What proportion of household dust is derived from outdoor soil?

An on-site inspection system for explosives in a container using dust collection through a vent cover and ion mobility spectrometric detection

Rapid and accurate cargo-inspection systems are required for shipping containers, because illegal and hazardous items, such as explosives and drugs, can be easily concealed in large containers. Dust in a container is suspended in the air and deposited in vent covers. The vapor and particulate matter of explosives can be adsorbed onto the dust. In this study, a model vent cover system was developed for a container and explosive-adsorbed dust specimens were modeled using organic [cotton fabric (CF) and sawdust (SD)] and inorganic substances [clay (CL) and silica (SL)]. The dust was placed in the inner part of the vent cover and collected through the vent holes, and the explosive components present in the dust were rapidly analyzed using ion mobility spectrometry (IMS). Three explosives, namely 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), and pentaerythritol tetranitrate (PETN), were investigated along with two different collection filters, namely poly(tetrafluoroethylene) (PTFE) and lens cleansing paper (LCP). The dust collection rate decreased when the dust was located away from the vent holes. The order of the dust collection rate was SD > CF > SL > CL. The limit of detection (LOD) of TNT was considerably lower than those of RDX and PETN. The LODs of the explosives when using the LCP collection filter were lower than those when using the PTFE filter. The LOD order was CF < SD < CL < SL for the PTFE collection filter and CL < SL < CF < SD for the LCP filter. The technique developed in this study can be employed as a rapid and accurate cargo inspection system for shipping containers.

House Sparrows as Sentinels of Childhood Lead Exposure

Our understanding of connections between human and animal health has advanced substantially since the canary was introduced as a sentinel of toxic conditions in coal mines. Nonetheless, the development of wildlife sentinels for monitoring human exposure to toxins has been limited. Here, we capitalized on a three-decade long child blood lead monitoring program to demonstrate that the globally ubiquitous and human commensal house sparrow (Passer domesticus) can be used as a sentinel of human health risks in urban environments impacted by lead mining. We showed that sparrows are a viable proxy for the measurement of blood lead levels in children at a neighborhood scale (0.28 km2). In support of the generalizability of this approach, the blood lead relationship established in our focal mining city enabled us to accurately predict elevated blood lead levels in children from another mining city using only sparrows from the second location. Using lead concentrations and lead isotopic compositions from environmental and biological matrices, we identified shared sources and pathways of lead exposure in sparrows and children, with strong links to contamination from local mining emissions. Our findings showed how human commensal species can be used to identify and predict human health risks over time and space.

Human Exposure to Pesticides in Dust from Two Agricultural Sites in South Africa

Over the last decades, concern has arisen worldwide about the negative impacts of pesticides on the environment and human health. Exposure via dust ingestion is important for many chemicals but poorly characterized for pesticides, particularly in Africa. We investigated the spatial and temporal variations of 30 pesticides in dust and estimated the human exposure via dust ingestion, which was compared to inhalation and soil ingestion. Indoor dust samples were collected from thirty-eight households and two schools located in two agricultural regions in South Africa and were analyzed using high-performance liquid chromatography coupled to tandem mass spectrometry. We found 10 pesticides in dust, with chlorpyrifos, terbuthylazine, carbaryl, diazinon, carbendazim, and tebuconazole quantified in >50% of the samples. Over seven days, no significant temporal variations in the dust levels of individual pesticides were found. Significant spatial variations were observed for some pesticides, highlighting the importance of proximity to agricultural fields or of indoor pesticide use. For five out of the nineteen pesticides quantified in dust, air, or soil (i.e., carbendazim, chlorpyrifos, diazinon, diuron and propiconazole), human intake via dust ingestion was important (>10%) compared to inhalation or soil ingestion. Dust ingestion should therefore be considered in future human exposure assessment to pesticides.

Key considerations for assessing soil ingestion exposures among agricultural workers

BACKGROUND

Soil ingestion is a critical, yet poorly characterized route of exposure to contaminants, particularly for agricultural workers who have frequent, direct contact with soil.

OBJECTIVE

This qualitative investigation aims to identify and characterize key considerations for translating agricultural workers' soil ingestion experiences into recommendations to improve traditional exposure science tools for estimating soil ingestion.

METHODS

We conducted qualitative in-depth interviews with 16 fruit and vegetable growers in Maryland to characterize their behaviors and concerns regarding soil contact in order to characterize the nature of soil ingestion in the agricultural context.

RESULTS

We identified and discussed four emergent themes: (1) variability in growers' descriptions of soil and dust, (2) variability in growers' soil contact, (3) growers' concerns regarding soil contact, (4) growers' practices to modify soil contact. We also identified environmental and behavioral factors and six specific agricultural tasks that may impact soil ingestion rates.

SIGNIFICANCE

Our investigation fills an important gap in occupational exposure science methodology by providing four key considerations that should be integrated into indirect measurement tools for estimating soil ingestion rates in the agricultural context. Specifically, a task-based framework may provide a structure for future investigations of soil contact that may be useful in other populations.

Spatial distribution and composition of mine dispersed trace metals in residential soil and house dust: Implications for exposure assessment and human health

Trace metal exposure from environmental sources remains a persistent global problem, particularly in communities residing adjacent to metal extraction and processing industries. This study examines front yard soil and house dust from 62 residences throughout the Australian Ag-Pb-Zn mining city of Broken Hill to better understand spatial variability in metal distributions, compositions and exposures across an industrially polluted urban environment. X-ray fluorescence analysis of paired soil/dust samples indicated that geomean concentrations (mg/kg) of Cu (32/113), Zn (996/1852), As (24/34) and Pb (408/587) were higher in house dust while Ti (4239/3660) and Mn (1895/1101) were higher in outdoor soil. Ore associated metals and metalloids (Mn, Zn, As, Pb) in soil and house dust were positively correlated and declined in concentration away from mining areas, the primary source of metalliferous emissions in Broken Hill. The rate of decline was not equivalent between soil and house dust, with the indoor/outdoor concentration ratio increasing with distance from mining areas for Zn/Pb (geomean = 1.25/1.05 (<1 km); 2.14/1.52 (1-2 km); 2.54/2.04 (>2 km)). House dust and Broken Hill ore Pb isotopic compositions (²⁰⁶Pb/²⁰⁷Pb; ²⁰⁸Pb/²⁰⁷Pb) were more similar in homes nearest to mining areas than those further away (geomean apportioned ore Pb = 88% (<1 km); 76% (1-2 km); 66% (>2 km)), reflecting spatial shifts in the balance of sources contributing to indoor contamination. Incorporation of house dust Pb reduced overestimation of IEUBK modelled blood Pb concentrations compared to when only soil Pb was used. These findings demonstrate that even in contexts where the source and environmental burden of metals are relatively apparent, geochemical relationships and exposures between outdoor and indoor environments are not always predictable, nor easily disaggregated.

The role of soil and house dust physicochemical properties in determining the post ingestion bioaccessibility of sorbed polychlorinated biphenyls

Ingestion of soils and house dusts is an important pathway for children's exposure to sorbed organic pollutants such as polychlorinated biphenyls (PCBs). To reduce the uncertainty of the exposure estimates, it is important to understand the extent to which chemicals desorb and become bioaccessible following ingestion. In this study we use a three compartment in vitro digestive system to model the role of soil and house dust physicochemical properties on the post ingestion bioaccessibility of PCBs. Matched pairs (n = 37) of soil and dust were characterized for percent carbon and nitrogen, pH, moisture content, and particle size distribution. They were then fortified with a mixture of 18 PCBs and processed through the assay. The percent bioaccessibility of each PCB was calculated, then modeled using individual PCB log K_ow values and the soil and dust properties. The bioaccessibility of the PCBs in soil (x̄ = 65 ± 16%) was greater (p < 0.001) than that of the PCBs in house dust (x̄ = 36 ± 14%). In the soil model, carbon was the sole statistically significant predictive (p ≤ 0.05) variable, while in house dust, both carbon and clay content were statistically significant (p ≤ 0.05) predictors.

The Critical Role of House Dust in Understanding the Hazards Posed by Contaminated Soils

The health risks posed by soil pollutants are generally thought to be due to soilingestion and have often resulted in massive regulatory efforts to remedy such contamination. The contribution of this route to the actual human health hazard has been questioned, however, as soil removal alone seems to have little influence on the body burdens of soil contaminants in exposed individuals. Ongoing research also has repeatedly and substantially reduced the estimates of soilingested daily. Because comparatively little time is spent outdoors by most individuals, exposure to soil brought indoors, present as house dust, is now thought to be nearly as important as the directingestion of soil. Exposure via house dust has not been studied specifically, but several observations suggest that it may be important. Dust is largely composed of fine particles of tracked-in soil. The smaller dust particles cling to surfaces better than soil, and contaminant concentrations are often higher in house dust. Fine particles are likely to be more bioavailable, and degradation is slower indoors. Contaminants thus may be concentrated and more readily available in the areas most frequented. In some studies, contaminant levels in dust are correlated more closely with body burdens of contaminants than other sources, suggesting that this route should be considered when assessing risks from soil. Until more research addressing exposure to dust is conducted, recommendations for assessing potential health risks from this pathway are provided.

Using paired soil and house dust samples in an in vitro assay to assess the post ingestion bioaccessibility of sorbed fipronil

For children, ingestion of soils and house dusts can be an important exposure pathway for regulated organic compounds. Following ingestion, the extent to which compounds desorb and become bioaccessible is a critical determinant of systemic adsorption. We characterized the physicochemical properties of 37 soil and house dust pairs collected during a national survey of United States homes. For each sample, we measured the bioaccessibility of fipronil, a phenylpyrazole insecticide using an in vitro, three- compartment digestive system, then modeled the physicochemical predictors of fipronil bioaccessibility. The properties of the soils and dusts were not correlated and percent carbon was the only significant predictor of bioaccessibility for both soils (p<0.001) and dusts (p<0.001). The carbon content of the soils (3.1±2.4%) was lower than that of the dusts (18.6±6.9%) Due to the lower carbon content, soil sorbed fipronil was more bioaccessible than dust sorbed fipronil. However, the slope of the bioaccessibility carbon regression line was steeper for the soils than for the house dusts. This suggested that, for soils having carbon percentages greater than those in this study, fipronil bioaccessibility may be less than that of house dusts having equal carbon content.

Potential sources and racial disparities in the residential distribution of soil arsenic and lead among pregnant women

Exposure to arsenic (As) or lead (Pb) has been associated with adverse health outcomes, and high-risk populations can be disproportionately exposed to these metals in soils. The objectives of this study were: to examine if predicted soil As and Pb concentrations at maternal residences of South Carolina (SC) low-income mothers differed based on maternal race (non-Hispanic black versus white), to examine whether differences in predicted residential soil As and Pb concentrations among black and white mothers differed by socioeconomic status (SES), and to examine whether such disparities persisted after controlling for anthropogenic sources of these metals, including direction from, and distance to industrial facilities. Kriged soil As and Pb concentrations were estimated at maternal residences in 11 locations in SC, and models with maternal race and individual and US Census block group level SES measures were examined. US Environmental Protection Agency Toxics Release Inventory (TRI) facility As and Pb releases categorized by distance and direction to block groups in which mothers resided were also identified, as were proxy measures for historic use of leaded gasoline (road density) and Pb-based paint (categories of median year home built by US Census block group). Consistent racial disparities were observed for predicted residential soil As and Pb concentrations, and the disparity was stronger for Pb than As (betas from adjusted models for black mothers were 0.12 and 2.2 for As and Pb, respectively, all p<0.006). Higher road density and older homes in block groups were more closely associated with higher predicted soil As and Pb concentrations than on-site releases of As and Pb categorized by facility location. These findings suggest that non-Hispanic black mothers in this study population had elevated residential As and Pb soil concentrations, after adjusting for SES, and that soil As and Pb concentrations were not associated with recent industrial releases.

Investigating relationships between biomarkers of exposure and environmental copper and manganese levels in house dusts from a Portuguese industrial city

This study reports on data obtained from a pilot survey focusing on house dust and toenail metal(loids) concentrations in residents living in the industrial city of Estarreja. The study design hereby described aims at investigating relationships between human toenails and both copper and manganese levels in settled house dusts. A total of 21 households and 30 individuals were recruited for the pilot study: 19 households corresponding to 27 residents living near the industrial complex, forming the exposed group, plus 2 households and 3 residents from residential areas with no anticipated environmental contaminants that were used for comparison. Factorial analysis was used for source identification purposes. Investigation on the potential influence of environmental factors over copper and manganese levels in the toenails was carried out via questionnaire data and multiple correspondence analysis. The results show that copper concentrations are more elevated in the indoor dusts, while manganese concentrations are more elevated in the outdoor dust samples. The geometrical relationships in the datasets suggest that the backyard soil is a probable source of manganese to the indoor dust. Copper and manganese contents in the toenail clippings are more elevated in children than in adults, but the difference between the two age groups is not statistically significant (p > 0.05). Investigation of environmental factors influencing the exposure-biomarker association indicates a probable relationship between manganese contents in indoor dust and manganese levels in toenail clippings, a result that is partially supported by the bioaccessibility estimates. However, for copper, no relationship was found between indoor dusts and the biomarkers of exposure.

A review of soil and dust ingestion studies for children

Soil and dust ingestion by children may be important pathways of exposure to environmental contaminants. Contaminated soil and dust may end up on children's hands and objects, because they play close to the ground. These contaminants can be ingested by children, because they have a tendency to place objects, including their fingers, in their mouths. Assessing exposure through this pathway requires information about the amount of soil and dust ingested by children. Estimates of soil and dust ingestion and information on the prevalence of the behavior have been published in the literature, but research in this area is generally limited. Three methodologies have been used to quantify soil and dust ingestion rates. In this paper, these are referred to as the tracer element method, the biokinetic model comparison method, and the activity pattern method. This paper discusses the information available on the prevalence of soil and dust ingestion behavior, summarizes the three methodologies for quantifying soil and dust ingestion, and discusses their limitations. Soil ingestion data derived from studies that use these methodologies are also summarized. Although they are based on different estimation approaches, the central tendency estimates of soil and dust ingestion derived from the three methodologies are generally comparable.

Bioaccessibility of pesticides and polychlorinated biphenyls from house dust: in-vitro methods and human exposure assessment

Semi-volatile chemicals like pesticides and polychlorinated biphenyls (PCB) tend to accumulate in house dust. This may result in residues of some parts per million (p.p.m.), closely associated with health impairments and diseases like cancer. To explain these associations, we must establish whether a relevant absorption from house dust into human organisms occurs, and most crucially the release of chemicals, that is, their bioaccessibility. Digestive as well as dermal bioaccessibilities were examined using in-vitro methods. On average, the digestive bioaccessibility was ~40% for the pesticides and ~60% for the PCB. The dermal penetration availability reached ~60% for the pesticides and ~70% for the PCB (percentages of the concentrations in the dust). Based on the bioaccessibility, an estimate of internal exposure was calculated and expressed as percentages of acceptable or tolerable daily intake (ADI/TDI) values. Exposure via the respiratory tract proved to be very low. Exposure via the digestive tract had maximum values of 4% for pesticides and 12% for PCB. Dermal exposure was much higher. Even for average concentrations in house dust (≈0.5 p.p.m.), children exposed to DDT and PCB showed up to 300% of the ADI/TDI values, and adults about 60%. With high concentrations of contaminants in house dust, the maximum doses absorbed through the skin reached 5000%.

A pilot study to assess the feasibility of using naturally-occurring radionuclides as mass balance tracers to estimate soil ingestion

The relatively few soil ingestion studies that have been conducted to date to support soil ingestion rate values used for contaminated site human health risk assessments (HHRAs) typically have measured mass balance elemental tracers (e.g., Al, Si, Ba, Ce, Mn, Ti, V, Zr), found in soil to estimate soil ingestion. This pilot study, involving a canine subject fed a known amount of tracer on a daily basis, assessed the use of alternative mass balance tracers, specifically naturally occurring radionuclides of the (238)U and (232)Th decay series, to estimate soil ingestion. A novel method of estimating soil ingestion via difference in isotopic ratios between the two decay series in food and soil was also assessed. The results of the study showed that the mean (214)Pb and (212)Pb activities measured in fecal samples were greater than what was contained in the soil inoculant, suggesting that the tracers were not being significantly absorbed in the GI tract. The mean daily soil ingestion rates, calculated after subtracting the contribution of tracers in the soil inoculant, were 3.9 g d(-1) (standard deviation 3.6 g d(-1)) for the isotope tracers, and 1.9 g(-1) (standard deviation 2.1 g d(-1)) for the 3 most reliable elemental tracers. The differences were not statistically significant and further evaluation of isotopic tracers for soil ingestion studies is warranted. Similarly, soil ingestion estimates calculated using the Isotope Ratio Method were not significantly different than when calculated using (212)Pb; however, the Isotope Ratio Method was observed to positively bias the soil ingestion estimates by approximately 50%.

Meta-analysis of mass-balance studies of soil ingestion in children

Ingestion of soil by young children may be an important source of exposure to environmental contaminants. Estimates of soil ingestion have been made by several studies using trace elements in a mass-balance approach, but differ substantially between studies and trace elements. We conduct a meta-analysis of four major mass-balance soil ingestion studies conducted on children between one and seven in the summer/fall in the northern United States. The analysis takes advantage of primary data from all studies, and provides a more complete description of soil ingestion among children. The meta-analysis uses data based on the two most reliable trace elements, Al and Si, that have passed a screening to identify and exclude measures with a high likelihood of bias. Details are described in a companion paper. The best linear unbiased predictor is used in a mixed model to estimate soil ingestion for study subjects. Overall, 11% of subject-periods are identified as outliers and excluded from the analysis. An analysis on 216 children based on Al and Si as tracer elements indicates that the mean (median) soil ingestion is 26 mg/day (33 mg/day), with the 95th percentile estimated as 79 mg/day. This systematic approach provides more reliable estimates than individual study results. There is some evidence that soil ingestion increases with a child's age, but insufficient data to distinguish soil ingestion by gender.

Potential for childhood lead poisoning in the inner cities of Australia due to exposure to lead in soil dust

This article presents evidence demonstrating that the historical use of leaded gasoline and lead (Pb) in exterior paints in Australia has contaminated urban soils in the older inner suburbs of large cities such as Sydney and Melbourne. While significant attention has been focused on Pb poisoning in mining and smelting towns in Australia, relatively little research has focused on exposure to Pb originating from inner-city soil dust and its potential for childhood Pb exposures. Due to a lack of systematic blood lead (PbB) screening and geochemical soil Pb mapping in the inner cities of Australia, the risks from environmental Pb exposure remain unconstrained within urban population centres.

Identifying soil cleanup criteria for dioxins in urban residential soils: how have 20 years of research and risk assessment experience affected the analysis?

This article reviews the scientific evidence and methodologies that have been used to assess the risks posed by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and presents a probabilistic analysis for identifying virtually safe concentrations of TCDD toxicity equivalents (TEQ) in residential soils. Updated data distributions that consider state-of-the-science cancer and noncancer toxicity criteria, child soil ingestion and dermal uptake, bioavailability in soil, and residential exposure duration are incorporated. The probabilistic analysis shows that the most sensitive determinants of dose and risk are childhood soil ingestion, exposure duration, and the selected TCDD cancer potency factor. It also shows that the cancer risk at 1 per 100,000 predicted more conservative (lower) soil criteria values than did the noncancer hazard (e.g., developmental and reproductive effects). In this analysis, acceptable or tolerable soil dioxin concentrations (TCDD TEQ) ranged from 0.4 to 5.5 ppb at the 95th percentile for cancer potency factors from 9600 to 156,000 (mg/kg/d)(-1) with site-specific adjustments not included. Various possible soil guidelines based on cancer and noncancer risks are presented and discussed. In the main, the current toxicology, epidemiology, and exposure assessment data indicate that the historical 1 ppb TEQ soil guidance value remains a reasonable screening value for most residential sites. This analysis provides risk managers with a thorough and transparent methodology, as well as a comprehensive information base, for making informed decisions about selecting soil cleanup values for PCDD/Fs in urban residential settings.

Distribution of pesticide residues within homes in central New York State

Residues for 17 pesticides were analyzed in 41 households in central New York State that represented farm, rural, and urban houses. Samples were taken in both summer and winter of 2000-2001 from the same households from four locations; family room carpet; adjacent smooth floor; flat tabletop surface; and settled dust collected in a Petri dish on a tabletop. Pesticide residues were analyzed to identity factors that influence both the transport into and the redistribution of pesticides in the indoor environment. Differences were observed between the various pesticides and pesticide classifications relative to location within and between households as well as by season. Variations in the pesticide residues were related to a number of factors. Higher residues were observed in the farm households, particularly in summer, with the highest amount observed for chloropyrifos in carpet (33 microg/m2). For many pesticides, the frequency of detection and the amount of residues were higher in summer, which relates to usage patterns in agriculture and horticulture; however, larger amounts of insecticides such as mecoprop, resmethrin, and tetramethrin were found on flat surfaces in winter, indicating household use and possible redistribution within the home. Distribution patterns suggest that routines within a household may cause high variation in residues; these practices include indoor pets and treatment for fleas and ticks, use of termiticides, and fastidiousness of occupants. Frequency of pesticide detection was highest in carpet for both summer and winter for all households, indicating that carpets hold pesticides over time. Adsorbent fibrous materials such as textiles hold pesticides by macro- and micro-occlusion in their complex structures. Amounts of pesticide residue were higher in carpets than on smooth floors, particularly for rural farm households where the farmer was a certified pesticide applicator. The maximum amount of pesticide residue on a smooth floor surface was 13.6 microg/m2 malathion while the maxima on wiped surfaces and in settled dust were 1.8 microg/m2 2, 4 D and 3 microg/m2 pendimethalin, respectively. Physical properties of individual pesticides such as vapor pressure influenced the distribution of the pesticide within the households. Evidence of volatilization of pesticides and redeposition on surfaces was observed, indicating that this is a mechanism for contamination of surfaces in addition to adsorption on airborne particles and tracking. High residues in winter are evidence that closure of households in winter that reduces ventilation results in redistribution of pesticides within households.

A risk-based methodology for deriving quality standards for organic contaminants in sewage sludge for use in agriculture—Conceptual Framework

This paper describes a systematic methodology (Conceptual Framework) to derive quality standards for organic (anthropogenic) contaminants in sewage sludge added to agricultural land, in the context of revision of EU Sludge Directive 86/278/EEC and the broader Soil Thematic Strategy. The overall objective is to ensure, based on a risk assessment approach, a sustainable use of sludge over a long time horizon. ILSI-Europe's Conceptual Framework is in essence consistent with the EU Technical Guidance Document (TGD) for Environmental Risk Assessment of Chemicals in the soil compartment, or US-EPA's Sewage Sludge Use and Disposal Regulations, Part 503 Standards. A 'checklist' of different exposure pathways and transfer processes for organic contaminants needs to be considered, and the most sensitive relevant toxicological endpoint and its PNEC need to be identified. The additional complexity specific to deriving Sludge Quality Standards (SQS) is that the toxicity results may need-e.g., for (indirect) human toxicity-to be related back to maximum acceptable soil exposure levels (PEC(soil)). In turn, the latter need to be back-calculated to the maximum acceptable levels in sewage sludge (PEC(sludge)) at the time of application. Finally, for a sustainable sludge use, the exposure from repeated addition and potential chemical build-up over time (e.g., 100 years) needs to be assessed. The SQS may therefore vary with the (local) sludge application regime, and/or sludge pretreatment processes.

Evaluation of landscape coverings to reduce soil lead hazards in urban residential yards: The Safer Yards Project

This study was designed primarily to evaluate the effectiveness of landscape coverings to reduce the potential for exposure to lead-contaminated soil in an urban neighborhood. Residential properties were randomized in to three groups: application of ground coverings/barriers plus placement of a raised garden bed (RB), application of ground coverings/barriers only (no raised bed, NRB), and control. Outcomes evaluated soil lead concentration (employing a weighting method to assess acute hazard soil lead [areas not fully covered] and potential hazard soil lead [all soil surfaces regardless of covering status]), density of landscape coverings (6 = heavy, > 90% covered; 1 = bare, < 10% covered), lead tracked onto carpeted entryway floor mats, and entryway floor dust lead loadings. Over 1 year, the intervention groups had significantly reduced acute hazard soil lead concentration (median change: RB, -478 ppm; NRB, -698 ppm; control, +52 ppm; Kruskal-Wallis, P = 0.02), enhanced landscape coverings (mean change in score: RB, +0.6; NRB, +1.5; control, -0.6; ANOVA, P < 0.001), and a 50% decrease in lead tracked onto the floor mats. The potential hazard soil lead concentration and the entryway floor dust lead loading did not change significantly. Techniques evaluated by this study are feasible for use by property owners but will require continued maintenance. The long-term sustainability of the method needs further examination.

The influence of soil remediation on lead in house dust

Lead in house dust has long been recognized as a principal source of excess lead absorption among children at the Bunker Hill Superfund Site (BHSS) in northern Idaho. House dust lead concentration from homeowner's vacuum cleaner bags has been monitored since the epidemic of childhood lead poisoning in 1974. Geometric mean house dust lead concentrations decreased from >10000 mg/kg in 1974 to approximately 4000 mg/kg in 1975, in response to air pollution control initiatives at the defective primary lead smelter. After smelter closure, 1983 mean dust lead concentrations were near 3000 mg/kg and were most dependent on soil sources. Following emergency soil removals from public areas and roadsides and fugitive dust control efforts in the mid-1980s, house dust lead decreased by approximately 40-60% to 1200-1500 mg/kg. In 1992, a cleanup goal of 500 mg/kg dust lead community average, with no individual home exceeding 1000 mg/kg, was adopted. This goal was to be achieved by a combination of contaminated soil removals and fugitive dust control efforts throughout the 21 square mile BHSS. Continual reductions in house dust lead concentrations have been noted throughout the residential area soil cleanup. Geometric mean house dust lead concentrations averaged approximately 500-600 mg/kg from 1996 to 1999 and dropped below 500 mg/kg in 2000. Analysis of these data indicates that approximately 20% of the variance in dust lead concentrations is attributed to yard, neighborhood, and community soil lead concentrations. Since 1996, dust lead concentrations and dust and lead loading rates have also been measured by dust mats placed at entryways into the homes. Neighborhood soil lead concentrations, household hygiene, the number of adults living in the home, and the number of hours a child spends outdoors in summer explain approximately 26% of the variance in mat dust lead loading rates. It is estimated that post-remedial house dust lead concentrations will stabilize at 400-500 mg/kg, as compared to approximately 200 mg/kg in socio-economically similar background communities; the difference possibly attributed to residual soil concentrations (3-6 times background), recontamination of rights-of-way, tracking of non-residential mining district soils and dusts, fugitive dusts associated with the remediation, and residual structural or carpet dusts.

Evaluation of four sampling methods for determining exposure of children to lead-contaminated household dust

Childhood exposure to lead has been demonstrated to result in health effects and lead-contaminated household dust is a primary exposure source. There is a need to establish reliable methods for sampling surfaces to determine levels of lead contamination. Three vacuums (HVS3, GS80, and MVM) and one wipe method were evaluated for the collection of household floor dust under field sampling conditions within a Superfund site and demographically similar control area. Side-by-side floor samples were taken from three locations within 41 randomly selected households between August and September 1995: a child's bedroom, primary play area, and primary entrance. Analysis was performed to assess the relative collection performance of each sampler, spatial distribution of lead within a household, and correlation of lead loading with observed blood lead level, and to determine if discrete or composites samples were more predictive of blood lead levels. Approximately 90% of the floor surfaces were carpeted. The rank order of sampling methods from greatest to lowest collection efficiency was HVS3 > GS80 > wipe > MVM. The HVS3 had the highest level of precision (CV=0.05), with the GS80 and wipe precisions 0.48 and 0.053, respectively. Lead loadings for samples collected in bedrooms and living areas and composite samples using the HVS3 and wipe methods were significantly correlated with blood lead levels. Correlations between blood lead levels and composite samples were stronger for the HVS3 (R(2)=0.33, P=0.003) and wipe (R(2)=0.25, P=0.002) methods than the respective discrete samples. Regression analysis indicated that a blood lead level of 10 microgram/dl corresponds to a carpet wipe sample geometric mean of 68 microgram/ft(2). For ongoing public health purposes, such as screening and clearance testing, use of the wipe sampling method is the most appropriate. This investigation supports findings by others that the present HUD risk levels for lead in floor wipe samples may not be adequate for reducing children's blood lead levels below 10 microgram/dl.

Relationship between lead mining and blood lead levels in children

The authors studied blood lead levels of 226 randomly selected children, aged 6-92 mo, who lived in either a lead-mining area or a nonmining area, and 69 controls. The authors sought to determine to what extent mining activities contributed to blood lead levels in the children. The mean blood lead levels in the study and control groups were 6.52 microg/dl and 3.43 microg/dl, respectively. The corresponding proportions of children with elevated blood lead levels were 17% and 3%. Soil and dust lead levels were up to 10 times higher in the study than the control group. Elevated blood lead levels appeared to result from exposure to both lead-mining waste and lead-based paint. Mining waste was the cause of the higher prevalence of elevated blood lead levels in these children.

Derivation of a target level of lead in soil at residential sites corresponding to a de minimis contribution to blood lead concentration

Inability to define either a clear toxicologic threshold or a stochastic all-or-nothing (cancer-type) response model for the noncarcinogenic effects of lead (Pb) in young children has posed difficulties for derivation of risk-based target levels of Pb in residential soil. Approaches based on empirical relationships between Pb levels in blood (PbB) and Pb in soil suffer from inability to specify the numerous variables which mediate between these two quantities. Approaches based on achieving a toxicologically de minimis target PbB level (e.g., 10 micrograms/dl) are subject to large uncertainty in estimating the distribution of existing PbB levels in a specific exposed population and in estimating the relative contribution from nonsoil sources of Pb. The multisource contribution to the distribution of PbB makes this approach unsuited for determination of a target Pb level in a single medium. An alternative approach is presented based on achieving a de minimis contribution to PbB (delta PbB) from soil. Contributions to Pb exposure from outdoor soil and indoor soil-derived dust (ISDD) are modeled and appropriate values are suggested for input parameters. This analysis predicts that chronic exposure of young children to 200 micrograms Pb/g (ppm) in residential soil will result in a delta PbB of 2 micrograms Pb/dl blood. This concentration of Pb in soil may provide an appropriate target level for residential soil when other significant sources of Pb exposure are present. In other cases, this approach can be used to predict a soil concentration of Pb corresponding to an appropriate non-de minimis delta PbB.