Hazard Quotients , Hazard Indexes , and Cancer Risks of Toxic Metals in PM 10 during Firework Displays

Bonfire night is a worldwide phenomenon given to numerous annual celebrations characterised by bonfires and fireworks. Since Thailand has no national ambient air quality standards for metal particulates, it is important to investigate the impacts of particulate injections on elevations of air pollutants and the ecological health impacts resulting from firework displays. In this investigation, Pb and Ba were considered potential firework tracers because their concentrations were significantly higher during the episode, and lower than/comparable with minimum detection limits during other periods, indicating that their elevated concentrations were principally due to pyrotechnic displays. Pb/Ca, Pb/Al, Pb/Mg, and Pb/Cu can be used to pin-point emissions from firework displays. Air mass backward trajectories (72 h) from the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model indicated that areas east and north-east of the study site were the main sources of the airborne particles. Although the combined risk associated with levels of Pb, Cr, Co., Ni, Zn, As, Cd, V, and Mn was far below the standards mentioned in international guidelines, the lifetime cancer risks associated with As and Cr levels exceeded US-EPA guidelines, and may expose inhabitants of surrounding areas of Bangkok to an elevated cancer risk.


INTRODUCTION
Over the last few decades there has been increasing interest in the adverse public health impacts of exposure to ambient toxic chemicals, principally in relation to carcinogenicity and mutagenicity (Pongpiachan, 2013a,b;Pongpiachan et al., 2013Pongpiachan et al., , 2015aPongpiachan et al., ,b, 2017a,b,c,d),b,c,d).Another central concern of ambient air quality studies is the chemical compositions of particulate metals.It is important to note that most toxic metals are preferentially present in finer aerosols, since they have lower densities and greater surface area per unit of volume and organic matter content (Charlesworth et al., 2003;Madrid et al., 2008;Yatkin and Bayram, 2008a,b).Recent studies underline the adverse health effects of exposure to particulate metals (Fang et al., 2013;Kong et al., 2011;Lu et al., 2014;Wu et al., 2015).To the best of our knowledge, there are very few publications on PM10-bound heavy metals in Southeast Asian countries, which have a combined population of 651 million.
Consequently, it seems profitable to monitor the levels of such metals in ambient air.Such data would be essential for public administrative bodies, such as the Environmental Protection Agency (EPA) or Pollution Control Department (PCD), for preparing amendments or revisions to air quality standards, as well as establishing baseline data for atmospheric research communities.
Previous studies have highlighted traffic emissions (Christoforidis and Stamatis, 2009;Duong and Lee, 2011;Johansson et al., 2009;Ndiokwere, 1984), solid waste incinerators (Jianguo et al., 2004;Zhang et al., 2008), thermal power plants (Meij and te Winkel, 2007;Reddy et al., 2005;Sushil and Batra, 2006), industrial boilers (Dahl et al., 2009;Yoo et al., 2002), open burning of e-waste and municipal solid waste (Fujimori et al., 2016;Wang et al., 2017), and forest fires (Betha et al., 2013;Breulmann et al., 2002) as major sources of heavy metals in ambient air.Despite the large number of published studies on the emission sources of selected metals in particulate matter, their behaviours in tropical regions remain unclear, especially in Southeast Asian countries, where few databases of particulate metals have been published and made publicly accessible.
During the past few years, several studies have examined the enhanced levels of toxic pollutants in ambient air during firework displays (Camilleri and Vella, 2010;Pongpiachan et al., 2017a;Seidel and Birnbaum, 2015).The literature also highlights the impacts of firework displays as one of the main contributors of specific metal particulates in ambient air (Feng et al., 2016;Tsai et al., 2012;Wang et al., 2007).Despite several investigations highlighting the importance of traffic emissions as a source of chemical pollutants in Bangkok (Pongpiachan, 2013b;Pongpiachan et al., 2013Pongpiachan et al., , 2015aPongpiachan et al., , 2017b)), there is no research on the effects of the "Loy Krathong Festival" (LKF) on the increase of selected metals in ambient air.It is a tradition for Thai people to float Krathongs (i.e., floating baskets) on a river, to pay respect to the spirit of the Thai river goddess or Phra Mae Kongkha.People generally launch elaborate fireworks on the evening of the full moon in the 12 th month of the traditional Thai lunar calendar.In Northern Thailand, LKF is locally acknowledged as "Yi Peng", which is one of the most memorable Lanna festivals throughout the year, combined with Khomloi (i.e., Lanna-style sky lanterns) and firework displays.Hence, it is evident that more field research is required to elucidate the influences of firework displays on selected metal profiles.The main goals of this study are to: (i) compare selected metal profiles before and after firework displays; (ii) investigate the influences of firework displays on the behaviours of selected PM10bound metals; and (iii) calculate hazard quotients, hazard indexes, and cancer risks associated with toxic metals in PM10 before and after the bonfire night episodes in the centre of Bangkok Metropolitan.

Air quality observation sites
This study examined the impacts of fireworks displays on ambient air quality in Bangkok according to the mass concentrations of PM10 and their chemical characteristics, including selected metals.
Data on ambient air quality were collected from four Pollution Control Department (PCD) Air Andersen Dichotomous Sampler" (Winberry et al., 1988).
The microwave oven was operated at 700 W for 10 min, and 1000 W for a further 10 min.
Hydrofluoric acid was evaporated by heating the sample solutions at 200 °C on a hot plate.The digested solutions were further diluted with 0.1 mol L -1 nitric acid (prepared from 60% nitric acid)

Health risk assessment of selected metals
Concentration and time are frequently used to depict exposure, where amount/mass characterises dose, and the time parameter allows calculation of the dose rate.In order to evaluate the health threats associated with PM10, the average exposure to selected metals by inhalation (Dinh) for both children and adults, based on individual's body weight during a given period, is computed using Eq.

Enrichment factors of selected metals
During the past few decades, enrichment factor (EF) has been comprehensively adopted to evaluate the influences of vehicular exhausts, industrial releases, and mining coupled with ore processing, on atmospheric metals (Li et al., 2012;Wang et al., 2014;Zhang et al., 2015).Despite some uncertainties regarding the selection criteria for the reference elements, Al, Fe, and Si are regularly employed for EF computations (López et al., 2005).In this investigation, Fe was selected as a reference element, presuming the subtle influences of contaminated Fe and the upper continental crustal composition provided by Rudnick (2003).The EF of an element E in a PM10 sample can be explained as Where R is a reference element.In the case of EF values close to one, the crust can be considered as the main contributor.Additionally, SPSS (version 13) was adopted for Pearson correlation analysis (SLRA) and t-tests.

Estimation of mineral matter in PM10
The evaluation of mineral matter (MIN) in PM10 was conducted using the common oxides of Ti, Al, Mn, Mg, Ca, Na, K, and Fe, which were summed (represented as MIN) and then subsequently computed using Eq. ( 8) (Feng et al., 2016;Kong et al., 2015;Terzi et al., 2010) For Ca, a factor of 1.95 is adopted herein because of the presence of CaO and CaCO3.

Back trajectory analysis
Backward trajectories, starting from each receptor site, were calculated using the HYSPLIT_4 (Hybrid Single-Particle Lagrangian Integrated Trajectory) Model with GDAS (Global Data Analysis System) one-degree gridded meteorological dataset (Draxler and Hess, 1998).The 72hour backward trajectories with 6 h temporal resolutions were computed at starting time of 02:00 UTC (local time in Thailand is UTC + 7 hours) on each sampling day.Since back trajectories are sensitive to differences in starting height (Draxler, 2003), the trajectories were tested starting from multiple heights of 1000 m, 1500 m, and 2000 m above sea level to confirm the uncertainty due to the inadequate spatial and/or temporal resolution of the input data.

RESULTS AND DISCUSSION
Table 1 shows the statistical descriptions of selected PM10-bound metals, as well as the t-test analysis during the FDP and NDP, as mentioned previously in section 2.1.The arithmetic means of the 31 selected metal particulate contents ranged from 0.061±0.0086ng m -3 (Be) to 2,096±548 ng m -3 (Ca) for FDP, and from 0.062±0.017ng m -3 to 2,794±929 ng m -3 (Ca) for NDP (see Table 1).
Of the 31 selected metals, although seven (Al, Ca, Sc, Cr, Y, Ba, and Pb) showed significant variations (p <0.05) in mean concentrations between FDP and NDP (see Table 1), only two of those (Ba and Pb) represented significant increases.These findings were in good agreement with previous studies highlighting that Ba and Pb can be considered as firework tracers (Kumar et al., 2016;Tsai et al., 2012;Vecchi et al., 2008).The fact that some earlier studies highlight Sr (Kumar et al., 2016; Vecchi et al., 2008), Mg and K (Kumar et al., 2016;Tsai et al., 2012), and K, Ba, Sr, Cd, S, and P (Kumar et al., 2016) as firework tracers may simply reflect the complexity of metal salts generally employed to generate colours in firework displays, which include SrCO3 (red), CaCl2 (orange), NaNO3 (yellow), BaCl2 (green), copper CuCl2 (blue), and a mixture of metal salts including Sr and Cu (purple colour).
It is well known that selected metals can be categorised into two clusters: crustal metals (including Al, Ca, Fe, Mg, K, and Na), which could be principally attributed to high loading of crustal dust; and anthropogenic metals (such as Zn, As, Pb, V, Ti, Cr, Mn, Ni, Sr, Cu, Li, Cd, and Co), which originate from human activities (e.g., traffic exhaust, industrial emissions, burning of fossil fuels) (Pan et al., 2015).In this study, the atmospheric concentrations of crustal metals (i.e., the sum of Al, Ca, Fe, Mg, K, and Na) were 6,582 ng m -3 for FDP and 7,438 ng m -3 for NDP.Interestingly, the particulate contents of anthropogenic metals (i.e., the sum of Zn, As, Pb, V, Ti, Cr, Mn, Ni, Sr, Cu, Li, Cd, and Co) were 510 ng m -3 for FDP and 468 ng m -3 for NDP.Since the anthropogenic metals were slightly higher in FDP, it seems rational to interpret particle injections triggered by firework displays as a main contributor to atmospheric concentrations of anthropogenic metals during the LKF period.These findings are also in good agreement with the finding that the TE concentration (see Eq. ( 9)) detected in FDP (i.e., 581 ng m -3 ) was slightly higher than in NDP (i.e., 526 ng m -3 ).
As illustrated in Fig. 2, two near-identical patterns (represented as percentage contributions) of particulate selected metals were detected during the FDP and NDP episodes, both of which followed the sequence: Ca>Na>K>Fe>Al>Mg>Zn>Cu>Pb. The similar decreasing sequence of selected metals between the two episodes highlights the relatively homogeneous distribution of the 31 target compounds throughout the ambient air of Bangkok during the observation periods.Since previous studies report that vehicle exhaust is the main emission source of air pollutants in Bangkok (Pongpiachan, 2013b;Pongpiachan and Iijima, 2016;Pongpiachan et al., 2013Pongpiachan et al., , 2015a, 2017a), it appears rational to interpret the identical distribution patterns of the selected metal compositions between the two episodes as resulting from the effects of road traffic emissions rapidly overwhelming other potential contributors during the sampling period.Further evaluations of particulate metal injections triggered by firework displays were conducted by applying the concept of diagnostic binary ratios, which will be described in section 3.1.

Diagnostic binary ratios of selected metals
Although the application of diagnostic binary ratios is frequently criticised as a reliable tool for identifying the emission sources of air pollutants (Galarneau et al., 2008), this technique has still been widely used in numerous studies tracing particulate metals in ambient air during the past few years (Font et al., 2015;Hieu and Lee, 2010;Weckwerth, 2001).A simple binary ratio of two or three selected metals is sensitive to physicochemical transformations, and emission source strengths that can occur in the ambient air commonly involve metal contributions from numerous sources; consequently, a comprehensive consideration of several metal ratios can provide a broader picture for potential source identification.In this study, Ba and Pb were the only two metals that showed significantly higher concentrations during the firework display episodes.It is also worth mentioning that Ca, Na, K, Fe, Al, Mg, Zn, and Cu were the eight most abundant metals observed in both episodes.For these reasons, 16 pairs of selected metals were selected as potential tracers for firework emissions (see Table 2).The 16 metal ratios fell within the ranges 0.033 0.56 and 0.017 0.35 for FDP and NDP, respectively.As illustrated in Fig. 3, emissions of Li-Cs showed strong correlation (R=0.92,n=50, p<0.0001) followed by n=50,p<0.0001),Al-Y (R=0.93,n=50, p<0.0001),V-Ni (R=0.91,n=50, p<0.0001),Mn-Cs (R=0.97,n=50, p<0.0001), n=50,p<0.0001),n=50,p<0.0001).Consequently, Li/Cs, Li/Tl, Al/Y, V/Ni, Mn/Cs, and Mn/Tl were tested to evaluate their potential as firework tracers.
Unfortunately, the FDP/NDP ratios of these six metal ratios were ≈1 and therefore inappropriate as firework tracers.Interestingly, only four metal ratios observed in FDP (i.e., Pb/Ca, Pb/Al, Pb/Mg, and Pb/Cu) were approximately two times higher than in NDP.Additionally, eight metal ratios (Ba/Ca, Ba/Na, Ba/K, Ba/Fe, Ba/Al, Ba/Mg, Ba/Zn, and Ba/Cu) had ratios <1.6.Since Se and Cd were the two metals with the largest enrichment factors (Fig. 4), a binary ratio of Se/Cd was also computed in both sampling campaigns.Regrettably, the FDP/NDP ratio of Se/Cd was close to one (i.e., 0.8), indicating its unsuitability for characterising firework displays.

Enrichment factors of selected metals
As shown in Fig. 4, the logarithmic EFs of the 31 selected metals in PM10 detected at both monitoring campaigns from November 2012 to December 2013 followed the sequence: Se>Sb>Cd>Bi>Cu>Sn>Pb>Mo>Zn>As>Tl>Ba>Ni>V>Rb>Ca>Cs>K>Na>Zr>Mn>Li>Be>Sr>Co >Cr>Fe>Y>Sc>Mg>Al.These findings can be categorised into four groups as an arbitrary scale, based on earlier reports (Karageorgis et al., 2009;Pongpiachan and Iijima, 2016) Crawford et al., 2007;Pongpiachan and Iijima, 2016).Additionally, the extremely low Log(EF) values observed for Al (i.e., FDP: -0.59; NDP: -0.54) observed here are consistent with earlier studies (Pongpiachan and Iijima, 2016;Wu et al., 1994).Terrestrial soil releases of particulate Al are the most reasonable interpretation of these exceedingly low Log(EF) values detected in both sampling episodes.
The back trajectory analysis reveals prevailing easterly and north-easterly surface winds during the observation period (see Figs. S1-S5).It is obvious that the majority of the prevailing winds passed over potential anthropogenic emission sources (i.e., the Special Economic Zones (SEZ) in Cambodia) before reaching the study site.Although total employment in all of Cambodia's SEZs is currently approximately 68,000, this represents just under 1% of total employment and 3.7% of total secondary industry employment in Cambodia (Warr and Menon, 2016) Hence, it appears reasonable to assume that the easterly winds were comparatively less influenced by anthropogenic emissions.These findings were consistent with the relatively low Log(EF) values observed in the present study, highlighting that crustal emissions dominated the ambient air quality of Bangkok during the observation period.

Hazard quotients, hazard indexes, and cancer risks of selected metals
Statistics for Dinh-children, Dinh-adults, HQchildren, HQadult, and LADD are shown in Table 3, including during both monitoring campaigns.As clearly displayed in Table 3, the risk levels of Pb, Cr, Co, Ni, Zn, As, Cd, V, and Mn through the inhalation exposure system in both FDP and NDP were in the range of 3.52×10 -7 ∼6.75×10 -3 and 1.99×10 -7 ∼3.80×10 -3 for children and adults, respectively.
Both values were much lower than the acceptance risk of 1.It should be noted that the sequence of risk levels for the non-carcinogenic heavy metals was Mn>Cr>Co>As>Pb>Cd>V>Zn>Ni, which differ from those for PM2.5 in Xinxiang (Zn>Pb>As>V>Cr>Mn>Ni>Cd>Co) reported by Feng et al. (2016).This discrepancy might be explained by some differences in particle size distributions (Marcazzan et al., 2001;Samara and Voutsa, 2005;Wang et al., 2013) and emission source characteristics between Bangkok and Xinxiang.It is worth mentioning that the sum of the risk levels (HI) for the nine heavy metals were 7.28×10 -3 and 4.10×10 -3 for children and adults, respectively.These are clearly less than 0.1 and much less than 1; moreover, they are 33 and 32 times lower than the risk levels reported for Xinxiang for children and adults, respectively.Since the HQchildren values were almost double the HQadult values for both monitoring periods, it appears reasonable to mention that children are more vulnerable than adults to the noncancerous effects of these nine non-carcinogenic heavy metals (Yang et al., 2014).This can be attributed to their mouthing behaviours, whereby children's hand-to-mouth activities represent a major pathway of chemical exposure (Pongpiachan, 2016;Pongpiachan et al., 2017a).
The LADD of Pb, Cr, Co, Ni, Zn, As, Cd, V, and Mn, and the cancer risk (Rt) (see Eq. 6) associated with As, Cd, Cr, Ni, and Co exposure via respiration, are also displayed in Table 3.The sequence of R values (see Eq. 5) during the FDP was As>Cr>Cd>Co>Ni, which differs from those observed in Xinxiang (i.e., As>Cd>Cr>Ni>Co) by Feng et al. (2016).It is important to note that the carcinogenic risks associated with As, Cd, Cr, Ni, and Co were all >10 -6 ; in particular, As and Cr were 180 and 145 times greater than internationally accepted precautionary or threshold values for cancer risk (Feng et al., 2012;Wang et al., 2007).Additionally, the carcinogenic risks for Ni (FDP: 5.94×10 -6 ; NDP: 4.51×10 -6 ) and Co (FDP: 6.51×10 -6 ; NDP: 6.55×10 -6 ) were slightly higher than the accepted value of 10 -6 .Overall, the lifetime cancer risks of particulate As, Cd, Cr, Ni, and Co noticeably surpass the US-EPA guidelines, and it seems rational to conclude that these may expose neighbouring residents in Bangkok to enhanced risk of cancer.

CONCLUSIONS
The atmospheric concentrations, hazard quotients, hazard indexes, and cancer risks of 31 selected metals present in PM10 were investigated during firework display and non-display periods in Bangkok.Only Ba and Pb were significantly higher during the firework display periods.These results were consistent with earlier findings indicating that these two metals can be acknowledged as firework tracers.Since the Pb/Ca, Pb/Al, Pb/Mg, and Pb/Cu ratios were approximately two times higher during the firework display period, it appears reasonable to apply these four diagnostic binary ratios as potential firework tracers, particularly in the case of Bangkok.Enrichment factors highlighted the importance of crustal emissions as a main contributor of particulate metals in ambient air of Bangkok regardless of firework events.No significant differences in risk levels were observed for Pb, Cr, Co, Ni, Zn, As, Cd, V, or Mn during firework episodes.Although HI values observed in both sampling campaigns were much lower than international guidelines, the carcinogenic risks associated with As, Cd, Cr, Ni, and Co all exceeded the acceptable level of 10 -6 , raising public health concerns over increased cancer risk among surrounding residents in Bangkok.

Table 2 .
Mean ratios of selected metals representative of FDP and NDP

Table 3 .
Hazard quotients, hazard indexes, and cancer risks for selected metals present in PM10 in Bangkok