scientificreports/ where BaP − TEQ is the total of converted PAHs levels based on toxic equivalents of BaP calculated by multiplying each PAH concentration (c i ) with the toxic equivalency factor (TEF). CSF is carcinogenic slope factor (mg kg −1 day −1 ), BW is body weight (kg), AT is the average life span (years), EF is the exposure frequency (day year −1 ), ED is the exposure duration (years), IR Inhalation is the inhalation rate (m 3 day −1 ), IR Ingestion is the soil intake rate (mg day −1 ), SA is the dermal surface exposure (cm 2 ), AF is the dermal adherence factor (mg cm 2 h −1 ), ABS is the dermal adsorption fraction, and PEF is particle emission factor (m 3 kg −1 ) 62 . CSF Ingestion , CSF Dermal and CSF Inhalation of BaP were addressed as 7.3, 25, and 3.85 (mg kg −1 day −1 ), respectively, determined by the cancercausing ability of BaP 63 . All of the parameters included in this model were based on the United States Environmental Protection Agency's (US EPA) Risk Assessment Guidance and associated publications 64,65 . Values of the parameters used in the above equations are presented in Table S1 ("Supplementary material S1").
Non-carcinogenic risk. The non-cancer risk assessment of PAHs is essentially examining the association between PAHs dose and unfavourable health effects. It primarily consisted of estimating PAHs exposure dose using various environmental matrices (in this study, dust and air), exposure pathways and exposure frequency 66 .
Although, other parameters such as age and body weight may influence the frequency and duration of exposure. The non-cancer risk of PAHs was evaluated in this study for five age groups based on ingestion and inhalation pathways using the Eqs. (5) and (6) to determine the health risk of daily oral and breathing intake of PAHs from dust and air, respectively 10,14,67 .
The equation to determine the health risk of breathing intake 10 :

Results and discussion
Distribution and levels of PAHs in air. The 70 . Due to the meteorological and geographical variations, the composition and concentration of PAHs vary in Lahore from the other urban environments of the world. The findings of the current study agreed with a study reported from twin cities of Pakistan (2132 pg m −3 ) 10 and Paris in France (1000 pg m −3 ) 71 (Table S6). 16 PAHs in road dust ranged from 245.7 ± 818.9 to 283.6 ± 945.2 (ng g −1 ) and 256.8 ± 853.4 to 89.0 ± 963.4 (ng g −1 ) in summer and winter seasons, respectively ( Table 2). The mean concentrations of ∑ LMPAHs and ∑ HMPAHs were ranged between 348.8 ± 104.6 and 387.3 ± 116.2 (ng g −1 ) and 470.1 ± 141.0 to 550.2 ± 165.1 (ng g −1 ) in summer while 359.8 ± 108.9 to 397.6 ± 119.3 (ng g −1 ) and 488.9 ± 146.7 to 565.8 ± 169.7 (ng g −1 ) during winter seasons. The mean ∑C 7 PAHs concentrations were ranged from 282.6 ± 84.8 to 331.3 ± 99.4 (ng g −1 ) and 285.2 ± 85.6 to 331.8 ± 99.5 (ng g −1 ) in summer and winter, respectively. Phe, Nap and Fla levels showed the highest concentrations in both seasons for dust samples. However, PAHs identified in dust samples showed higher winter concentrations than summer during the study period. The concentrations range of PAHs parameters for summer and winter dust are mentioned in Tables S4 and S5, respectively.
Spatial distribution pattern of PAHs in Lahore. Lahore, the country's biggest traffic hub, had a tremendous influx of traffic every day, causing traffic congestion and eventually increasing vehicular emissions 28 . Furthermore, poor car engines maintenance and fuel quality had a substantial role in PAHs emissions 77 . The (4) ILCR Inhalation : www.nature.com/scientificreports/ concentrations of PAHs in Lahore city air and dust varied with seasons. The greatest PAHs concentrations in the air were detected in Shah Jamal and UET, followed by Ravi road, where traffic pollution is to blame for the increasing PAHs levels in these areas. Among all study sites, the dust samples taken from UET road had the highest PAH concentrations. It is the oldest and largest high-traffic area globally, with heavily inhabited streets on both sides of a 2-km span 78 . Due to the high population density in the city, the sites are subjected to considerable traffic and domestic heating 77 . The study's findings revealed that the PAHs distribution in the dust around Lahore is not uniform but rather the result of a number of contributing elements like heavy traffic and distance from industry.
Profile of PAHs in air and dust samples. A significant variation was detected among PAHs concentrations in air, and dust samples. The PAHs profile distributions was in order of 4 rings (air: 31%, dust: 32%) > 3 rings (air: 26%, dust: 27%) > 5 rings (air: 19%, dust: 20%) > 2 rings (air: 14%, dust: 13%) and > 6 rings (air: 10%, dust: 8%) PAHs, respectively. The most dominant PAHs in this investigation were 3 rings (Acy, Flu, Phe, and Ant) and 4 rings (Pyr, BAA, and CHR) with a cumulative percentage contribution of 55% in air and 57% in dust (Fig. 2a,b). This compositional pattern is similar to previous studies showing 3 and 4 ring PAHs as the main contributors of PAHs 2, 79, 80 . The observed trend can be explained by considering the physicochemical characteristics of PAHs and their nearness to their origins 79 . In the present study, 4 rings PAHs were found in relatively higher concentrations, representing the pyrogenic origin and biomass fuel combustion, followed by 3 rings PAHs, suggesting the markers of petroleum-derived residues 22,41 . The dominance of HMW (4, 5 and 6 rings) PAHs was similar to the results conducted by Najmeddin and Keshavarzi 55 in Ahvaz city where HMW PAHs (dust: 68.8%) showed higher concentrations. The amount of PAHs in the atmosphere is also influenced by several variables such as emission sources and meteorological characteristics, including rainfall, temperature, wind speed and direction, resulting in seasonal variation in PAHs levels 81 . Higher PAHs emissions in the winter season are due to increased combustion of biomass and fossil fuels for household heating and the usual increase in primary pollutants in the colder months due to poorer dispersion conditions and lower atmospheric temperature compared to the summer (high-temperature) seasons 82,83 . Thus, the overall results of the PAHs profile revealed not only biomass combustion but also vehicular emission as the source of PAHs deposition in Lahore because all of the selected sampling sites are known for traffic pollution due to the high rate of daily traffic flow on the roadways 1, 4 .

PAHs isomeric ratio in air and dust.
In the present study, PAHs ratios such as Phen/Anth, Flan/Pyr, BaA/Chry, and BaP/BghiP, IP/(IP + BghiP), Flu/(Flu + Pyr) and Anth/(Anth + Phen) were determined to predict the origins of PAHs, which possesses a significant hazard to the population 46,54,58 . PAHs generated from various sources have considerable diverse compositional patterns 84,85 . Ant/(Ant + Phe) ratios less than 0.1 suggest a petroleum source, whereas ratios greater than 0.1 implies that combustion is dominant 65,86 . In the present study, Ant/(Ant + Phe) ratios ranged from ( 86 . As the ratio of BaA/(BaA + Chr) in this study ranged from (0.31-0.49; 0.49-0.53), representing vehicular emission and grass and wood combustion are highlighted as key sources of PAHs in air and dust of Lahore city (Fig. 3a,b). Furthermore, the LMW/HMW PAHs ratio was employed to estimate the extent of burning activities and pyrogenic and petrogenic sources of PAHs 10 . The current study's findings revealed that petroleum combustion was a prominent PAHs contributor since LMW/HMW PAHs ratio was less than 1 during both summer and winter seasons in air (0.68-0.74) and dust samples (0.69-0.75), respectively. It is further supported by the results of Spearmen Correlations, showing the negative relationship between LMW/HMW PAHs ratio for air (summer: R2 = 0.9691; winter: R2 = 0.9968) (Fig. 4a,b) and dust (summer: R2 = 0.9649; winter: R2 = 0.872) (Fig. 4c,d). Current results are consistent with the findings reported by He et al. 84 from Nanjing, China, where fossil fuel burning was recognized as the primary source of PAHs and Hamid et al. 10 , where the indoor and outdoor air PAHs relation with fuel combustion in Pakistan's twin cities was found (Rawalpindi and Islamabad).
Health risk assessment of PAHs in air and dust. Carcinogenic risk. The total BaPeq of ∑16PAHs in air and dust samples ranged from 92.5 to 122.4 pg m −3 and 91.9-117.5 ng g −1 during the study period, respectively (Tables 1 and 2). The current study's findings were comparable to the PAHs levels in street dust of Nanjing, China, from 25.9 to 90.8 (ng g −1 ) 84 . The cancer risk from various routes of exposure was found to be in the order: dermal contact > ingestion > inhalation. Total ILCR estimates utilizing maximum concentrations showed a possible cancer risk for persons residing in sampling areas. The total ILCR values in air samples were children (summer: 9.61E − 02, winter: 2.09E − 02) and adults (summer: 1.45E − 01, winter: 3.14E − 02) and in dust samples of selected areas, children summer: 9.16E − 03, winter: 8.80E − 03 and adults summer: 1.38E − 02, winter: 1.33E − 02 during the study period (Table 3). According to the present study, both ingestion and dermal contact increased cancer with the magnitude of 1E − 02 and 1E − 03 in air and dust samples, respectively, contributing significantly to cancer risk in children and adults (Table 3).
According to epidemiological research, long-lasting PAHs exposure has been linked to increased skin, lung and gastrointestinal malignancies 5,22,54 . For adults, skin contact was the most common exposure route because PAHs enter in the body very easily by the dermal contact with soil, contaminated water, soot, tar or by applying few oils on the body that contain high levels of PAHs, which resulted in a substantially increased risk, followed Non-carcinogenic risk. PAHs can induce health hazards that are not always linked to cancer but can show significant consequences for non-cancerous health risks such as asthma, heart problems, acute lung dysfunction, jaundice, kidney and liver failure 51,52 . Microbial diversity and metabolic profiles may serve as response markers to PAHs exposure in children with asthma. Inhaling PAHs causes hypersensitivity of immunoglobulin E (IgE) substance associated with increasing the asthma emergency department visits in all age groups 87,88 . PAHs, such as Naphthalene, are extremely carcinogenic, can induce kidney and liver damage, cause redness and irritation of skin through dermal contact and cause red blood cell destruction when breathed. Primarily the industrial workers exposed to PAHs and other chemicals were shown to have an elevated risk of skin, lung, bladder, and gastrointestinal malignancies in many studies 89 . PAHs metabolites are related to increased risk of atherosclerotic   www.nature.com/scientificreports/ cardiovascular disease (ASCVD) in the general population, changing the heart rate variability (HRV), an early marker of cardiac autonomic imbalance 90 . Chronic exposure to PAHs induced oxidative stress, involved in the development of diabetes 91,92 . Additionally, additive effect of reduced lung function and urinary OH-PAHs on diabetes was also found 93 . Total estimated daily intake (EDI) PAHs of oral intake in air samples ranged from 2.7 to 17.9 ng kg −1 day −1 for two age cohorts in the air samples, i.e. children: 17.9 ng kg −1 day −1 and adults: 2.7 ng kg −1 day −1 in summer seasons and a similar trend was observed during winter seasons, exhibiting relative higher winter EDI in children than adults. The oral PAHs intake in the dust samples varied as children summer: 16.2 ng kg −1 day −1 , winter: 16.3 ng kg −1 day −1 , adults summer: 0.01 ng kg −1 day −1 , winter: 2.5 ng kg −1 day −1 (Fig. 5a). Total EDI range of breathing intake of air and dust PAHs for two age groups ranged from 695.1 to 1362.7 ng kg −1 day −1 during the study period. EDI for breathing intake of PAHs in air samples varied as children summer: 1317 ng kg −1 day −1 , winter: 1362.7 ng kg −1 day −1 and adults summer: 695.1 ng kg −1 day −1 , winter: 719.2 ng kg −1 day −1 . Whereas, total EDI in dust samples were found to be children: 1234.5 ng kg −1 day −1 , 1235.8 ng kg −1 day −1 and adults: 260 ng kg −1 day −1 , 652.2 ng kg −1 day −1 during summer and winter seasons, respectively (Fig. 5b). The pattern for PAHs intake values through both oral and breathing exposure was higher in children than adults because PAHs can enter the body by breathing in the air contaminated with dust, cigarette smoke, wood, coal, or any other anthropogenic activity such as mining, oil refining, metallurgy, chemical manufacture, transportation, and the electrical sector. PAHs are inhaled through the lungs and are largely deposited in the kidneys, liver and fat. The present study's findings were consistent with previous research conducted in China and Pakistan 10,94 .

Practical implications of this study
As a developing country, Pakistan is facing serious energy crises, and to generate required energy has adopted alternate inefficient fuels, which results in environmental deterioration, which substantially increases the PAHs emissions. Furthermore, the paradigm shift of natural gas vehicles to gasoline/diesel fuel engines has also substantially contributed to the outdoor concentration of PAHs 10 . Additionally, due to a shortage of natural gas (comparatively cleaner fuel), biomass combustion became the main source of household required energy, resulting in an upsurge of pyrogenic PAHs 41,72 . As few past studies highlighted the significance of long-term strategies that intend to transition from allocating subsidies to unsustainable, environmentally-degrading fossil fuels to sustainably-produced renewable energy carriers 10,42,95,96 . Therefore, The green technologies and dissemination of alternative fuels mainly biodiesel and solar power could be one of the environment-friendly alternatives and planning infrastructure, fuel quality, fuel subsidies, renewable energy industry, energy price change and abatement of industrial emissions will be highly essential to reduce the PAHs pollution and maintain the air quality to boost up the economy and achieve energy security of the city.

Conclusions and recommendations
The present study assessed sixteen US EPA priority listed PAHs in outdoor air and dust environments from ten selected areas of Lahore city, Pakistan. Results derived from the comparative analysis identified relative higher PAHs concentrations during the winter season in both air and dust matrices, with air being a more contaminated environmental compartment, which can be attributed to diesel combustion and heavy traffic. Naphthalene, Phenanthrene and Pyrene were the primary PAHs contributors to the air and dust PAHs in Lahore City. According to the particular isomer ratios, PAHs in the investigated region were largely produced by fuel combustion as well Table 3. Total risk (∑ILCR) in ∑ 16 PAHs in air and dust samples of Lahore City, Pakistan. www.nature.com/scientificreports/ as petroleum emissions. Ingestion and dermal contact were the primary exposure routes for PAHs long term exposure. In comparison, inhalation was the least significant contributor to air and dust matrices. The estimated total ILCR from Σ16PAHs exposure signifies a high health risk to the exposed population. This research identifies the need for immediate actions of legislation to limit the semi-volatile organic compounds, particularly PAHs, in urban cities of the developing world and enhance environmental management and health conditions. Overpopulation, rapid industrialization and urbanization have challenged the energy resources and resulted in a dramatic shift of non-environment friendly fuel choice, which has elevated the PAHs levels in the city. Therefore, the investigation of the gaseous PAHs in the atmosphere and dust of the second largest city of Pakistan suggests that green technologies should be introduced in the market to reduce the gap between energy need and supply and ensure public health. Furthermore, the government should formulate policies to minimize pollution load and improve air quality and associated health risks. Moreover, comprehensive research, including a wide range of environmental matrices with varying socioeconomic factors, is needed to determine the extent of chemical contamination in the world's worst air quality affected city, i.e. Lahore.

Data availability
All data generated or analyzed during this study are included in this article (and its supplementary information file).