Evacuation characteristics of released airborne TiO2 nanomaterial particles under different ventilation rates in a confined environment

A specially designed 32 m3 airtight chamber that allows the implementation of various ventilation strategies was utilised to study the evacuation characteristics of airborne sub-micron particles generated from TiO2 nanopowder in a potential indoor accidental release situation. Following the release using a heated line from a nebuliser system, the spatial and temporal variations in particle number concentration were recorded by three condensation particle counters (CPCs) distributed at specific locations in the chamber. A differential mobility spectrometer was co-located with one of the CPCs for the measurement of particle size distributions (PSDs). The different modal groups present within the measured PSDs were determined through a log10-normal fitting program. Of the ventilation rates evaluated, the greatest relative improvement in particle concentration and clearance time occurred at the highest rate (12 air changes per hour, ACH). At the same time, indications of cross-contamination from regions with strong mixing conditions to regions where mixing was poor, were obtained showing that the latter could operate as particle traps where localised poor ventilation might occur. However, reducing the ventilation rate led to: i) an increase of leftover particles in the air of the chamber when the cleaning process had been completed, and more specifically to an increased ratio of ultrafine particles over fine ones, resulting in the potentially dangerous accumulation of contaminants with high exposure hazards, ii) a decrease in ventilation efficiency, which at low evacuation rates became independent of distance from the inlet diffuser, iii) slower clearance of resuspended particles, with the lowest efficiency at the moderate ventilation rate.