Indoor intake fraction considering surface sorption of air organic compounds for life cycle assessment

Life cycle assessment (LCA) has largely focused on characterizing the impact of outdoor emissions. However, the intake fraction (iF) of indoor air emissions could be more important. The present paper aims to determine the long-term intake fractions of indoor emissions, including multiple indoor removal pathways such as sorption on indoor surfaces, and to compare it to the outdoor intake fraction. The developed model accounts for the different removal pathways in buildings, including air exchange, degradation in the gas phase, degradation on surfaces, and finally partitioning between air, walls, and furniture assuming a kinetically limited material transfer between gas phase and a near-surface film. The indoor intake fraction is presented as a function of the adsorption and degradation rate on surfaces. The intake fraction of volatile substances is only affected by the ventilation rate, with a constant intake fraction of 1 x 10(-2). For ozone-sensitive substances, indoor gas phase reactions can significantly reduce the intake fraction. Semi-volatile substances are affected by the adsorption and degradation on room surfaces. For highly adsorbing substances, the decrease in intake fraction is limited to a minimum value of 2.5 x 10(-4) by the mass transfer rate between air and room surfaces for a typical office or residence room in developed countries with temperate climate. Indoor intake fraction is compared to outdoor intake fraction calculated using the Impact 2002 multimedia model. Typical calculated indoor intake fraction values are in a significantly higher range (2.5 x 10(-4) to 1 x 10(-2)) than inhalation outdoor values (1 x 10(-9) to 1 x 10(-6)). This paper opens new possibilities to assess the health impact of indoor and outdoor air emissions in a consistent way, including surface sorption-a major removal pathway for semi-volatile compounds. By combining the newly calculated intake fractions with effect factors and with indoor and outdoor emissions per functional unit, it becomes possible to consistently account for indoor exposure in methods such as LCA.