Age Specific Estimates of Hourly Dust and Soil Ingestion Rates in a Residential Area Johan Bierkens, Christa Cornelis, Mirja Van Holderbeke, Rudi Torfs INTRODUCTION Indoor dust highly contributes to an unhealthy indoor environment. When settled on surfaces, it may be ingested via hand-mouth or hand- object contact. When re-suspended, it may pose a serious source of inhalation exposure. Few studies are available on outdoor (contaminated) soil as a source of indoor dust. Information on the contribution of dust in total soil&dust ingestion is often lacking. However, in view of the large contribution of indoor contaminants in the total exposure, this information is essential. Therefore a mathematical model to calculate hourly soil and dust ingestion rates was developed which uses estimates of daily soil&dust ingestion rates from tracer studies, estimates of the time spend in- and outdoors and the indoor/outdoor ingestion ratio as input variables. The dust ingestion rate indoor is the difference between the total soil and dust ingestion rate and the soil ingestion (indoor + outdoor) rate based on estimates of tracer studies: Equation 2 with: DI in = the dust ingestion rate indoor (mg/day) The soil ingestion rate outdoor can be derived from the total soil and dust ingestion rate, the time spent indoor and outdoor and the ratio between the ingestion indoor and outdoor: Equation 3 with: SI out = the soil ingestion rate outdoor (mg/day) The soil ingestion rate indoor can be calculated as the difference between the total soil ingestion rate and the soil ingestion rate outdoor: Equation 4 with: SI in = the soil ingestion rate indoor (mg/day) The total ingestion rate of soil and dust indoor is the sum of both ingestion rates: Equation 5 with: I in = soil and dust ingestion rate indoor (mg/day) RESULTS AND DISCUSSION For children a mean soil ingestion (TRACER in the equations) of 63 mg/day (based on arithmetic mean values from literature for children 1-6 years old) was assumed. For adults a mean soil ingestion of 46 mg/day (based on arithmetic mean values from literature) was used in the calculations. The fraction exterior soil in interior dust was set at 0.5 (assuming residences with garden) as recommended by Cornelis and Swartjes (2007). To determine the ratio ingestion indoor/ ingestion outdoor the default value 45/55 used by US-EPA in the IEUBK model was recalculated on an hourly basis. For the current calculations the time spent outdoors and indoors by children and adults belonging to different age bins defined in the harmonized method for risk assessment (Cornelis & Swartjes, 2007) was adopted. The approach developed here results in rather high estimates of the hourly ingestion rate of soil outdoors for children of 1 to 3 years old, for 15 to 21 year olds, and also for adults of 21 to 31 year in a residential setting with garden, due to the specific time patterns in these cases. The daily time spent outdoors is very low, relative to the total awake time, resulting in a low fr and hence a high soil ingestion outdoors. It might be considered pooling the result of the 1 to 3 year group with the 3 to 6 year group, to get a more reasonable estimate of 39 mg/h of ingested soil outdoors. CONCLUSIONS Hourly soil and dust ingestion rates calculated with the proposed mathematical model allow the risk assessor to predict exposure to soil- and dust- borne pollutants for a particular setting in residential areas with garden or vegetable garden for different age categories more accurately, i.e. on an hourly basis. Especially on dust these data have long been missing. However, the method cannot be used in a residential setting without garden. Hourly ingestion rates (mg/h) for adults and children. METHODOLOGY The total ingestion rate of soil and dust can be split up into the intake of soil outdoor, dust indoor and soil indoor. Bierkens and Cornelis (2006) have developed a model that allows to derive ingestion rate values for soil indoor and outdoor and dust indoor from tracer study data, and both for daily ingestion as for hourly ingestion rates. In this model, the total dust and soil indoor and outdoor ingestion rate can be calculated as: Equation 1 with: SI total = the total daily ingestion rate of soil and dust indoor and outdoor (mg/day) TRACER = the total daily ingestion rate of soil indoor and outdoor (mg/day) – taken from literature T out = the awake time outdoor (hours/day) T in = the awake time indoor (hours/day) f r = the ratio between ingestion indoor and outdoor (-) f r soil = the fraction soil in (soil + dust) indoor (-) Bierkens, J., C. Cornelis Opstellen van gebruiksafhankelijke waarden voor bodemingestie (No2006/IMS/R/387). Mol, België: VITO Cornelis, C., F.A. Swartjes Ontwikkeling van een geharmoniseerde methodiek voor beoordeling van gezondheidsrisico's door bodemverontreiniging in de Kempenregio: VITO, Mol, Belgium and RIVM, Bilthoven, the Nederlands.