The effect of chlorine substitution on the dermal absorption of polychlorinated biphenyls

Penetration pathways, influencing factors and predictive models for dermal absorption of exobiotic molecules: A critical review

This review provides a comprehensive summary of the skin penetration pathways of xenobiotics, including metals, organic pollutants, and nanoparticles (NPs), with a particular focus on the methodologies employed to elucidate these penetration routes. The impacts of the physicochemical properties of exogenous substances and the properties of solvent carriers on the penetration efficiencies were discussed. Furthermore, the review outlines the steady-state and transient models for predicting the skin permeability of xenobiotics, emphasizing the models which enable realistic visualization of pharmaco-kinetic phenomena via detailed geometric representations of the skin microstructure, such as stratum corneum (SC) (bricks and mortar) and skin appendages (hair follicles and sebaceous gland units). Limitations of published research, gaps in current knowledge, and recommendations for future research are highlighted, providing insight for a better understanding of the skin penetration behavior of xenobiotics and associated health risks in practical application contexts.

Percutaneous Penetration of Liquid Crystal Monomers (LCMs) by In Vitro Three-Dimensional Human Skin Equivalents: Possible Mechanisms and Implications for Human Dermal Exposure Risks

Liquid crystal monomers (LCMs) are indispensable materials in liquid crystal displays, which have been recognized as emerging persistent, bioaccumulative, and toxic organic pollutants. Occupational and nonoccupational exposure risk assessment suggested that dermal exposure is the primary exposure route for LCMs. However, the bioavailability and possible mechanisms of dermal exposure to LCMs via skin absorption and penetration remain unclear. Herein, we used EpiKutis 3D-Human Skin Equivalents (3D-HSE) to quantitatively assess the percutaneous penetration of nine LCMs, which were detected in e-waste dismantling workers' hand wipes with high detection frequencies. LCMs with higher log K_ow and greater molecular weight (MW) were more difficult to penetrate through the skin. Molecular docking results showed that ABCG2 (an efflux transporter) may be responsible for percutaneous penetration of LCMs. These results suggest that passive diffusion and active efflux transport may be involved in the penetration of LCMs across the skin barrier. Furthermore, the occupational dermal exposure risks evaluated based on the dermal absorption factor suggested the underestimation of the continuous LCMs' health risks via dermal previously.

Inhalation and dermal absorption as dominant pathways of PCB exposure for residents of contaminated apartment buildings

Applications of polychlorinated biphenyls (PCBs) in buildings and their persistence in indoor environments have led to cases of current and highly elevated exposure in humans, despite the global cease of production decades ago. Personal exposure to PCBs was assessed among residents in a social housing estate in Denmark containing both contaminated (n = 67) and non-contaminated (n = 23) apartments. Samples and estimated daily intakes (EDIs) were assessed for 15 PCB congeners, and body burden, which was limited by the dietary data availability, was compared across 7 indicator PCBs, with its sum (PCB_sum7) often applied in European regulation of PCBs. Median PCB_sum7 EDI across measured pathways for exposed residents was 101 ng· (kg bodyweight)^-1· day^-1, with the majority of exposure (60%) coming from inhalation of contaminated indoor air. Calculated from both PCBs measured in indoor air and on hand wipes, dermal absorption estimates showed comparable results and served as a secondary exposure pathway, accounting for 35% of personal exposure and considering selected assumptions and sources of physical-chemical parameters. Estimates revealed that diet was the primary PCB source among the reference group, accounting for over 75% of the PCB_sum7 EDI across exposure routes. When evaluating overall EDIs across the two study groups and including dietary estimates, PCB exposure among exposed residents was around 10 times higher than the reference group. Solely within the exposed population, pathway-specific body burdens were calculated to account for exposure across years of residence in contaminated apartments, where lower chlorinated PCBs were dominant in indoor air. Among these dominant congeners, estimated body burdens of PCB-28 and -52 were significantly correlated with measured serum (r_s = 0.49, 0.45; p < 0.001). This study demonstrates that inhalation and dermal absorption serve as dominant exposure pathways for residents of apartments contaminated with predominantly lower chlorinated PCBs and suggest that predictions of body burden from indoor environment measurements may be comparable to measured serum PCBs.

Insights into the Dermal Absorption, Deposition, and Elimination of Poly- and Perfluoroalkyl Substances in Rats: The Importance of Skin Exposure

Humans are frequently exposed to poly- and perfluoroalkyl substances (PFASs) via direct skin contact with personal care and consumer products containing them. Here, we used a rat model to estimate the dermal penetration efficiency of 15 representative PFASs. After 144 h post-dosing, 4.1-18.0 and 5.3-15.1% of the applied PFASs in the low (L) and high (H) groups, respectively, were absorbed into the rats. PFAS absorption and permeation were parabolically associated with the perfluorinated carbon chain length (C_F), peaking for perfluoroheptanoic acid (PFHpA). The lipid-rich stratum corneum of the skin barrier substantially suppressed the penetration of less hydrophobic short-chain PFASs, whereas the water-rich viable epidermis and dermis served as obstacles to hydrophobic long-chain PFAS permeation. However, the renal clearance (CL_renal) of the target PFAS decreased with increasing C_F, suggesting that urinary excretion is crucial to eliminate less hydrophobic short-chain PFASs. Notably, the peak times of PFASs in the systemic circulation of rats (8-72 h) were remarkably longer than those after oral administration (1-24 h). These results suggest that dermal penetration can be long-lasting and contribute considerably to the body burden of PFASs, especially for those with moderate hydrophobicity due to their favorable skin permeation and unfavorable urinary excretion.

Percutaneous penetration and dermal exposure risk assessment of chlorinated paraffins

The widespread occurrence of chlorinated paraffins (CPs) in environmental matrices has resulted in a high frequency exposure to CPs via dermal contact. To quantitatively estimate percutaneous penetration of CPs, Episkin® human skin equivalents (HSE) was applied as an in vitro model to evaluate the mechanism of percutaneous penetration of CPs. The co-exposure of CPs mixtures to HSE showed that about 11.7% and 10.2% of short-chain CPs (SCCPs) and medium-chain CPs (MCCPs) could penetrate the HSE and enter the receptor fluid, while no long-chain CPs (LCCPs) (C>17) were able to penetrate the HSE during the 36-h assay period. The experimentally obtained permeability coefficient (K_p) values for CPs were significantly (p < 0.01) negatively correlated with their log octanol-water partition coefficient (log K_ow). Furthermore, 24 participants were recruited to assess direct human dermal exposure to CPs in China with the total CPs collected onto hand wipes and forehead wipes being 96,600 and 30,400 ng/person, respectively. The proportion of total SCCPs and MCCPs intake via dermal penetration (skin area investigated in this study) accounting for 2.0% of the total intake of CPs. Considering the total skin surface of human body is around 20 times of the area studied, the total intake of CPs through dermal penetration could be a significant exposure pathway.

Chlorinated paraffins and tris (1-chloro-2-propyl) phosphate in spray polyurethane foams - A source for indoor exposure?

In this study, we investigated chemical additives present in new and used spray polyurethane foams (SPFs) and assessed the dermal transfer through direct contact. This first study shows that cured do-it-yourself spray one-component SPFs (OCFs) often contain chlorinated paraffins (C₁₄-C₃₇), and tris (1-chloro-2-propyl) phosphate (TCIPP), ranging 0.2-50%, and 0.9-30% w/w, respectively. Six OCFs contained CP levels ranging 22-50% w/w, whereas nine OCFs used for similar applications only contained CP levels ranging 2-17% w/w. It is unclear if the combination CPs/TCIPP is meant to improve the flame retardancy of products, and could suggest an unnecessary use of high CPs/TCIPP concentrations in OCFs. The two-component SPFs (TCFs) contained only TCIPP with levels ranging from 7.0% to 9.0%. The CPs and TCIPP were easily transferred from cured OCFs to the hands. Levels up to 590 µg per hand for CPs and up to 2.7 µg per hand for TCIPP were found. After end-of-life, it is challenging to recycle used SPFs. They may, therefore, end up at landfills where the TCIPP/CPs may leach into the environment. Therefore, further investigation is needed to assess potential exposure risks associated with general and occupational use, and the impact of landfill leaching on the environment.

Human health risks due to airborne polychlorinated biphenyls are highest in New Bedford Harbor communities living closest to the harbor

In response to concerns raised by communities surrounding the New Bedford Harbor Superfund site, we completed a field and modeling study that concluded the harbor is the primary source of polychlorinated biphenyls (PCBs) in air around the harbor. The follow-up question from residents was whether the PCBs measured in air pose a risk to their health. The US Environmental Protection Agency focuses their site-specific, risk-based decisions for site clean-up on cancers. We focused our assessment on the non-cancer effects on the thyroid based on the congener specific patterns and concentrations of PCBs measured in air near and distant to the harbor. Human and animal studies of PCB-induced effects on the thyroid provide evidence to support our analysis. Drawing from the published toxicological data, we used a Margin of Exposure (MOE) approach to derive a human-equivalent concentration in air associated with human health effects on the thyroid. Based on the MOEs calculated herein, evaluation of the MOE indicates that changes in thyroid hormone levels are possible among people living adjacent to the Harbor. Changes in thyroid hormone levels are more likely among people who live near the harbor compared to residents living in areas distant from the harbor. This risk assessment documents potential health risks associated with proximity to a marine Superfund Site using site-specific ambient air PCB congener data.

Dermal Uptake from Airborne Organics as an Important Route of Human Exposure to E-Waste Combustion Fumes

Skin absorption of gaseous organic contaminants is an important and relevant mechanism in human exposure to such contaminants, but has not been adequately examined. This article demonstrates that dermal uptake from airborne contaminants could be recognized as a significant exposure route for local residents subjecting to combustion fume from e-waste recycling activities. It is particularly true for organic pollutants which have high dermal penetration rates and large skin-air partition coefficients, such as low molecular weight plasticizers and flame retardants.

Cytochrome c adducts with PCB quinoid metabolites

Polychlorinated biphenyls (PCBs) are a group of 209 individual congeners widely used as industrial chemicals. PCBs are found as by-products in dye and paint manufacture and are legacy, ubiquitous, and persistent as human and environmental contaminants. PCBs with fewer chlorine atoms may be metabolized to hydroxy- and dihydroxy-metabolites and further oxidized to quinoid metabolites both in vitro and in vivo. Specifically, quinoid metabolites may form adducts on nucleophilic sites within cells. We hypothesized that the PCB-quinones covalently bind to cytochrome c and, thereby, cause defects in the function of cytochrome c. In this study, synthetic PCB quinones, 2-(4'-chlorophenyl)-1,4-benzoquinone (PCB3-pQ), 4-4'-chlorophenyl)-1,2-benzoquinone (PCB3-oQ), 2-(3', 5'-dichlorophenyl)-1,4-benzoquinone, 2-(3',4', 5'-trichlorophenyl)-1,4-benzoquinone, and 2-(4'-chlorophenyl)-3,6-dichloro-1,4-benzoquinone, were incubated with cytochrome c, and adducts were detected by liquid chromatography-mass spectrometry (LC-MS) and matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI TOF). Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) was employed to separate the adducted proteins, while trypsin digestion and liquid chromatography-tandem mass spectrometry (LC-MS/MS) were applied to identify the amino acid binding sites on cytochrome c. Conformation change of cytochrome c after binding with PCB3-pQ was investigated by SYBYL-X simulation and cytochrome c function was examined. We found that more than one molecule of PCB-quinone may bind to one molecule of cytochrome c. Lysine and glutamic acid were identified as the predominant binding sites. Software simulation showed conformation changes of adducted cytochrome c. Additionally, cross-linking of cytochrome c was observed on the SDS-PAGE gel. Cytochrome c was found to lose its function as electron acceptor after incubation with PCB quinones. These data provide evidence that the covalent binding of PCB quinone metabolites to cytochrome c may be included among the toxic effects of PCBs.

Effect of Bromine Substitution on Human Dermal Absorption of Polybrominated Diphenyl Ethers

Human dermal absorption of eight mono- to deca-brominated diphenyl ethers (PBDEs) was investigated for the first time using EPISKIN human skin equivalent tissue. Using a standard in vitro protocol, EPISKIN tissues mounted in specially designed diffusion cells were exposed to the target PBDEs for 24 h. Estimated steady-state flux (Jss) and permeation coefficients (Papp) across the skin increased with decreasing bromine substitution from BDE-153 (Papp = 4.0 × 10(-4) cm/h) to BDE-1 (Papp = 1.1 × 10(-2) cm/h). This was accompanied by an increase in the time required to traverse the skin tissue into the receptor fluid (lag time) from 0.25 h for BDE-1 to 1.26 h for BDE-153. Papp values for the studied PBDEs were correlated significantly (P < 0.05) with physicochemical parameters like water solubility and log KOW. While less brominated congeners achieved faster dermal penetration, higher PBDEs displayed greater accumulation within the skin tissue. The PBDEs thus accumulated represent a contaminant depot from which they may be slowly released to the systemic circulation over a prolonged period. Maximal percutaneous penetration was observed for BDE-1 (∼ 30% of the applied 500 ng/cm(2) dose). Interestingly, BDE-183 and BDE-209 showed very low dermal absorption, exemplified by a failure to reach the steady state within the 24 h exposure period that was studied.

Evaluation of in vitro vs. in vivo methods for assessment of dermal absorption of organic flame retardants: a review

There is a growing interest to study human dermal exposure to a large number of chemicals, whether in the indoor or outdoor environment. Such studies are essential to predict the systemic exposure to xenobiotic chemicals for risk assessment purposes and to comply with various regulatory guidelines. However, very little is currently known about human dermal exposure to persistent organic pollutants. While recent pharmacokinetic studies have highlighted the importance of dermal contact as a pathway of human exposure to brominated flame retardants, risk assessment studies had to apply assumed values for percutaneous penetration of various flame retardants (FRs) due to complete absence of specific experimental data on their human dermal bioavailability. Therefore, this article discusses the current state-of-knowledge on the significance of dermal contact as a pathway of human exposure to FRs. The available literature on in vivo and in vitro methods for assessment of dermal absorption of FRs in human and laboratory animals is critically reviewed. Finally, a novel approach for studying human dermal absorption of FRs using in vitro three-dimensional (3D) human skin equivalent models is presented and the challenges facing future dermal absorption studies on FRs are highlighted.

Hydrocarbon products and their derivatives in fish of the Pechora River, North-Eastern European Russia

Analysis of one of the major surface oil spill consequences, which took place in 1994 on the territory of the Pechora River (Russia) catchment area, was carried out. Data on hydrocarbon products and their derivatives concentrations in muscle and liver tissues of several fish species living in the Pechora's Delta and the river tributaries were obtained. Comparative analysis of OCCs, PCBs and PAHs concentrations in 1997 and 2008 showed that levels of all examined pollutants in fish tissues are very low. At the same time, in 2008 OCCs and PCBs concentrations in fish were lower compared to 1997, except for PAHs concentrations in Pechora Delta whitefish, which demonstrate the opposite tendency in relation to two compounds for the last 11 years.

Congener profiles of occupational PCB exposure versus PCB exposure from fish consumption

The composition of polychlorinated biphenyl (PCB) congeners in serum samples is compared between a cohort previously exposed to PCBs from working at a capacitor plant (n=180) and a cohort of Great Lakes sport-caught fish eaters (n=217). Fourteen congeners were measured in both samples. A multiple logistic regression model differentiating the two groups as a function of relative proportions amongst congeners 74, 138, 153, 180, and 201 correctly classifies more than 99% of the people (395/397); higher proportions of congeners 74, 153, and 201 characterize capacitor plant workers, while higher proportions of congeners 138 and 180 characterize fish eaters. The pattern is driven by the relative amounts of 74+153+201 compared to 138+180; all of the fish eaters, but only 5% of the capacitor plant workers, have a greater percent of 138+180 than 74+153+201. Consideration of combinations of congener levels and their relative proportions is relevant to tracking route of exposure and may also be relevant to modeling effects on health outcomes.

Polychlorinated naphthalenes and coplanar polychlorinated biphenyls in tissues of harbor seals (Phoca vitulina) from the northern Gulf of Alaska

Blubber, liver and kidney samples of harbor seals (Phoca vitulina) from the northern Gulf of Alaska were collected during 2000-2001 for the analysis of polychlorinated naphthalenes (PCNs) and coplanar polychlorinated biphenyls (CoplPCBs). On the lipid weight (lw) base, the total concentrations of PCNs (Sigma PCNs) ranged from 0.3 to 27 ng/g lw, and the total concentrations of CoplPCBs (Sigma CoplPCBs) were 3.6-546 ng/g lw in all the tissue samples. Di-ortho PCBs and mono-ortho PCBs were dominant followed by non-ortho PCBs and PCNs. Sigma Mono-ortho PCBs and Sigma di-ortho PCBs in nursing seals were apparently lower than those in male adult seals, but Sigma PCNs and Sigma non-ortho PCBs in female adults were not significantly different from those in male adults. Differences in PCNs and CoplPCBs congener profiles in female and male adult seals are apparently related to their chemical structure and properties, animal's developmental stages and physiological conditions. A large quantity of mono-ortho and di-ortho PCBs might be transferred to newborns from the nursing seals during lactation, while non-ortho PCBs and PCNs were relatively accumulative in the mother seals. Sigma PCNs and Sigma CoplPCBs in the harbor seals correlated with ages, gender, body weight and blubber thickness, but the blubber Sigma PCNs and Sigma CoplPCBs in Kodiak Island and Southern Alaska Peninsula (KIAP) did not significantly differ from those in Prince William Sound (PWS). In addition to the new PCB data, this first report on PCN contamination in Alaskan harbor seal tissues is useful for the wildlife and ecosystem management and human health protection.

Distribution of organochlorine pollutants in ovine dental tissues and bone

The distribution of selected lipophilic organochlorine pollutants, including two pairs of tetra- and hexa-chlorobiphenyl isomers (PCB-54, -80, -155, -169) and organochlorine pesticides [hexachlorobenzene (HCB) and 1,1-bis(4-chlorophenyl)-2,2-dichloroethene (4,4'-DDE)], in ovine dental pulp, dentine, enamel and mandibular bone was examined. Sheeps were given a single dose of individual organochlorine (1-4μmol/kg) in olive oil by intramuscular injection and sacrificed 2 months later. Organochlorine residues were determined by gas chromatography. The highest levels of organochlorines were found in bone. The lipid adjusted levels varied significantly between tissues. Lower chlorinated, metabolically unstable, non-planar PCB-54 was enriched in enamel, while higher chlorinated, metabolically resistant, planar PCB-169 was preferentially retained in pulp. The ratio of planar to non-planar PCB homologues and pesticides (PCB-169/-155, PCB-80/-54, HCB/4,4'-DDE) decreased in the order pulp>dentine>enamel, suggesting that besides compound's lipophilicity and the tissue's lipid content, other physicochemical properties of organochlorines and tissue specific characteristics affect their distribution.

Time- and tissue-dependent polychlorinated biphenyl residues in hairless mice after exposure to polychlorinated biphenyl-contaminated soil

Four groups of 16 age-matched female Crl:SKH1-hrBR hairless mice were exposed to either control soil or polychlorinated biphenyl (PCB)-contaminated soil (retrieved from an electrical waste landfill in Southern Illinois) for 11 weeks. The mice were exposed in a study to determine interactions between environmental PCBs and ultraviolet radiation (UVR), but the UVR group did not differ and provided a replicate for the residue study. Ear biopsies were performed immediately after the termination of soil exposure. The mice were maintained in regular bedding for 37 weeks thereafter. The ear-skin, trunk-skin, fat-pad, and liver samples were collected and weighed at the end of the study (week 48) and analyzed for PCB residues. A total of 141 PCB congeners were target analytes. There were significant differences in body weights and food consumption from week 2 to 28. The liver weights of mice treated with PCB only were significantly greater than those of UVR-treated mice. The fat-pad weight did not differ among treated groups. PCB residues in the ear biopsies specimens of mice exposed to contaminated soil were 342.3 and 317.2 ppm in the PCB- and PCB + UVR-treated groups, respectively, and contained both persistent and episodic congeners. After 37 weeks of isolation from soil, the ear PCB residues decreased to 21.5 ppm (PCB group) and 14.5 ppm (PCB + UVR group), and only persistent congeners contributed to the total PCB residues. The accumulation of PCB residues was highest in the fat pad (fat pad > ear skin > trunk skin > liver) in both PCB +/- UVR groups at the end of the study. However, the percentage of individual congeners contributing to total PCBs in these different tissues did not differ.

Excretion pattern of co-planar and non-planar tetra- and hexa-chlorobiphenyls in ovine milk and faeces

This study employed the gas chromatography with electron capture detection to determine residual levels and excretion patterns of two pairs of structurally diverse polychlorinated biphenyl (PCB) congeners (IUPAC Nos. 54, 80, 155, and 169) administered to lactating sheep by intramuscular injection. PCB levels and excretion patterns in blood, milk, and faeces were time-dependent and differed from the composition of PCB congeners administered. Lactational transfer substantially exceeded the faecal transfer. Between days 3 and 7, the amount of PCB congeners 54 and 169 excreted in milk was around 50- and 800-fold higher than the amount of these two congeners excreted via faeces. During the same period, the relative contribution of co-planar PCB congeners (80 and 169) in PCB pattern decreased in blood and increased in milk and faeces compared with non-planar PCBs (54 and 155). On day 3, the ratio PCB 169 to 54 was 7-fold higher in milk than in faeces. PCB congeners with log Kow values under 6.5 reached peaks of their excretion in milk within the first three days after administration, while the super-lipophilic PCB 169 congener with log Kow value of over 7 has not reached the plateau until day 10, but afterwards, its level remained relatively high throughout the observation period. During the 57-day follow-up period, the excretion of PCB 80, 155, and 169 in milk was 4.5-, 14-, and 46-fold greater compared with PCB 54. Differences in levels and patterns were explained with some physico-chemical properties of individual PCB congeners, such as lipophilicity, planarity, metabolic stability, sorption/diffusion properties.

PCB profiles in mouse skin biopsies and fat from an environmental mixture

Fat, ear skin and trunk skin were collected from mice exposed to PCB-contaminated soil for PCB specific congener analysis. The soil had been retrieved from a Superfund site prior to remediation. Mice were exposed to either contaminated soil or control soil for 4 weeks. Skin samples were collected immediately after the 4-week exposure and again after 4-week recovery. Out of 141 PCB congeners measured, only 91 peaks representing 103 congeners were above quantification limits. Relative liver weights in PCB-exposed mice doubled indicating significant enzyme induction. Consistent with this observation, total PCB residues declined about 80% during the 4-week recovery. The PCB residue profiles in the ear skin (208mg/kg total) and trunk skin (129mg/kg) after 4-week exposure were identical and similar to those in body fat (370mg/kg) and in the contaminated soil. The results confirmed that both skin tissues are important reservoirs and can serve as useful biopsy media.

Exposure to soil contaminated with an environmental PCB/PCDD/PCDF mixture modulates ultraviolet radiation-induced non-melanoma skin carcinogenesis in the Crl:SKH1-hrBR hairless mouse

Chlorinated aromatic contaminants are active in carcinogenic processes within the skin and may have the potential to modulate ultraviolet radiation (UV)-induced skin carcinogenesis. Exposure to a complex environmental PCB/PCDD/PCDF mixture (polychlorinated biphenyls/polychlorinated dibenzo-p-dioxins/polychlorinated dibenzofurans) during the irradiation phase of photocarcinogenesis was associated with significant (P < or = 0.001) reductions in papilloma incidence and squamous cell carcinoma multiplicity at irradiated skin sites. This protective effect was associated with significantly (P < 0.0001) reduced chronic epidermal thickening in UV and contaminant-exposed mice compared with mice exposed to UV only. Contaminant exposure was also associated with increased UV absorbance of skin methanol extracts implying a sunscreen-like effect.

Dermal absorption in rhesus monkeys of polychlorinated biphenyls from soil contaminated with Aroclor 1260

Human health risk assessments involving contaminated soil include dermal absorption as a potential pathway contributing to the total exposure burden. For PCB-contaminated soil, the U.S. Environmental Protection Agency uses a dermal absorption factor of 14%, based on a 1993 study of dermal absorption in rhesus monkeys. The current study examined several parameters that can influence the dermal absorption of lipophilic hydrocarbons, including soil organic content, particle size, skin residence time, and contaminant "aging" in the soil. Four groups of four female rhesus monkeys each were exposed to radiolabeled Aroclor 1260 either intravenously (100% absorption) or dermally with PCB-spiked soil. Groups exposed for 12 or 24 h to PCBs aged in soil exhibited percutaneous absorption values of 3.43 +/- 0.35 and 4.26 +/- 0.52%, respectively, while a group exposed for 24 h to soil freshly spiked with PCBs exhibited a dermal absorption value of 4.07 +/- 0.46%. Evidence strongly suggests that the factor most responsible for modulating the percutaneous absorption of highly lipophilic compounds from soil is its organic content. The base soil used in the current study with Aroclor 1260 had an organic content of 5-6% (< or =2 mm particle fraction), a value typical for U.S. soil. The organic content of the soil applied to the skin was 8.7% (<150 microm particle fraction), a value that contrasts sharply with the soil containing 0.9% organics used in the 1993 study with Aroclors 1242 and 1254 that produced a dermal absorption value of 14% for PCBs.

In vitro dermal absorption of flame retardant chemicals

Flame retardant chemicals may be used in furniture fabric in the future to reduce the flammability of the fabric. As a part of the process to evaluate the potential for exposure to these chemicals, this study examined the in vitro dermal absorption of two flame retardant chemicals. The chemicals were [14C]decabromodiphenyl oxide (DBDPO) and [14C]tris-(1,3-dichloro-2-propyl)phosphate (TDCP). Skin from the adult hairless female mouse (SKH1) was removed and mounted in flow-through diffusion cells. The chemicals, at three dose levels (DBDPO: 6, 30 and 60 nmol; TDCP: 20, 100 and 200 pmol), were applied in a volatile vehicle (tetrahydrofuran for DBDPO; acetone for TDCP) to the skin. Fractions of receptor fluid, pumped below the skin, were collected over a 24-h period. The skin was washed with solvent (tetrahydrofuran for DBDPO; ethanol for TDCP) to remove unabsorbed chemical 24 h after application. The receptor fluid, skin wash and skin were analyzed for chemical-derived radioactivity. The skin from the high-dose group of both chemicals, and the receptor fluid from TDCP high-dose samples, were analyzed for parent compound and metabolites by HPLC. The 24-h cumulative percent of the dose of DBDPO in the receptor fluid was very low (0.07-0.34%). The applied dose of DBDPO detected in the skin ranged from 2 to 20%. The lowest dose of DBDPO had the highest percentage of the dose (20%) in the skin. The major portion of the applied dose was removed by washing the skin 24 h after application of DBDPO, and ranged from 77 to 92%. HPLC analysis of homogenate extract prepared from the high-dose of DBDPO-treated skin showed the presence of DBDPO and a minor unknown peak. TDCP was readily detected in the receptor fluid; 39-57% of the applied dose of TDCP was in the receptor fluid by 24 h. The solvent wash removed 11-25% of the dose from the skin and 28-35% remained in it. HPLC analysis of the skin homogenate extract and receptor fluid extract from the TDCP high-dose treated samples showed the presence of parent compound and a minor unknown peak. TDCP more readily penetrated hairless mouse skin and diffused into the receptor fluid than DBDPO. TDCP has a lower molecular weight and log octanol:water partition coefficient than DBDPO. The differences in the physico-chemical properties of these two chemicals most likely explains their dissimilar absorption through hairless mouse skin.

Bioconcentration of lipophilic organochlorines in ovine dentine

The bioconcentration of lipophilic organochlorines in ovine dentine in comparison to adipose tissue was examined. Sheep were given a single dose (0.2-1.4 mg/kg body wt) of individual polychlorobiphenyl (PCB) congeners (tetrachlorobiphenyls IUPAC Nos. 54 and 80, and hexachlorobiphenyls IUPAC Nos. 155 and 169) and organochlorine pesticides 1,1-bis(4-chlorophenyl)-2,2-dichloroethene) (4,4'-DDE) and hexachlorobenzene (HCB). They were killed 2 months after the dose and teeth and adipose tissue were collected. For the extraction of organochlorines, dentine was treated ultrasonically with sulfuric acid and hexane. The concentration of organochlorines in dentine and adipose tissue was determined by high-resolution gas chromatography with electron-capture detection. The bioconcentration of 'planar' compounds (PCB-169, HCB) in dentine, compared to adipose tissue, was lower than that of 'non-planar' (PCB-155, 4,4'-DDE). Ratios of the non-planar to planar compounds PCB-155/-169 and 4,4'-DDE/HCB in dentine were 1.4 and 7.2, and in adipose tissue 0.5 and 2.4, respectively. The dentine:adipose tissue bioconcentration ratios (on a lipid basis) of PCB-54, -80, -155, -169, 4,4'-DDE and HCB were 47, 16, 0.3, 0.1, 3.2 and 1.0, respectively. The results indicate that the bioconcentration of organochlorines in different tissues cannot be based on lipid content only. It is suggested that the physicochemical properties of individual organochlorines, i.e. lipophilicity (K(ow)), diffusivity, metabolism and tissue-specific interactions, play a part in the different bioconcentrations of individual organochlorine pollutants in dentine compared to adipose tissue.

Enhanced systemic tissue distribution after dermal versus intravenous 3,3',4,4'-tetrachlorobiphenyl exposure: limited utility of radiolabel blood area under the curve and excretion data in dermal absorption calculations and tissue exposure assessment

As a dioxin-like polychlorinated biphenyl (PCB), 3,3',4,4'-tetrachlorobiphenyl (TCB) is receiving increasing research and regulatory interest due to its high toxicity and persistence in the environment. (14)C-TCB was administered at an identical dose of 300 microg via the intravenous (iv) or dermal route to swine to examine the exposure route dependency of the relationship between tissue exposure and blood area under the curve (AUC) and the relationship between dermal absorption and excretion of radiolabel. After iv and dermal exposure, blood, urine, and feces samples were collected during the 11-day in vivo studies. At the end of the experiments, full mass balance studies were conducted to characterize tissue distribution of label. On average, over 70% of the applied dermal and iv doses were recovered. As expected, more than a 10-fold increase in blood AUC (0.49 vs 0.031, h x % dose/ml), plasma AUC (0.40 vs 0.038, h x % dose/ml), urine excretion (29 vs 2.3% of the applied dose), and fecal (30 vs 3.0% of the applied dose) excretion was determined after iv exposure compared to dermal exposure. However, we unexpectedly found that the tissue residue following iv exposure (8.0% of the applied dose) was only half that following dermal exposure (16% of the applied dose). Significantly larger (20- to 30-fold) ratios of blood AUC:tissue residue and excretion:tissue residue were observed after iv exposure compared to dermal exposure. This may indicate a route-related concentration-dependent blood-to-tissue partition process of pooled label, unique skin metabolism, or saturable hepatic metabolism of TCB. Thus, a long-term, low-input exposure pattern similar to this dermal exposure could be more harmful to systemic tissues than a short-term, high-dose exposure similar to this iv exposure. One should be aware that greater absorption, higher blood concentrations, greater blood and plasma AUCs, and greater excretion of label do not necessarily result in a greater overall tissue exposure and that some conventional approaches using label determination in blood and excreta without full mass balance studies may underestimate dermal absorption of chemicals similar to TCB.

Dermal absorption and tissue disposition of 3,3',4, 4'-tetrachlorobiphenyl (TCB) in an ex-vivo pig model: assessing the impact of dermal exposure variables

TCB is one of the dioxin-like polychlorinated biphenyls (PCBs). This research was designed to help assess the risk of occupational and environmental TCB exposure. To evaluate exposure variables' effects on dermal absorption and cutaneous disposition, 14C-TCB (40 microg/cm(2)) in acetone, methylene chloride, a water-acetone mixture, and a soil-based mixture were applied in an ex-vivo pig-skin-flap model (n = 4-5/treatment). Dermal absorption (0.11-0.66%, 8 hr) and penetration (1.14-2.48%) varied according to exposure conditions. Acetone and methylene chloride vehicles differed in absorption profiles and skin penetration patterns but were similar in absorption amounts. Adding water to the acetone did not change absorption but did alter the penetration pattern. The non-occluded soil-based mixture showed more absorption than did the liquid vehicles (p<0.05), but occlusion significantly (p<0.05) decreased that absorption (0.66-->0.29%, 8 hr) and penetration (2.48-->1.11%). In conclusion, dermal absorption data from liquid-organic or aqueousorganic mixtures may underestimate the risk of exposure to TCB-contaminated soil.