Ten questions concerning indoor ultrafine particles

Ultrafine particles (<= 100 nm in diameter) are the smallest condensed-phase materials in air. By number, they are the most abundant particles both outdoors and indoors. Because of their small size, they contribute little to mass concentrations of fine particles, a regulated air pollutant with large associated health risks. Inhaling ultrafine particles raises health concerns, in part owing to their ability to migrate from the site of deposition in the respiratory tract. Buildings provide partial protection against ultrafine particles of outdoor origin. The degree of that protection is broadly variable, influenced by particle size and by building design and operational factors, such as ventilation rates and the efficiency of particle filters. Because of the large proportion of time spent by people indoors, much exposure to ultrafine particles occurs in buildings, even for particles of outdoor origin. Diverse indoor sources, including cooking and the use of candles, contribute materially to indoor ultrafine particle concentrations. Indoor sourced ultrafine particle concentrations are also influenced by building design and operational factors. Whether of indoor or outdoor origin, dynamic processes influence the concentrations, sizes, and chemical composition of ultrafine particles. Such processes include deposition to indoor surfaces, coagulation, and sorptive partitioning of semivolatile organic compounds. Advancing scientific and technical knowledge about building factors and dynamic processes that influence indoor ultrafine particle concentrations can contribute (a) to better understanding of associated health risks and (b) to the more effective design and implementation of mitigation strategies and technologies.