Congener Specific Distribution and Health Risk Assessment of Polychlorinated Biphenyls in Urban Soils

Abstract

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 ΣPCBs 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 6.04 × 10⁻⁸ (ΣPCBs) to 1.57 × 10⁻⁵ (ΣTEQ) and 3.13 × 10⁻⁷ (ΣPCBs) to 8.15 × 10⁻⁵ (ΣTEQ) 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 desig- nated as persistent organic pollutants by the Stockholm Convention. PCBs were primarily used in transformers and capacitors as dielec- tric and coolant fluids and in lubricants, flame retardants, plasticizers, paint, etc.[1] Even though the production has been banned since the 1970s and usage of these contaminants are restricted in many countries, PCBs continue to be detected in environmental samples from around the world.[2]

PCBs are toxic, bioaccumulative, and can undergo long-range atmospheric transport world-wide to regions such as the Arctic, Antarctic and high altitude regions of Mount Everest.[3,4,5] PCBs are characterized by high thermal and chemical stabilities, low vapor pressures, high dielectric constants, hydropho- bicity, high lipophilicity, and extreme resistant to degradation. Because of these characteris- tics, PCBs accumulate in soil, sediments and biota.[6]

Soils of urban areas are usually known as the source as well as sink for environmental pollutants, where cumulative effects of long- range atmospheric transport and local sources are the important factors.[7,8] As a result, com- paratively higher concentrations of PCBs have been reported in urban soils than suburban or rural soils.[9,10] Humans are exposed to PCBs mainly through the consumption of contami- nated food, and occupational exposure to PCBs occurs mainly via the inhalation and dermal routes. Higher amount of PCBs in soils, espe- cially dioxin-like PCBs (dl-PCBs), may cause toxicological health risks to urban residents through ingestion, inhalation and skin con- tact.[11] In earlier studies, the potential risk to human health from PCBs in urban soils has been reported.[12,13] Therefore, more attention should be given to the status of PCBs in urban soils and the possible risk to human and envi- ronmental health.

In earlier investigations, occurrence of PCBs had been reported in environmental and biological samples from India.[14,15,16,17,18,19,20] 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 pur- pose, PCBs were extracted from soils using accelerated solvent extraction (ASE) and quantified.

Materials and Methods

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 km2 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.[21]

The 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-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, wide- mouthed 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 sodi- um 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 stan- dard solutions of PCBs were purchased from Dr. Ehrenstorfer (Dr. Ehrenstorfer GmbH, Augsburg, Germany) and used for the instru- ment calibration and quantification.

Sample extraction, clean up and analysis

Sample extraction and clean up was carried out according to validated methods.[22] Briefly, 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 extrac- tion was carried out with accelerated solvent extractor (ASE-350, Dionex Corp.) using ace- tone/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 vacu- um 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 instru- mental analysis are reported elsewhere.[20]

Quality control analysis

Appropriate quality assurance quality con- trol 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%).[20] PCB congeners were identified in the sample extract by comparing the accu- rate retention time from the standard mixture and quantified using the response factors from multi-level calibration curves of the standards (r2 value 0.999).

A signal to noise ratio of 3:1 was used to cal- culate instrument detection limits by using a valid quantifiable peak. Each sample was ana- lyzed in duplicate and the average was used in calculations. Method detection limits were established by processing 8 aliquots of a sam- ple 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 individ- ual congeners being less than 0.01ng g⁻¹). Furthermore, it may be noted that our labora- tory is ISO 17025 accredited and had been par- ticipating in proficiency testing exercises con- ducted by international agencies, including the Centre d’expertise en analyse environ- nementale du Québec, and performance scores were satisfactory for PCBs.

Toxic equivalent quotients (TEQ) of each congener were calculated by multiplying the concentration of individual dl-PCB congener with the toxic equivalent factors (TEFs)[11] with reference to 2,3,7,8-tetrachloro dibenzo-p- dioxin (TCDD) and reported as pg WHO₂₀₀₅- TEQ g⁻¹ dry-weight (dw).

Health risk assessment for poly- chlorinated 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 poten- tial for developing non-carcinogenic health effects after averaged exposure period. Risk is the probability of cancer development in a life- time after a uniform exposure.[23] We estimated the human exposure to PCBs through soil ingestion and the consequent health risk by using equations presented by United States Environmental Protection Agency.[24,25,26] 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 expo- sure 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. Summary of input parameters used in calculations for health risk assessment.

. The equations used for estimating LADD, HQ (non-cancer risk) and probable can- cer 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);
• 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 estimat- ed to cause no adverse health effects over a specific exposure duration. Non-carcinogenic risks (HQ) were assessed by comparing expo- sure 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 multi- plying 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 can- cer 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 expo- sure from consumption of foods, sediment or soil ingestion, and dust or aerosol inhalation.

Results

Polychlorinated biphenyls concen- trations in soil

The concentrations of ΣPCBs and individual 28 congeners in urban soils from Delhi, India, are presented in

Table 2. Polychlorinated biphenyl congener concentrations (ng/g dw) in urban soils from Delhi, India.

. The observed levels of total PCBs ranged between 1.08- 100.67 ng g⁻¹ (dw) with the mean and median of 21.16 ng g⁻¹ and 10.16 ng g⁻¹ (±5.24 ng g⁻¹), respectively, which is at par with those of other cities around the world. The most common congener num- bers among studied PCBs were CB-44, CB-52 and CB-138. The concentration range of 12 dl- PCBs was 0.10-22.57 ng g⁻¹ (mean 5.33 ng g⁻¹±0.55 ng g⁻¹, median 2.00 ng g⁻¹±0.55 ng g⁻¹) and accounted for approximately 25% of total PCBs. TEQ of 12 dl-PCBs, calculated using WHO ₂₀₀₅-TEFs, ranged from less than 1 to 387 pg-TEQ g⁻¹ (mean 73±28 pg-TEQ g⁻¹). Non-ortho-PCBs (CB-77, CB-81, CB-126 and CB-169) were the sole contributors and accounted for over 99% of total TEQ (

Table 3. Toxic equivalent quotients of dioxin-like polychlorinated biphenyl congeners in urban soils from Delhi, India.

). Tetrachorinated, pentachlo- rinated and hexachlorinated biphenyls were the dominant homologues groups of PCBs (

Table 4. Polychlorinated biphenyl homologues (3-7 CB) in urban soils from Delhi, India (ng/g dw).

, Figure 1).

Health risk assessment

Adults and children may be exposed to con- taminants in soil and may also take chemicals from soil through different intake pathways, such as ingestion, inhalation and dermal con- tact. Figure 2 shows average of ΣPCBs in urban soil at different locations in Delhi, India. The calculated average daily intake of PCBs through soil ingestion by 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. Calculated polychlorinated biphenyl exposure from urban soil and risk for adults and children.

), 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 (ΣPCBs) to 1.57×10⁻⁵ (ΣTEQ) and 3.13x10⁻⁷ (ΣPCBs) to 8.15×10⁻⁵ (ΣTEQ) for Delhi adults and children, respec-tively (Table 5). The quantified non-carcino- genic HQ for soil ingestion pathway of PCBs was 0.105 and 0.330 for Delhi adults and chil- dren, respectively (Figure 3).

Discussion

Concentrations and possible sources

The PCBs contamination levels in soils from Delhi were much lower than soil quality guide- lines (1.3 mg/kg or 1300 ng g⁻¹).[27] The results of this study were similar to observations reported for urban soils from Kathmandu,[7] urban soils of Beijing[28] and from Tibet.[29] However, the concentrations of PCBs in Delhi soils were lower than those of Vietnam,[30] Switzerland,[31] Romania,[32] China,[33] San Felipe, Nuevo Mercurio, Zacatecas, Mexico,[34] Moscow[35] and Guangdong Province, South China,[36] but higher than PCBs reported in soils from Turkey.[37]

The PCB patterns show that the low molecu- lar weighted PCBs were major contributors (60%) compared to high molecular weight PCBs (with 40%). Low molecular weight PCBs were primarily used in electrical equipments while high molecular weight PCBs were main- ly used as additives in various applications.[38] PCBs had never been produced in India, but are used in industrial applications.[39] Possible sources of PCBs releases in the study area may be from electronic and electrical waste recy- cling and industrial emission depositions. During the burning of different mixtures of waste content, garden waste, paper, plastics, PVC (polyvinyl chloride) and painted wood may produce relatively large amounts of dioxin like- PCBs.[40,41,42]

Risk estimates

To ensure public health safety, a tolerable daily intake (TDI) has been set by internation- al agencies. The TDI is the amount of intake per kg of body weight per day of a chemical substance suspected of having adverse health effects when absorbed into the body over a long period of time. The joint FAO/WHO Expert Committee on Food Additives[43] recommended a TDI of 1.0 pg TEQ kg⁻¹d⁻¹ for acute, sub- chronic and chronic exposures to dioxins. The European Commission[44] established an accept- able weekly intake of 14 pg WHO-TEQ/kg body weight for dioxins and 12 pg dioxin-like PCBs. The Japanese government[45] established the TDI of dioxins at 4 pg-TEQ kg⁻¹d⁻¹.

The observed LADD of TEQ intake for adults (1.05×10⁻¹0 mg TEQ kg⁻¹ d⁻¹) and children (5.43×10⁻¹0 mg TEQ kg⁻¹ d⁻¹) are much lower than recommended acceptable daily intake. The acceptable risk distribution is equal or lower than 10⁻⁶ for carcinogens and may be up to 10⁻⁴. The observed ILCR (10⁻⁸-10⁻⁵) and HQ values less than the acceptable risk level (HQ=1) (Figure 3) suggests minimum risk to the adults and children due to exposure to PCBs through urban soils in Delhi.

Conclusions

Low concentrations of PCBs in Delhi soils, and subsequently their intake values, were lower than those established in international guidelines. Therefore, estimates of cancer and non-carcinogenic risk through soil ingestion were low, indicating no harmful effects on the population of Delhi. In future, it may be useful to conduct more intensive assessments for persistent organic pollutants, in response to human health and environmental concerns.