Congener specific distribution and health risk assessment of polychlorinated biphenyls in urban soils

Polychlorinated biphenyls (PCBs) were primarily used in transformers and capacitors, lubricants, flame retardants, plasticizers, paint, carbonless papers, etc. These are capable of long-range atmospheric transport and have been designated as persistent organic pollutants by the Stockholm Convention. Due to their characteristic properties, PCBs are found worldwide in all environmental matrices (including human) and biota. Soils are usually considered to be the source as well as sink for environmental pollutants, with cumulative effects of long-range atmospheric transport and local sources. Around the world, comparatively higher concentrations of PCBs have been reported in urban soils than suburban or rural soils. Higher amount of PCBs in urban soils may cause toxicological health risks to urban residents through ingestion, inhalation and skin contact. This paper presents the PCB distribution in soils from Delhi, India, and exposure risk estimates for human health through soil ingestion. The concentration of ∑PCBs ranged between 1.08-100.67 ng g–1 (mean 21.16 ng g–1±5.24 ng g–1), which was much lower than the Canadian soil quality guideline value of 1.3 mg/kg or 1300 ng g–1. Human health risk estimates through the soil ingestion pathway were made in terms of lifetime average daily dose (LADD), incremental lifetime cancer risks and non-carcinogenic hazard quotient (HQ). The LADD for Delhi adults and children was 3.02¥10–8 mg kg–1 d–1 and 1.57¥10–7 mg kg–1 d–1, respectively, which corresponds to toxic equivalent quotients (TEQ) intake of 0.105 pg TEQ kg–1 d–1 (0.735 pg TEQ kg–1 week–1) and 0.543 pg TEQ kg–1 d–1 (3.801 pg TEQ kg–1 week–1), respectively. The estimated LADD for Delhi residents was lower than the acceptable intake values recommended by the World Health Organization (1 pg TEQ kg–1 d–1), the European Commission (14 pg TEQ kg–1 week–1) and by the Japanese government (4 pg TEQ kg–1 d–1). The probability of cancer risk ranges from 6.04x10–8 (∑PCBs) to 1.57¥10–5 (∑TEQ) and 3.13x10–7 (∑PCBs) to 8.15¥10–5 (∑TEQ) for adults and children, respectively, and was within acceptable ranges of 10–6 to 10–4. The non-carcinogenic risk in terms of health HQ was 0.105 and 0.330 for adults and children, respectively, which was lower than the acceptable limit of 1. The study found lower concentrations of PCBs than guideline values and low health risk estimates through the soil ingestion pathway within acceptable levels, indicating a minimum risk for Delhi residents.


Introduction
Polychlorinated biphenyls (PCBs) are a group of 209 congeners and have been designated as persistent organic pollutants by the Stockholm Convention.4][5] PCBs are characterized by high thermal and chemical stabilities, low vapor pressures, high dielectric constants, hydrophobicity, high lipophilicity, and extreme resistant to degradation.Because of these characteristics, PCBs accumulate in soil, sediments and biota. 6oils of urban areas are usually known as the source as well as sink for environmental pollutants, where cumulative effects of longrange atmospheric transport and local sources are the important factors. 7,8As a result, comparatively higher concentrations of PCBs have been reported in urban soils than suburban or rural soils. 9,10Humans are exposed to PCBs mainly through the consumption of contaminated food, and occupational exposure to PCBs occurs mainly via the inhalation and dermal routes.Higher amount of PCBs in soils, especially dioxin-like PCBs (dl-PCBs), may cause toxicological health risks to urban residents through ingestion, inhalation and skin contact. 113] Therefore, more attention should be given to the status of PCBs in urban soils and the possible risk to human and environmental health.
5][16][17][18][19][20] In this study we quantified the levels of PCBs and their risk estimates in urban soils from the Delhi metro-politan area.This is probably the first study to present the 28 PCB-congener levels, including dioxin-like PCBs, in soils from Delhi, and may provide the base-line statistical data needed for health effect assessments.For this purpose, PCBs were extracted from soils using accelerated solvent extraction (ASE) and quantified.

Study area and sampling
The sampling area was the national capital territory (NCT), Delhi, which is the capital of India with a population of 1.67 million.The total area of NCT, Delhi, is approximately 1483 km 2 and is located in northern India between 28°24'17"N to 28°53'00"N and between 76°50'24"E to 77°20'37"E.Average annual rainfall is 714 mm.The climate in Delhi is hot and humid.During summer, temperatures rise up to 40-45°C and in winters temperatures fall to 4-5°C. 21he soil samples were collected during June 2011 from 14 urban locations on streets and roads near residential areas in Delhi.Approximately 1/2 kg of soil sample was collect-N o n -c o m m e r c i a l u s e o n l y ed from each sampling location, and after removing pebbles, sticks and leaves the sample was mixed thoroughly until homogenized.One part was then transferred to clean, widemouthed amber glass containers that were then labeled.After collection, samples were transported to the laboratory and kept at 4°C until analysis.

Chemicals and solvents
Chemicals (sodium sulfate, potassium hydroxide and sulfuric acid) and solvents (high performance liquid chromatography grade acetone, hexane and dichloromethane) were purchased from Merck India.Pre-cleaned silica gel 60 (0.063-0.100 mm) was obtained from Supelco (Sigma-Aldrich Corp., St. Louis, MO, USA) and used as adsorbent in column chromatography.Prior to use, anhydrous sodium sulfate was cleaned separately with methanol, dichloromethane and acetone in Soxhlet extractor for 8 h each, and stored in air-tight conditions at 130°C.Reference standard solutions of PCBs were purchased from Dr. Ehrenstorfer (Dr.Ehrenstorfer GmbH, Augsburg, Germany) and used for the instrument calibration and quantification.

Sample extraction, clean up and analysis
Sample extraction and clean up was carried out according to validated methods. 22Briefly, a homogenized 20 g sample was dried by mixing with diatomaceous earth (ASE prep DE, Dionex Corp., Sunnyvale, CA, USA) until a free-flowing powder was obtained.The extraction was carried out with accelerated solvent extractor (ASE-350, Dionex Corp.) using acetone/hexane (v/v, 1:1) solvent mixture in two cycles with 5 min static time.The ASE was operated at 1500 psi and the oven temperature was maintained at 100°C.The extracts were concentrated to 2.0 mL using a rotatory vacuum evaporator (Eyela, Tokyo, Japan).Multilayered silica gel column chromatography on a tri-functional column with neutral, basic and acidic silica was performed to remove interfering organic and polar compounds.Details of methodology for clean up and instrumental analysis are reported elsewhere. 20

Quality control analysis
Appropriate quality assurance quality control analysis was performed, including analysis of procedural blanks (analyzed concentra-tions<method detection limit, MDL), random duplicate samples (standard deviation, SD<5%), calibration standard verification (SD<15%) and matrix spike recovery (100±20%). 20PCB congeners were identified in the sample extract by comparing the accurate retention time from the standard mixture and quantified using the response factors from multi-level calibration curves of the standards (r 2 value 0.999).
A signal to noise ratio of 3:1 was used to calculate instrument detection limits by using a valid quantifiable peak.Each sample was analyzed in duplicate and the average was used in calculations.Method detection limits were established by processing 8 aliquots of a sample spiked with a quantity sufficient to produce a detectable response (s/n>3) and multiplying the standard deviation by the t students value (3.0 for 8 replicates).For statistical calculations the non-detect values of PCB congeners have been reported as <0.01 ng g -1 (MDL of all 28 individual congeners being less than 0.01ng g -1 ).Furthermore, it may be noted that our laboratory is ISO 17025 accredited and had been participating in proficiency testing exercises conducted by international agencies, including the Centre d'expertise en analyse environnementale du Québec, and performance scores were satisfactory for PCBs.

Health risk assessment for polychlorinated biphenyls exposure
In this study, assessment of human health risk is discussed as the calculated estimates of the upper-bound excess probabilistic lifetime cancer risk and non-carcinogenic hazards.Hazard is the exact measure of the magnitude of exposure potential or a quantifiable potential for developing non-carcinogenic health effects after averaged exposure period.Risk is the probability of cancer development in a lifetime after a uniform exposure. 235][26] For this purpose, the LADD (lifetime average daily dose), non-cancer risk HQ (hazard quotient) and probabilistic incremental lifetime cancer risks (ILCR) were calculated.LADD is an amount a person takes in as a result of exposure to a chemical in contaminated air, water, soil or food.The input parameters used in the health risk estimation for PCBs are given in Table 1.The equations used for estimating LADD, HQ (non-cancer risk) and probable cancer risk were as follows: LADD (mg kg -1 day -1 )=(Cs x IR x F x EF x ED)/(BW x LT) (1) HQ=LADD/RfD (2) ILCR=LADD x cancer slope factor oral (3)   where: Cs is the total PCBs concentration in soil (µg kg -1 ); IR is the soil ingestion rate (mg day -1 ); F is the unit conversion factor (10 -9 ); EF is exposure frequency (days/year); ED is the exposure duration (year); BW is the body weight (kg); LT is the lifetime which is equal to exposure duration x 365 days (days); RfD is the reference dose for individual dl-PCB congener (mg kg -1 day -1 ).An RfD is a daily intake rate that is estimated to cause no adverse health effects over a specific exposure duration.Non-carcinogenic risks (HQ) were assessed by comparing exposure with RfD of each dl-PCB and the total was reported for each sampling location.Cancer risk for dioxin-TEQ was calculated for each of the 12 dl-PCB congeners from LADD by multiplying slope factor for dioxin (150,000/mg kg -1 day -1 ) and the total was reported.Cancer risk for non-dl-PCBs was calculated from LADD of 28 PCBs by multiplying the upper bound cancer slope factor (2/mg kg -1 d -1 ).The upper bound slope factor is used for those exposure pathways for which environmental processes are likely to increase health risk, such as exposure from consumption of foods, sediment or soil ingestion, and dust or aerosol inhalation.

Health risk assessment
Adults and children may be exposed to contaminants in soil and may also take chemicals from soil through different intake pathways, such as ingestion, inhalation and dermal contact.Figure 2 shows average of ∑PCBs in urban soil at different locations in Delhi, India.The calculated average daily intake of PCBs through soil ingestion adults and children in Delhi was 3.02¥10 -8 mg kg -1 d -1 and 1.57x10 -7 mg kg -1 d -1 , respectively (Table 5), for a soil PCB concentration of 21.2 µg kg -1 .With respect to RfD, the average LADD for adults and children was less than 1% and less than 5%, respectively, of the RfD.The calculated value of TCDD substituted WHO-TEQ daily intake for adults and children was 1.05x10 -10 mg TEQ kg -1 d -1 and 5.43¥10 -10 mg TEQ kg -1 d -1 , respectively, when soil TEQ was 0.073 µg kg -1 (Table 5).The calculated probability cancer risk estimate for PCB exposure from ingestion of soil was 6.04x10 -8 (∑PCBs) to 1.57¥10 -5 (∑TEQ) and 3.13x10 -7 (∑PCBs) to 8.15¥10 -5 (∑TEQ) for Delhi adults and children, respectively (Table 5).The quantified non-carcinogenic HQ for soil ingestion pathway of PCBs was 0.105 and 0.330 for Delhi adults and children, respectively (Figure 3).

Table 5 .
Calculated polychlorinated biphenyl exposure from urban soil and risk for adults and children.average daily dose (mg/kg bw/d); dl-PCB, dioxin-like polychlorinated biphenyls; TEQ, toxic equivalent quotients.

Figure 1 .
Figure 1.Polychlorinated biphenyl homologs in soil at different locations in Delhi, India.

Figure 2 .
Figure 2. Average of ∑polychlorinated biphenyls in urban soil at different locations in Delhi, India.

Figure 3 .
Figure 3. Health risk hazards of polychlorinated biphenyls for adults and children through soil ingestion.