How air cleaners, ventilation, and outdoor air pollution influence air quality in European hospitals: Case studies from Finland and Romania

• First-time indoor/outdoor black carbon (BC) concentrations reported in hospitals. • Lung-deposited surface area (LDSA) infiltration factors varied between 0.04 and 0.64. • Indoor BC, LDSA, and PM 2.5 concentration varied by factor of 100 between hospitals. • Air cleaners reduced indoor/outdoor ratios of particulate pollution by 70–90 %. • Air purifiers are effective in both mechanically and naturally ventilated spaces. In healthcare facilities, maintaining a controlled, contaminant-free environment is essential. This involves eliminating airborne contaminants and ensuring a continuous supply of clean air. The objectives of this study were to understand the differences in the indoor and outdoor characteristics of particulate matter pollution (fine particle mass (PM 2.5 ), lung-deposited surface area (LDSA), and black carbon mass (BC) concentrations) and environmental conditions (air temperature, relative humidity and carbon dioxide concentration) in hospital buildings in Romania and Finland. Additionally, the effectiveness and impact of ventilation and air cleaning technologies on the indoor air quality were assessed. The highest mean concentrations of outdoor PM 2.5 , LDSA, and BC were observed in Bucharest, with values of 32.7 µg/m³, 59.4 µm²/cm³, and 3.3 µg/m³, respectively. The use of air cleaners effectively reduced indoor particulate concentrations in both naturally and mechanically ventilated buildings. In the naturally ventilated hospital in Bucharest, Romania, the use of air cleaners resulted in reductions of up to 93.8 % and 89.3 % in the median PM 2.5 and LDSA indoor/outdoor (I/O) ratios, respectively. In the mechanically ventilated hospital in Espoo city in Finland, corresponding I/O-ratio reductions were 78.6 % and 69.9 %. These results highlight that indoor air quality is influenced by both indoor and outdoor air characteristics, as well as the building's ventilation and filtration systems. In addition, reduction in indoor concentration values emphasize the effectiveness of using portable air cleaners as a local solution for reducing particulate pollution when integrated with an appropriate natural or mechanical ventilation system.