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The paediatric intensive care unit (PICU) provides care to critically ill neonates, infants and children. These patients are vulnerable and susceptible to the environment surrounding them, yet there is little information available on indoor air quality and factors affecting it within a PICU. To address this gap in knowledge we conducted continuous indoor and outdoor airborne particle concentration measurements over a two-week period at the Royal Children's Hospital PICU in Brisbane, Australia, and we also collected 82 bioaerosol samples to test for the presence of bacterial and viral pathogens. Our results showed that both 24-hour average indoor particle mass (PM 10 ) (0.6-2.2μgm -3 , median: 0.9μgm -3 ) and submicrometer particle number (PN) (0.1-2.8×10 3 pcm -3 , median: 0.67×10 3 pcm -3 ) concentrations were significantly lower (p<0.01) than the outdoor concentrations (6.7-10.2μgm -3 , median: 8.0μgm -3 for PM 10 and 12.1-22.2×10 3 pcm -3 , median: 16.4×10 3 pcm -3 for PN). In general, we found that indoor particle concentrations in the PICU were mainly affected by indoor particle sources, with outdoor particles providing a negligible background. We identified strong indoor particle sources in the PICU, which occasionally increased indoor PN and PM 10 concentrations from 0.1×10 3 to 100×10 3 pcm -3 , and from 2μgm -3 to 70μgm -3 , respectively. The most substantial indoor particle sources were nebulization therapy, tracheal suction and cleaning activities. The average PM 10 and PN emission rates of nebulization therapy ranged from 1.29 to 7.41mgmin -1 and from 1.20 to 3.96pmin -1 ×10 11 , respectively. Based on multipoint measurement data, it was found that particles generated at each location could be quickly transported to other locations, even when originating from isolated single-bed rooms. The most commonly isolated bacterial genera from both primary and broth cultures were skin commensals while viruses were rarely identified. Based on the findings from the study, we developed a set of practical recommendations for PICU design, as well as for medical and cleaning staff to mitigate aerosol generation and transmission to minimize infection risk to PICU patients., (Copyright © 2017 Elsevier Ltd. All rights reserved.)