Human Health Risk Assessment associated with contaminants in the finest fraction of sidewalk dust collected in proximity to trafficked roads

The objective of the study was to determine concentration of metals in sidewalk dust collected in close vicinity to heavily congested roads in Poland in order to assess non-carcinogenic and carcinogenic health risk for both children and adults associated with the ingestion, dermal contact and inhalation of sidewalk dust. Results revealed that sidewalk dust from Warsaw, Krakow, Wroclaw and Opole is heavily contaminated especially with Sb, Se, Cd, Cu, Zn, Pb, considered as indicators of traffic emission. Hazardous indices determined for different exposure pathways indicated that the greatest health risk for both children and adults is associated with the ingestion of sidewalk dust. Carcinogenic risk associated with the ingestion of sidewalk dust by children, calculated for As, Cd, Ni and Pb exceeded safe level of 1 × 10−4 in all cities except for Warsaw. Non-carcinogenic risk of ingestion for children was two orders of magnitude higher than dermal risk and four to five orders of magnitude higher than risk of inhalation. Non-carcinogenic risk associated with the ingestion of sidewalk dust by adults is comparable with dermal contact risk and five orders of magnitude higher when inhalation risk.

can undergo resuspension 20,22,24 and be deposited on road surface, sidewalks or nearby, they should also be considered as a secondary source of pollution. Moreover considering the fact that road dust is enriched with heavy metals especially in close vicinity to heavily congested and stop/start areas of the cities 25,26 , it seems therefore reasonable to assess potential health risk associated with the exposition of inhabitants of big, congested cities to sidewalk dust.
The main objective of the study was (1) to determine concentration of metals in sidewalk dust collected in close vicinity to main roads in four big cities of Poland and to calculate its pollution indices (PI); (2) to assess non-carcinogenic and carcinogenic health risk for both children and adults associated with the ingestion, dermal contact and inhalation of sidewalk dust by calculating hazard quotient (HQ) and hazard index (HI) for each metal and each exposition pathway.

Materials and Methods
Samples locations and sampling collection. Total of 56 samples of sidewalk dust were collected from four big cities of Poland, that is from Warszawa -the capital of Poland, Krakow -second biggest city with 769 498 inhabitants, Wroclaw-the fourth biggest city of Poland with the population of 639 258 and Opole-much smaller city, estimated as 26 th biggest, with 128 224 inhabitants. In Warszawa live approximately 1.770 million residents, however the whole great metropolitan area of Warszawa is the 8 th most populous capital city in EU with 3.1 mln residents. It is estimated that daily approximately 690 000 cars is entering Warszawa, in Kraków 246 000 cars, in Wroclaw 238 000 and in Opole 42 000 cars respectively. From all cities of Poland Krakow is regarded as the most congested city, mostly due to unfavorable road grid.
In Warszawa samples of sidewalk dust were collected from two locations, from Flotyli Wislanej Boulevard (52°23,5084′N 21°03,454′E), Zbigniew Religa Boulevard (52°26,2333′N 21°00,0710′E). In Kraków samples were All samples locations were situated in proximity to main roads. After sprinkling with ultra pure water dust was swept from sidewalks (rectangle 4 m x 1 m) using brush, then placed in Ziploc bag and transported to the laboratory.
Methods. Metals were extracted from fine fraction (<20 µm) of the sidewalk dust samples using aqua regia, according to microwave digestion protocol 3050A 27 . Concentrations of the following metals Al, As, Ba, Cd, Co, Cr, Cs, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Rb, S, Sb, Se, Si, Sn, Sr, Ti, U, V, W, Zn and Zr were there determined via ICP-MS (ELAN 6100 PerkinElmer) according to USEPA method 6020B 28 . In order to obtain unambiguous and unbiased results of ICP-MS analysis, reagent blanks as well as certified international reference materials (METRANAL TM 32, ERM-CZ120 as well as SRM 1848a) were used.
The concentration of Cu, Pb, Zn, Cr, Mn and Cd in sidewalk dust was determined using ICP-MS and/or ICP-OES as well as AAS methods.
Concentrations were then compared with the World Average Shale Value (AVE), considered as a geochemical background values (BV) for the fraction <20 µm according to Turekian & Wedepohl 29 . Furthermore PI (pollution index) was calculated according to the Eq. 1: where Ci refers to metal concentration and Ni-is the geochemical background value according to Turekian & Wedepohl 29. Phase composition of sidewalk dust were carried out via X-ray Diffractometers (SmartLab (9 kW) RIGAKU with a high temperature camera HTK 1200, Miniflex 600 RIGAKU, APD X'Pert Pw3020 PHILIPS).The interplanar distances obtained from the X-ray patterns were used for identifying crystalline phases based on the data of the ICDD (International Centre for Diffraction Data) catalogue and the XRAYAN software.
In order to assess non-carcinogenic risk for children and adults an average daily intake dose of deleterious substances and exposure through ingestion (ADD ing ), dermal contact (ADD derm ) and inhalation (ADD inh ) were calculated for sidewalk dust according to USEPA 30 based on exposure factors provided by Regional Screening Levels (RSLs) -Generic tables 31 and data as well as formulas provided by authors 25,[32][33][34]36 (Table 1) (Eqs 2, 3 and 4).
Potential non-carcinogenic risk related to specific metals were then assessed for each pathway using hazard quotient (HQ) according to the Eqs 5, 6 and 7: 1 Where: RfD ing -oral reference dose (mg/kg per day) obtained by Regional Screening Levels (RSLs)-Generic tables 31 RfD inh -inhalation reference concentration (mg/m 3 ) RfD derm -dermal reference dose (mg/kg per day) GIABS-gastrointestinal absorption factor If HQ exceeds threshold value of 1, potential adverse health effect may occur. The greater the value of HQ above unity, the greater the level of concern 34,37 .
Moreover hazard index (HI) as a sum of individual HQ was calculated in order to assess the overall potential of non-carcinogenic effects posed by more than one deleterious substances.
If HI < 1 then no significant risk occurs, but when HI > 1 chronic risk more likely occurs. Carcinogenic risk for individual pathways were calculated according to Eqs 8,9 and 10 and as total carcinogenic risk was calculated was according to Eq. 11: Where: SF-slope factor SF o -oral slope factor ((mg·kg −1 ·day −1 ) −1 ) IUR -inhalation unit risk (µg·m −3 ) −1 = + + CRisk CRing Crinch CRderm (11) If the risk is higher than threshold value of 10 −4 -10 −6 , the risk is considered as unacceptable according to US EPA 38 .

Results
Geochemical composition of sidewalk dust. Phase analysis conducted via XRD method (Fig. 1A) revealed that sidewalk dust is predominantly comprised of quartz, then to a lesser extent it consists of minerals such as potassium feldspars (microcline), plagioclases (albite), chlorite (clinochlore), calcite and dolomite, as well as a group of clay minerals (such as smectite, illite, kaolinite and mica).
Results of SEM-EDS analysis (Fig. 1B) confirmed the presence of quartz and aluminum silicates as main constituents of sidewalk dust, however other compounds such as titanates or elements like Ag and some Rare Earth Elements (REE) such as La, Ce, Nd and Sm were also detected. These above mentioned secondary constituents of sidewalk dust could be of probable anthropogenic origin since alkali metal titanates (e.g. potassium and sodium titanates) are commonly used as inorganic fillers, which promotes stability of the friction coefficient 39

Concentration of heavy metals in sidewalk dust. Concentration of metals in sidewalk dust from
Warszawa, Krakow, Wroclaw and Opole are presented in Table 2.
The concentration of metals considered as key tracers of non-exhaust emission in sidewalk dust collected from all cities were significantly elevated or even extreme when compared to background values. Pollution indices (PI) have exceeded value of 1 (unpolluted) with respect to most of the analyzed metals thus confirming contamination of the sidewalk dust with: As, Cd, Co, Cr, Cu, Fe, Mg, Mn, Mo, Pb, Sb, Sn, Se and Zn. The highest PI indices were Human health risk assessment for inhabitants of warszawa, krakow, wroclaw and opole. Ingestion pathway. The potential non-carcinogenic human health risk related to ingestion of sidewalk dust by both children and adults (Table 3) are presented in the descending order of deleterious substances: HQ ing indices calculated for individual metals in sidewalk has not exceeded acceptable level of 1, thus indicating negligible non-carcinogenic toxic risk, except for concentration of antimony in Warszawa's sidewalk dust, which can pose health risk for children when ingested. Considering an average HQ indices calculated for all cities it can be concluded that the potential non-carcinogenic human health risk is higher for children than it is for adults. Values of hazard quotient (HQ) calculated for children were one magnitude higher than those determined for adults and were as follows: Sb (   www.nature.com/scientificreports www.nature.com/scientificreports/ Carcinogenic risk (CR) calculated for As, Cd, Ni and Pb in sidewalk dust (Table 4) has exceeded safe level of 1 × 10 -4 in all cities except for Warszawa thus indicating that potential carcinogenic risk posed by those metals to children via ingestion occurs and it is not negligible.
Furthermore carcinogenic health risk associated with the ingestion of sidewalk dust by adults was not found ( Table 4). Values of CR calculated for As, Cd, Ni and Pb has not exceeded safe acceptable range of 1 × 10 -6 -1 × 10 -4 .
Dermal contact pathway. The results presented in Table 5 has revealed a greater risk associated with dermal contact with sidewalk dust for adults rather than for children. These findings are consistent with the results of other researchers 34,45 . The highest HI was determined for adults exposed to sidewalk dust via dermal contact in Wroclaw (1. 19  www.nature.com/scientificreports www.nature.com/scientificreports/ The potential non-carcinogenic human health risk related to dermal contact with individual deleterious substances in sidewalk dust for both children and adults are presented in the descending order: Cd > Pb > Cu > Zn. Furthermore, no carcinogenic health risk associated with sidewalk dust dermal contact for children was found, as the CR values for As and Pb did not exceed acceptable level of 1 × 10 −4 ( Table 6). Carcinogenic risk was however significant for adults, since CR values calculated for As and Pb were 3.04 E-04.   Table 5. Non-carcinogenic health risk associated with dermal contact with sidewalk dust by children and adults. * ADD (average daily dose via dermal contact). **RfD derm (mg/kg body weight per day). HQI (unitless).  www.nature.com/scientificreports www.nature.com/scientificreports/ Inhalation pathway. The potential non-carcinogenic human health risk related to the inhalation of sidewalk dust for both children and adults are presented in the descending order of individual deleterious substances: Mn > N i > Co > Ba > As > Cd > Se. Values of Hazardous Indices (HI inh ) determined for sidewalk dust in all four cities were below the unity (Table 7), thus indicating negligible potential non-carcinogenic health human risk associated with the inhalation of sidewalk dust by both children and adults.
It was found that non-carcinogenic risk associated with the inhalation of sidewalk dust is two times higher for children than it is to adults due to the fact that kids are introducing 50% more air into their lungs per body mass and moreover their respiratory systems are not entirely developed yet so they are more easily to be damaged 45 .
Consequently no carcinogenic health risk associated with the inhalation of sidewalk dust was found for children and adults (Table 8), as the CR values for As, Cd, Ni and Pb did not exceed acceptable treshold level of 10 -4 . Carcinogenic health risk was however higher for children than for adults, but in both cases was considered as negligible.   [47][48][49] , since lanthanum is used in batteries, catalyst and lenses, cerium is currently used as fuel additive, catalyst or optical polish, neodymium and samarium are commonly used in lasers, magnets or in car computers and LCD screens 40 . It can be concluded that the exposition to sidewalk dust via all pathways is especially dangerous in heavily congested urban areas, in close vicinity to traffic lights (thus confirming findings of previous researches 50 .
Hazardous indices determined for various exposure pathways for inhabitants of Warszawa, Krakow, Wroclaw and Opole indicate that the greatest health risk is associated with the introduction of heavy metals from sidewalk dust through ingestion. Ingestion is the primary pathway which has the most harmful effect on both children and the adults when compared to other exposure pathways like dermal and inhalation.
Non-carcinogenic risk associated with the ingestion of sidewalk dust by children was two orders of magnitude higher than the corresponding dermal risk and was four to five orders of magnitude higher than inhalation value. These findings are consistent with the reports of other, multiple researchers 32,34,45 . For adults non-carcinogenic risk associated with the ingestion of sidewalk dust is comparable with dermal contact risk and five orders of magnitude higher when compared to the risk of dust inhalation.
Children are especially exposed to metals in sidewalk dust due to the fact that on average, the dose introduced into the child's body by ingestion is nearly ten times greater than that of an adult. The hazards indices (HI) calculated for the following metals (Ba, Cd, Co, Cu, Mn, Mo, Pb, Sb, Se, Sn, Zn and Zr) for the ingestion of sidewalk dust by children has significantly exceeded safe level of 1 (from 4.1 in Opole to 6.03 in Warszawa) in all cities, thus confirming that the exposure to sidewalk dust can cause potential health hazard to children.
Values of hazard indices determined for adults revealed potential non-carcinogenic risk for sidewalk dust ingestion only for Krakow inhabitants (HI 2.37). The total non-carcinogenic risk calculated as the sum of HI values for the individual exposure pathways was as high as 2.59 with respect to children and 0.524 with respect to adults. Therefore it can be concluded that the sidewalk dust can cause a higher potential health risk for children than for adults. Especially concern should be paid to the contamination of sidewalk dust with Cd, Zn since those elements are the most mobile and consequently bioavailable.
The results of human health risk assessment related to sidewalk dust exposure may confirm the purposefulness of creating areas of limited traffic in the city centers, where pedestrians, tourists prevails and they are potentially exposed to the risk of ingestion, dermal contact or inhalation of sidewalk dust.   Table 8. Carcinogenic health risk for the inhalation of sidewalk dust by children and adults.