Integrating human health impact from indoor emissions into an LCA: a case study evaluating the significance of the use stage

Indoor emissions of toxic substances from products can have a negative effect on human health. These are typically not considered in a life cycle assessment (LCA), potentially underestimating the importance of the use stage. The purpose of this paper is to develop a method that, based on a set of measured emission rates, calculates the impact on human health during the use stage of products that are used indoors and that emit volatile organic compounds (VOCs).

Emissions from a product are measured in a test chamber and reported as a set of emission rates (microgrammes per hour) at specific points in time (hour/day). Constrained non-linear regression (CNLR) analysis is then used to determine parameters for three emission models, and a model is selected based on goodness of fit with the measured emission rates (R ² and expert judgement). The emission model is integrated over a defined time period to estimate the total use stage emissions per functional unit (FU). The total emissions are subsequently integrated in a homogeneously mixed one-box model within the USEtox model. Intake fraction (iF) is calculated based on size of residential home, inhalation rate, exposure time, ventilation rate, mixing factor and number of people exposed.

The method is tested in a case study of a chair, with the results showing that the impacts in the use stage are in most cases significantly higher than from the production and disposal stages combined. The sensitivity to parameter variations is evaluated. Intake fraction (factor of 761), replacement frequency (factor of 70) and emission model (factor of 24) are found to be the most important model parameters. Limiting early exposure (>14 % of emissions may occur in the first month and >50 % in the first year) and replacing furniture less frequently will reduce exposure.

The case study shows that the impact on human health from indoor emissions can be of significance, when compared to the impact on human health from total outdoor emissions. Without specific exposure data (e.g. ventilation rates) the uncertainty will be high. The developed method is applicable to all products that emit VOCs, provided that the emission rate can be modelled using an exponential decay model and that the product amount is related to a meaningful functional unit. It is recommended that when performing an LCA of products that emit VOCs, the indoor use stage is included in the life cycle impact assessment.