Estimation of ambient concentrations and human health risks from emissions of particulate matter and associated metals from unpaved rural roads

Jim Kacer

doi:10.25820/etd.007259

The impetus for this study was the application of electric arc furnace (EAF) slag to unpaved roads in Muscatine County, Iowa by the Muscatine County Secondary Road Department as a supplement to the crushed stone also applied to the roads. Various metals have toxic effects by the inhalation route, and electric arc furnace (EAF) steel slag is known to contain metals with a potential for toxicity to humans. Manganese, for example, has been designated by EPA as a hazardous air pollutant (HAP). In some states, EAF slag is applied to unpaved (gravel) roads as a low-cost supplement to limestone and other crushed stone, where it may be a public health concern for the local population. A group of Muscatine County residents had concerns about direct ingestion of slag-related metals in the road dust by pica children (children who eat dirt and other non-food substances). In response to these concerns, the Iowa Department of Public Health (IDPH) stated that manganese in the steel slag dust could potentially present a hazard to pica children by the ingestion pathway. A follow-up health consultation completed by IDPH concluded that adults are not at risk from incidental ingestion or inhalation of the slag and that children near several sample locations may be at risk from incidental ingestion. No air samples were collected, and the inhalation hazard was not evaluated in detail.Fugitive emissions are defined by EPA as “those emissions which could not reasonably pass through a stack, chimney, vent, or other functionally-equivalent opening”, for example, dust emissions from roads. A research project was designed to address various gaps in the knowledge concerning fugitive emissions of particulate matter (PM) from the unpaved roads to which slag had been applied. Aim 1 of the research was to compare particulate matter (PM10 and PM2.5) concentrations and airborne metals concentrations near roads to which EAF slag had been applied to those near roads to which the slag had not been applied. A sub-aim was to evaluate several factors potentially affecting the metal content in the airborne particulates, such as traffic, relative humidity, wind direction, and wind velocity. Manganese, one of the primary metals of concern in the slag, was 1.3 times more concentrated in the PM10 fraction from the slag-covered roads as compared to the PM10 fraction from the non-slag-covered roads, but that increase was not significant (p = 0.26). Other metals detected in the airborne dust from both slag-covered and non-slag-covered roads that are also designated as HAPs are antimony, arsenic, chromium, cobalt, lead, nickel, and selenium. In addition, hourly sampling of PM10 and metals in the PM10 fraction was conducted at one of the sample locations where slag had been applied to the road. Manganese concentrations in the PM10 were positively correlated (Spearman r = 0.86) with the particulate concentrations in the PM10. Wind direction and the interaction of traffic and wind direction were found to be statistically significant factors (α = 0.05) affecting manganese concentrations in the fugitive emissions from the road to which EAF slag had been applied. This research demonstrated that application of steel slag can result in elevated levels of manganese in the airborne dust generated by vehicular traffic on the unpaved roadway. Aim 2 was to use the EPA AERMOD plume dispersion model to estimate a fugitive PM10 emission rate from particulate concentrations measured near unpaved roads. One sub-aim was to determine if the emission factor estimated with AERMOD is comparable to the emission factor estimated using EPA AP-42 methodology. The AP-42 emission factor is calculated in units of grams/vehicle kilometer traveled (g/VKT) or pounds/vehicle mile traveled (lb/VMT). A second sub-aim was to compare transient fugitive PM concentrations to hourly average concentrations to determine the degree to which hourly measurements underestimate transient concentrations and to provide data for evaluation of human health risks from transient exposures. Transient is defined here as the time it takes for a plume generated by passage of a single vehicle to increase from and return to the background concentrations, generally between 30 and 90 seconds. Hourly averages of PM10 were measured in the field using an EPA reference method. For each hour when PM10 was measured, the emission rate in AERMOD was adjusted until the PM10 concentration at the modeled receptor matched that of the field measurement from the same location for that hour. There was a wide variation in the inversely modeled emission rates; however, the geometric mean of the model-derived emission factors (444 g/VKT, with a geometric standard deviation of 4.33) was reasonably close to the emission factor calculated using EPA’s AP-42 guidance for unpaved roads (795 g/VKT). Direct-reading measurements of transient total PM concentrations due to road traffic were also collected, from which it was determined that vehicle speed and wind speed were significant (α = 0.05) determinants of PM concentration, average PM concentration, and total PM mass for each plume. Several measures were used to characterize each plume, including average concentration, maximum concentration, and cumulative PM mass in the plume. Wind speed was also a significant (α = 0.05) determinant for time to maximum concentration and PM residence time. Wind direction was only significant for residence time. The geometric mean of the hourly per vehicle PM10 concentrations was 12.4 µg/m3 and the geometric mean of the average concentration for the duration of the PM plume associated with one vehicle was 4096 µg/m3, indicating that residents near the road would be exposed to substantially higher short-term concentrations than would be indicated by hourly averages. Aim 3 was to assess risk to human health due to exposure through the inhalation and direct ingestion pathways to ambient dust near rural unpaved roads, based on sampling 11 meters from the edge of the road.. The human health risk assessment was completed using the field data described above using several methodologies. For PM10 and lead, median concentrations were compared to the applicable National Ambient Air Quality Standard established under the requirements of the Federal Clean Air Act (CAA). The hourly maximum lead concentration measured using an ambient continuous metals monitor (ACMM) exceeded the NAAQS 0.15 μg/m3, while the lead in the dichotomous sampler 24-hour samples did not. Although the lead results indicate the potential for a human health risk, compliance with the NAAQS for lead is based on a rolling three-month average, so these results cannot be used to determine compliance with the NAAQS. The median PM10 result did not exceed the applicable NAAQS. However, the PM10 results exceeded the 24-hour NAAQS of 150 μg/m3 in 7 of the 63 24-hour samples indicating a potential risk to human health. For metals defined as hazardous air pollutants (HAPs) under the CAA, risk assessment methodologies specified in Superfund and CAA guidance documents were followed. For cancer effects, the CERCLA/CAA guidance assumes exposure over a 70-year lifetime at an estimated average exposure concentrations and also assumes a linear dose-response relationship for the carcinogenic substance for a specified exposure range. For non-cancer effects, the CERCLA/CAA guidance provides reference concentrations (inhalation pathway) and reference doses (ingestion pathway) for acute and chronic effects, as applicable. For both cancer and non-cancer risks, total risk is determined by addition of risks from individual substances unless there are data supporting synergistic or antagonistic effects from exposure to multiple pollutants. Risks were calculated assuming the receptor was located 11 meters from the edge of the unpaved road. Using EPA methodologies, the hazard index (HI) for non-cancer risk from exposure to the median of 24-hour HAP metals concentrations was 3.7 for the slag-covered road and 1.5 for the non-slag-covered (background) road, indicating the potential for non-cancer risk on both types of road. Cancer risk was 2.4 x 10-4, at the slag-covered road and 1.5 x 10-4 at the non-slag-covered road, primarily due to the chromium concentrations, greater than the 10-4 that EPA considers indicative of unacceptable cancer risk; however, this is based on the assumption that all chromium detected in the PM was hexavalent, since the metals were not speciated. Personal monitoring of individuals living or working near unpaved roads could be conducted to develop more realistic exposure data, and chromium should be speciated as part of this effort to determine the concentration of hexavalent chromium. A non-cancer risk assessment for direct ingestion of HAP metals in the road dust was also completed. The maximum cadmium concentration in the dust from the slag-covered roads, 82.5 mg/kg, presents a potential risk to adults, while chromium (III) in the dust from the slag-covered roads, at a maximum of 444 mg/kg, presents a potential risk to children. No unacceptable risk from ingestion of HAP metals from the dust from non-slag-covered roads was found. The conclusion of the research is that application of EAF slag to unpaved rural roads presents potential risks to the health of exposed population due to ingestion risk, while unacceptable cancer risks from inhalation of PM-associated HAP metals exist at both slag-covered roads and non-slag-covered roads. Unacceptable non-cancer risk from inhalation of PM10 exists at both slag-covered roads and non-slag-covered roads.

Estimation of ambient concentrations and human health risks from emissions of particulate matter and associated metals from unpaved rural roads

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