In order to help correlate the exposure of a population to a toxicant directly to the source of exposure, researchers have developed a concept called the intake fraction (iF), which is a dimensionless variable. The estimation of the iF relationship is an important tool for the risk assessment of chemicals. Essentially, this tool relates the amount per unit emission that is taken in by the exposed population. Once iF values have been determined for a set of different types of pollutants, sources, and pathways, new population-level exposures can be predicted with surprisingly limited additional data.
The purpose of this project was to investigate and further develop the iF concept as a risk assessment tool, by reviewing existing data and studies, modelling methods, and developing a database for future analyses. The study also applied the iF concept to a varied set of case studies to test utilisation possibilities for different types of sources and substances. The exposure assessments conducted in these case studies covered a wide area of substances (volatile, semi-volatile, metals and particles), source types (point, mobile and area) and intake routes (inhalation and ingestion). The results of the case studies were predominantly within the ranges found in previous iF estimates for similar types of pollutants and sources, and support the utility of iF as an efficient, relatively consistent, and comparable metric for evaluating risks across sources, pollutants, and intake routes. It also makes this concept a useful tool for screening and prioritisation of chemicals to evaluate. This project successfully added estimates for several compounds with differing intake routes and source types, and collated all iF estimates published thus far into a database.
As more studies are done, the Intake Fraction Database (iFDb) will grow and provide new opportunities for comparative and exploratory analyses. For example, intake fractions may be estimated for some scenarios where data are limited by using information in the iFDb to provide a plausible range of values. Additionally, intake fractions could be estimated for new substances based on basic information about the substance’s chemical and physical properties and the exposed population and area. Overall, as previously mentioned, this concept would be useful for conducting a screening level assessment and comparison between substances for prioritisation of further investigation.
MM Loh, J Soares, A Karppinen, J Kukkonen, L Kangas, K Riikonen, A Kousa, A Asikainen, MJ Jantunen, Intake Fraction Distributions for Benzene from Vehicles in the Helsinki Metropolitan Area, Atmospheric Environment 2008, in print.