Impacts of hydraulic fracturing wastewater from oil and gas industries on drinking water: Quantification of 69 disinfection by-products and calculated toxicity.

Oil and gas production generates large amounts of brine wastewater called "produced water" with various geogenic and synthetic contaminants. These brines are generally used in hydraulic fracturing operations to stimulate production. They are characterized by elevated halide levels, particularly geogenic bromide and iodide. Such salt concentrations in produced water may be as high as thousands of mg/L of bromide and tens of mg/L of iodide. Large volumes of produced water are stored, transported, reused in production operations, and ultimately disposed of by deep well injection into saline aquifers. Improper disposal may potentially contaminate shallow freshwater aquifers and impact drinking water sources. Because conventional produced water treatment typically does not remove halides, produced water contamination of groundwater aquifers may cause the formation of brominated and iodinated disinfection by-products (I-DBPs) at municipal water treatment plants. These compounds are of interest because of their higher toxicity relative to their chlorinated counterparts. This study reports a comprehensive analysis of 69 regulated and priority unregulated DBPs in simulated drinking waters fortified with 1 % (v/v) oil and gas wastewater. Impacted waters produced 1.3×-5× higher levels of total DBPs compared to river water after chlorination and chloramination. Individual DBP levels ranged from (<0.1-122 µg/L). Overall, chlorinated waters formed highest levels, including trihalomethanes that would exceed the U.S. EPA regulatory limit of 80 µg/L. Chloraminated waters had more I-DBP formation and highest levels of haloacetamides (23 µg/L) in impacted water. Calculated cytotoxicity and genotoxicity were higher for impacted waters treated with chlorine and chloramine than corresponding treated river waters. Chloraminated impacted waters had the highest calculated cytotoxicity, likely due to higher levels of more toxic I-DBPs and haloacetamides. These findings demonstrate that oil and gas wastewater if discharged to surface waters could adversely impact downstream drinking water supplies and potentially affect public health.

Impact of workplace displacement during a natural disaster on computer performance metrics: A 2-year interrupted time series analysis.

BACKGROUND: Remote working may enhance company resiliency during natural disasters and other events causing workplace displacement. OBJECTIVE: We conducted an interrupted time series analysis to investigate the impact of Hurricane Harvey on employee computer use during and after a seven-month displacement period from the physical workplace. METHODS: Ergonomic software was used to collect information on employees' computer usage. RESULTS: Although there was no change in total computer use in response to the hurricane (ß 0.25), active computer use significantly declined (ß -0.90). All measured computer use behaviors returned to baseline prior to the complete return to the physical workspace. CONCLUSION: Despite a transient period of reduced activity during closure of the workplace building, productivity returned to normal prior to the employees' return to a commercial workspace. The ability to work remotely may improve resiliency of employees to perform workplace tasks during events causing workplace displacement.

Oxidation of antibiotics by ferrate(VI) in water: Evaluation of their removal efficiency and toxicity changes.

Antibiotics in water and wastewater have been determined extensively. The treatment of antibiotics in water needs evaluation of possible harmful effects on aquatic ecosystems and human health. This paper presents the toxicity evaluation of antibiotics after their treatment with ferrate (VI) (FeVIO42-, Fe(VI)) in water. The antibiotics (sulfamethoxazole (SMX), erythromycin (ERY), ofloxacin (OFL), ciprofloxacin (CIP), tetracycline (TET), oxytetracycline (OXY), and trimethoprim (TMP)) were treated at pH 8.0 by applying two concentrations of Fe(VI) to have molar ratios of 5:1 and 10:1 ([Fe(VI)]:[antibiotic]). Under the studied conditions, incomplete removal of antibiotics was observed, suggesting that the treated solutions contained parent antibiotics and their transformation products. The toxicity of antibiotics without Fe(VI) treatment was tested against freshwater green alga Raphidocelis subcapitata and cyanobacterium Synechococcus elongatus, which were determined to be generally sensitive to antibiotics, with EC50 < 1.0 mg/L. The toxicity of Fe(VI) treated solution was tested against R. subcapitata. Results found no toxicity for the treated solutions of OFL, CIP, and OXY. However, SMX, ERY, and TET remained toxic after Fe(VI) treatment (i.e., more than 75% growth inhibition of R. subcapitata). Results demonstrated that R. subcapitata may be applied to test the toxicity of antibiotics after oxidative treatments.

High-Resolution Mass Spectrometry Identification of Novel Surfactant-Derived Sulfur-Containing Disinfection Byproducts from Gas Extraction Wastewater.

Introduction of oil and gas extraction wastewaters (OGWs) to surface water leads to elevated halide levels from geogenic bromide and iodide, as well as enhanced formation of brominated and iodinated disinfection byproducts (DBPs) when treated. OGWs contain high levels of chemical additives used to optimize extraction activities, such as surfactants, which have the potential to serve as organic DBP precursors in OGW-impacted water sources. We report the first identification of olefin sulfonate surfactant-derived DBPs from laboratory-disinfected gas extraction wastewater. Over 300 sulfur-containing DBPs, with 43 unique molecular formulas, were found by high-resolution mass spectrometry, following bench-scale chlor(am)ination. DBPs consisted of mostly brominated species, including bromohydrin sulfonates, dihalo-bromosulfonates, and bromosultone sulfonates, with chlorinated/iodinated analogues formed to a lesser extent. Disinfection of a commercial C12-olefin sulfonate surfactant mixture revealed dodecene sulfonate as a likely precursor for most detected DBPs; disulfur-containing DBPs, like bromosultone sulfonate and bromohydrin disulfonate, originated from olefin disulfonate species, present as side-products of olefin sulfonate production. Disinfection of wastewaters increased mammalian cytotoxicity several orders of magnitude, with chloraminated water being more toxic. This finding is important to OGW-impacted source waters because drinking water plants with high-bromide source waters may switch to chloramination to meet DBP regulations.

Occurrence and toxicity of antibiotics in the aquatic environment: A review.

In recent years, antibiotics have been used for human and animal disease treatment, growth promotion, and prophylaxis, and their consumption is rising worldwide. Antibiotics are often not fully metabolized by the body and are released into the aquatic environment, where they may have negative effects on the non-target species. This review examines the recent researches on eight representative antibiotics (erythromycin, trimethoprim, sulfamethoxazole, tetracycline, oxytetracycline, ofloxacin, ciprofloxacin, and amoxicillin). A detailed overview of their concentrations in surface waters, groundwater, and effluents is provided, supported by recent global human consumption and veterinary use data. Furthermore, we review the ecotoxicity of these antibiotics towards different groups of organisms, and assessment of the environmental risks to aquatic organisms. This review discusses and compares the suitability of currently used ecotoxicological bioassays, and identifies the knowledge gaps and future challenges. The risk data indicate that selected antibiotics may pose a threat to aquatic environments. Cyanobacteria were the most sensitive organisms when using standard ecotoxicological bioassays. Further studies on their chronic effects to aquatic organisms and the toxicity of antibiotic mixtures are necessary to fully understand the hazards these antibiotics present.

Vulnerable Populations Exposed to Lead-Contaminated Drinking Water within Houston Ship Channel Communities.

Recent events have drawn increased attention to potential lead exposures from contaminated drinking water. Further, homes with older infrastructure are at greatest risk due to the presence of the disinfectant chemical chloramine, which can leach lead from older pipes. There is a growing need to determine the extent of lead leaching especially within vulnerable communities and homes with children. This pilot study collected survey data and performed lead analysis on drinking water in the small community of Manchester in Houston, TX. Manchester is characterized by industrial sites, flooding, and a low socioeconomic population. Surveys and water analyses were completed on randomly selected homes (N = 13) and documented perceptions of participants on their drinking water regarding presence and concentration of lead. Lead was discovered in 30.8% of homes ranging from 0.6 to 2.4 (µg/L), all below the US Environmental Protection Agency action level of 15 ppb, but above the water standard goals. These findings further suggest that contaminated water is a broad issue requiring concerted efforts to ensure the health of US residents.

Exploring Demographic and Environmental Factors Related to Unintentional Pesticide Poisonings in Children and Adolescents in Texas.

There is limited literature on the frequency and distribution of pesticide exposures, specifically with respect to demographic and environmental factors in the United States. The purpose of this exploratory study was to investigate geographic trends and factors associated with unintentional pesticide exposures in children and adolescents in Texas. The study used an ecological design with secondary data. A spatial scan statistic, based on a Poisson regression model, was employed to identify spatial clusters of unintentional pesticide-related poison center exposures. Next, logistic regression models were constructed to identify potential demographic and environmental factors associated with unintentional pesticide-related poison center exposures. There were 59,477 unintentional pesticide-related poison center exposures from 2000 to 2013. The spatial scan statistic found a change in the number of counties in the identified clusters (e.g. , aggregation of counties with higher than expected exposures) for two time periods (2000-2006; 2007-2013). Based on the logistic regression models, factors associated with unintentional pesticide-related poison center exposures were percent black or African American population, year structure built, and percent moved in the past 12 months. In conclusion, this study found certain demographic and environmental factors may be associated with unintentional pesticide-related poison center exposures. Through understanding trends and associated factors, public health professionals can design interventions for populations at higher risk of pesticide exposures. This study also supports the use of spatial methods being utilized to expand upon current analysis of poison center data. Future research should confirm and build upon these findings.

Occurrence, distribution and composition of aliphatic and polycyclic aromatic hydrocarbons in sediment cores from the Lower Fox River, Wisconsin, US.

The Lower Fox River is a 39 mile section which supports approximately 270,000 rural inhabitants across 18 counties, 303,000 metropolitan residents in Green Bay and Appleton, Wisconsin, and several large industrial complexes such as paper mills and power plants. This paper presents the distribution and concentrations of aliphatic (n-alkanes) and aromatic hydrocarbons (polycyclic aromatic hydrocarbons [PAHs]) as well as total organic carbon (TOC) in the Lower Fox River to identify the sources of hydrocarbon contamination. Excluding one outlier, percent TOC values were between 0.73 and 19.9% with an average value of 6.74%. Total n-alkanes ranged from 3.51 µg/g to 117 µg/g and showed a strong presence of odd carbon-numbered n-alkane ratios (range of C25 to C35), suggesting source input from terrestrial biomass. The mean polycyclic aromatic hydrocarbon (PAH) concentration was 24,800 ng/g. High molecular weight PAH concentrations dominated the distribution of hydrocarbon contaminants. Cross-plots of PAHs were used to compare diagnostic source ratios of benz[a]anthracene (BaA), chrysene (Chr), fluoranthene (Flu), pyrene (Pyr), anthracene (Ant), phenanthrene (Phe), indeno[1,2,3-cd]pyrene (InP), and benzo[g,h,i]perylene (BghiP) by depth and area. PAH ratios varied slightly with the core depth. Deeper core sections indicated the presence of biomass combustion while the upper core sections indicated combustion of both petroleum and biomass. The PAH toxicity of one core was estimated using toxicity equivalency factors, and the benzo[a]pyrene toxic equivalence quotient totaled 2,293 ng/g-dry wt. Levels of PAHs in sediments are compared with established regulatory values and recommendations are made.

Chlorine decay and trihalomethane formation following ferrate(VI) preoxidation and chlorination of drinking water.

This paper presents the effect of preoxidation with ferrate(VI) (FeVIO42-, Fe(VI)) prior to chlorination on chlorine decay and formation of disinfection by-products in filtered raw water from a full-scale drinking water treatment plant. The rate of chlorine decay became significantly faster as the concentration of ferrate(VI) increased. Chlorine degradation followed two first-order decay reactions with rate constants k1 and k2 for fast and slow decay, respectively. Kinetic modeling established the relationships between k1 and k2 and varying dosages of chlorine and ferrate(VI). When ferrate(VI) was used as a pre-oxidant, the levels of trihalomethanes (trichloromethane (TCM), dichlorobromomethane (DCBM), dibromochloromethane (DBCM), and tribromomethane (TBM)) in water samples decreased as the ferrate(VI) concentration increased. The concentrations of these trihalomethanes followed the order TCM > DCBM ≈ DBCM > TBM.

Synergistic effect of aqueous removal of fluoroquinolones by a combined use of peroxymonosulfate and ferrate(VI).

This paper examines for the first time the combined use of ferrate(VI) (FeVIO42-, Fe(VI)) with peroxymonosulfate (PMS) to oxidize four fluoroquinolones (FQs) (flumequine (FLU), enrofloxacin (ENR), marbofloxacin (MAR), and ofloxacin (OFL)) at pH 7.0 and 25.0 °C. The results demonstrate that a combination of PMS and Fe(VI) synergistically enhance removal of FQs compared to the removal seen with either PMS or Fe(VI) alone. The oxidized products (OPs) of the reaction of FLU with the PMS-Fe(VI) system, identified by liquid chromatography high-resolution mass spectrometry (LC-HRMS) techniques, showed transformation products resulted from hydroxylation, ring cleavage, and de-fluorination processes. Thus, the combination of PMS and Fe(VI) can be developed as an innovative oxidative technology to remove contaminants from water.

Chloramines in a pilot-scale water distribution system: Transformation of 17ß-estradiol and formation of disinfection byproducts.

The degradation and transformation products of 17ß-estradiol (E2) by chloramines in a pilot-scale water distribution system (WDS) were investigated using varying conditions including multiple mass ratios of chlorine to nitrogen (Cl/N), changing concentrations of chloramines, and different pH and pipe materials. The degradation of E2 was complete in ≤9 h in both deionized water (DW) and in the WDS under studied conditions. When the degradation rate of E2 was compared in WDS and DW, the degradation rate was appreciably greater in the WDS than in the DW at Cl/N mass ratios of 3, 4 and 6. However, at Cl/N mass ratios of 8 and 9, degradation was faster in the DW than in the WDS. The degradation rate of E2 was greatly affected by the initial total chloramine concentration, and the degradation of E2 in DW was consistent with second-order kinetics. The degradation rate of E2 in both the DW and the WDS increased with increasing pH. The order of degradation rate of E2in different pipes was: ductile iron loop (loop A) > polyethylene (PE) loop (loop B)> stainless steel loop (loop C). Ten specific degradation products of E2, produced by chloramination, were identified. Most of the degradation products of E2 chloramination were stable for more than 10 h. The degradation pathways of E2 in the WDS are proposed and briefly discussed. The concentrations of trihalomethanes (THMs), haloacetic acids (HAAs), and halogenated nitromethane (HNMs) during the degradation E2 in WDS were also determined.

Pesticide-Related Hospitalizations Among Children and Teenagers in Texas, 2004-2013.

OBJECTIVE: Acute exposure to pesticides is associated with nausea, headaches, rashes, eye irritation, seizures, and, in severe cases, death. We characterized pesticide-related hospitalizations in Texas among children and teenagers for 2004-2013 to characterize exposures in this population, which is less well understood than pesticide exposure among adults. METHODS: We abstracted information on pesticide-related hospitalizations from hospitalization data using pesticide-related International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes and E-codes. We calculated the prevalence of pesticide-related hospitalizations among children and teenagers aged #19 years for all hospitalizations, unintentional exposures, intentional exposures, pesticide classifications, and illness severity. We also calculated age- and sex-specific prevalence of pesticide-related hospitalizations among children. RESULTS: The prevalence of pesticide-related hospitalizations among children and teenagers was 2.1 per 100,000 population. The prevalence of pesticide-related hospitalizations per 100,000 population was 2.7 for boys and 1.5 for girls. The age-specific prevalence per 100,000 population was 5.3 for children aged 0-4 years, 0.3 for children and teenagers aged 5-14 years, and 2.3 for teenagers aged 15-19 years. Children aged 0-4 years had the highest prevalence of unintentional exposures, whereas teenagers aged 15-19 years had the highest prevalence of intentional exposures. Commonly reported pesticide categories were organophosphates/carbamates, disinfectants, rodenticides, and other pesticides (e.g., pyrethrins, pyrethroids). Of the 158 pesticide-related hospitalizations, most were coded as having minor (n=86) or moderate (n=40) illness severity. CONCLUSION: Characterizing the prevalence of pesticide-related hospitalizations among children and teenagers leads to a better understanding of the burden of pesticide exposures, including the type of pesticides used and the severity of potential health effects. This study found differences in the frequency of pesticide-related hospitalizations by sex, age, and intent (e.g., unintentional vs. intentional).

Degradation of fluoroquinolone antibiotics by ferrate(VI): Effects of water constituents and oxidized products.

The degradation of five fluoroquinolone (FQ) antibiotics (flumequine (FLU), enrofloxacin (ENR), norfloxacin (NOR), ofloxacin (OFL) and marbofloxacin (MAR)) by ferrate(VI) (Fe(VI)O4(2-), Fe(VI)) was examined to demonstrate the potential of this iron-based chemical oxidant to treat antibiotics in water. Experiments were conducted at different molar ratios of Fe(VI) to FQs at pH 7.0. All FQs, except FLU, were degraded within 2 min at [Fe(VI)]:[FQ] ≤ 20.0. Multiple additions of Fe(VI) improved the degradation efficiency, and provided greater degradation than a single addition of Fe(VI). The effects of anions, cations, and humic acid (HA), usually present in source waters and wastewaters, on the removal of FLU were investigated. Anions (Cl(-), SO4(2-), NO3(-), and HCO3(-)) and monovalent cations (Na(+) and K(+)) had no influence on the removal of FLU. However, multivalent cations (Ca(2+), Mg(2+), Cu(2+), and Fe(3+)) in water decreased the efficiency of FLU removal by Fe(VI). An increase in the ionic strength of the solution, and the presence of HA in the water, also decreased the percentage of FLU removed by Fe(VI). Experiments on the removal of selected FQs, present as co-existing antibiotics in pure water, river water, synthetic water and wastewater, were also conducted to demonstrate the practical application of Fe(VI) to remove the antibiotics during water treatment. The seventeen oxidized products (OPs) of FLU were identified using solid phase extraction-liquid chromatography-high-resolution mass spectrometry. The reaction pathways are proposed, and are theoretically confirmed by molecular orbital calculations.

Pesticide-related poison center exposures in children and adolescents aged ≤19 years in Texas, 2000-2013.

CONTEXT: Although national poison center data show that pesticides were the 8th most commonly reported substance category (3.27%) for children aged ≤5 years in 2014, there is limited information on childhood and adolescent pesticide exposures. OBJECTIVE: This study assessed pesticide-related poison center exposures in children and adolescents aged ≤19 years from 2000-2013 in Texas to characterize the potential burden of pesticides. MATERIALS AND METHODS: Pesticide-related poison center exposures among children and adolescents aged ≤19 years reported to Texas poison centers were identified. The distribution of exposures was estimated by gender, age category, medical outcome, management site, exposure route, and pesticide category. RESULTS: From 2000 to 2013, there were 61,147 pesticide-related poison center exposures in children and adolescents aged ≤19 years. The prevalence was highest among males at 864.24 per 100,000 population. The prevalence of unintentional exposures was highest among children aged ≤5 years at 2310.69 per 100,000 population, whereas the prevalence of intentional exposures was highest among adolescents aged 13-19 years at 13.82 per 100,000 population. A majority of medical outcomes reported were classified as having no effect (30.24%) and not followed, but minimal clinical effects possible (42.74%). Of all the exposures, 81.24% were managed on site. However, 57% of intentional exposures were referred to or treated at a health-care facility. The most common routes of exposure were ingestion (80.83%) and dermal (17.21%). The most common pesticide categories included rodenticides (30.02%), pyrethrins/pyrethroids (20.69%), and other and unspecified insecticides (18.14%). DISCUSSION: The study found differences in the frequency of exposures by intent for sex and age categories, and identified the most common medical outcomes, management site, exposure route, and pesticide category. CONCLUSION: Through characterizing pesticide-related poison center exposures, future interventions can be designed to address groups with higher prevalence of exposure.

A review of the influence of treatment strategies on antibiotic resistant bacteria and antibiotic resistance genes.

Antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARG) in the aquatic environment have become an emerging contaminant issue, which has implications for human and ecological health. This review begins with an introduction to the occurrence of ARB and ARG in different environmental systems such as natural environments and drinking water resources. For example, ARG or ARB with resistance to ciprofloxacin, sulfamethoxazole, trimethoprim, quinolone, vancomycin, or tetracycline (e.g., tet(A), tet(B), tet(C), tet(G), tet(O), tet(M), tet(W), sul I, and sul II) have been detected in the environment. The development of resistance may be intrinsic, may be acquired through spontaneous mutations (de novo), or may occur due to horizontal gene transfer from donor bacteria, phages, or free DNA to recipient bacteria. An overview is also provided of the current knowledge regarding inactivation of ARB and ARG, and the mechanism of the effects of different disinfection processes in water and wastewater (chlorination, UV irradiation, Fenton reaction, ozonation, and photocatalytic oxidation). The effects of constructed wetlands and nanotechnology on ARB and ARG are also summarized.

A Pilot Study of Changes in Environmental Knowledge and Behaviors among Head Start Employees and Parents Following Environmental Health Training in Webb County, TX.

Head Start centers in Webb County, Texas primarily serve low-income Hispanic families disproportionately affected by environmental exposures. A total of 560 parents and employees attended environmental trainings. Pre- and post-assessments measured whether the trainings were effective at improving related knowledge and behaviors. A total of 152 parents and 94 employees signed consent forms. Only the 64 parents and 50 employees who completed all questionnaires were included in the data analysis. Paired t tests and McNemar tests found significant improvements in knowledge and behaviors related to multiple environmental topics (p < 0.05). Mean scores out of eleven for knowledge before and immediately after were 9.69 (95 % CI 9.44, 9.94) and 10.58 (95 % CI 10.42, 10.74), respectively. Mean scores out of ten for behavior before and 1 month after training were 8.00 (95 % CI 7.71, 8.29) and 9.29 (95 % CI 9.10, 9.48), respectively. This pilot study found improved knowledge and behaviors following environmental health training.

Pharmaceuticals and personal care products in waters: occurrence, toxicity, and risk.

Pharmaceuticals and personal care products (PPCP) are compounds with special physical and chemical properties that address the care of animal and human health. PPCP have been detected in surface water and wastewater in the ng/L to µg/L concentration range worldwide. PPCP ecotoxicity has been studied in a variety of organisms, and multiple methods have been used to assess the risk of PPCP in the environment to ecological health. Here we review the occurrence, effects, and risk assessment of PPCP in aquatic systems, as well as the sustainability of current methods for managing PPCP contamination in aquatic systems. The major points are the following: (1) a number of PPCP present potential concerns at environmentally relevant concentrations. PPCP mixtures may produce synergistic toxicity. (2) Various methods have been used for the ecological risk assessment of PPCP in aquatic systems. There are similarities in these methods, but no consensus has emerged regarding best practices for the ecological risk assessment of these compounds. (3) Human health risk assessments of PPCP contamination in aquatic systems have generally indicated little cause for concern. However, there is a lack of information regarding whether antibiotic contamination in wastewater and aquatic systems could lead to an increase in clinically relevant antibiotic-resistant bacteria and antibiotic-resistant genes. (4) Over the next century, the combination of increasing global population size and potential droughts may result in reduced water availability, increased need for water reuse, and increasing concentrations of PPCP in wastewaters. The current wastewater treatment methods do not remove all PPCP effectively. This, coupled with the possibility that antibiotics may promote the development of antibiotic-resistant bacteria and antibiotic-resistant genes, leads to concerns about the sustainability of global water supplies.

Mechanistic relationships between hepatic genotoxicity and carcinogenicity in male B6C3F1 mice treated with polycyclic aromatic hydrocarbon mixtures.

The genotoxicity of a complex mixture [neutral fraction (NF)] from a wood preserving waste and reconstituted mixture (RM) mimicking the NF with seven major polycyclic aromatic hydrocarbons (PAHs) and benzo(a)pyrene (BaP) was investigated by determining DNA adducts and tumor incidence in male B6C3F1 mice exposed to three different doses of the chemical mixtures. The peak values of DNA adducts were observed after 24 h, and the highest levels of PAH-DNA adducts were exhibited in mice administered NF + BaP, and the highest tumor incidence and mortality were also observed in this group. DNA adduct levels after 1, 7, or 21 days were significantly correlated with animal mortality and incidence of total tumors including liver, lung, and forestomach. However, only hepatic DNA adducts after 7 days significantly correlated with liver tumor incidence. Most proteins involved in DNA repair including ATM, pATR, Chk1, pChk1, DNA PKcs, XRCC1, FANCD2, Ku80, Mre11, and Brca2 were significantly lower in liver tumor tissue compared to non-tumor tissue. Expressions of proteins involved in apoptosis and cell cycle regulation were also significantly different in tumor versus non-tumor tissues, and it is possible that PAH-induced changes in these gene products are important for tumor development and growth.

A comparison of concentrations of polycyclic aromatic compounds detected in dust samples from various regions of the world.

Settled house dust can be a source of human exposure to toxic polycyclic aromatic hydrocarbons (PAHs) through non-dietary ingestion and dermal contact. Information regarding the concentrations of various contaminants in house dust would be useful in estimating the risk associated with exposure to these compounds. This study reports on the surface loading, variability and distribution of PAHs in settled house dust collected from homes in three locations: Sumgayit, Azerbaijan; Shanxi Province, China; and southern Texas, United States. The highest PAH floor surface loadings were observed in China, followed by Azerbaijan and Texas. Median concentrations of high molecular weight (four ring and larger) PAHs ranged from a low of 0.11 microg/m(2) in Texas, to 2.9 microg/m(2) in Azerbaijan and 162 microg/m(2) in China. These trends in total surface loading and relative carcinogenicity indicate that the risk of health effects from exposure to PAHs in house dust is highest in the Chinese population and lowest in the Texas population. As anticipated, variability among dust samples from different houses within the same region was high, with coefficients of variation greater than 100%. Alkylated PAHs comprised 30-50% of the total mass of PAHs. Based on a comparison of the composition of specific components, PAHs in China and Azerbaijan were determined to be derived mainly from combustion sources rather than from unburned fossil fuels such as petroleum. These results, coupled with ongoing investigation of appropriate PAH exposure biomarkers in humans, will guide future efforts to identify ways to reduce exposures in the study areas.