VOC injection into a house reveals large surface reservoir sizes in an indoor environment

The total partitioning capacity of indoor surface reservoirs determines the mechanism by which humans receive nondietary exposure to organic contaminants, via inhalation, dermal uptake, and dust ingestion. And yet, this capacity is largely unknown. Surface organic films are ubiquitously present but have very low partitioning volume being only 10’s of nanometer thick, whereas other surface reservoirs such as building materials and furnishings can be permeable or porous with large surface areas at the molecular level. Here, we assess the total partitioning capacity of volatile organic compounds (VOCs) in an indoor environment from the measured kinetics of VOC surface uptake after injection of compounds with variable volatility into a well-characterized, unoccupied test house. We show that the size of the indoor surface reservoirs is very large with an octanol-equivalent average thickness on the order of micrometers, indicating that permeable/porous materials such as painted surfaces and wood are likely the major surface reservoirs in the house rather than organic surface films. Large surface reservoirs result in compounds with octanol-air partition coefficients (K_OA) larger than 10⁵ being predominantly partitioned to indoor surface reservoirs, making them hard to be removed via ventilation. This result significantly impacts our understanding of VOC fate and human exposure in indoor environments. With such a large partitioning capacity, organic contaminants will have much longer indoor residence times than previously predicted.