Indoor air measurement campaign in Helsinki, Finland 1999 – the effect of outdoor air pollution on indoor air

Wintertime indoor and outdoor particle size distributions were studied in an office building near Helsinki downtown by measuring the particle size distributions with two similar differential mobility particle sizer systems (DMPS). Measurements were made simultaneously at two places; on the rooftop of a building (30 m above the ground level) in front of a ventilation system corresponding to outdoor concentration, and an office room (first floor). The ventilation rate was also continuously monitored. Indoor particle concentrations were observed to vary from 500 to 104 cm−3 with a high dependence on the outdoor concentrations. This indicates that in this scenario, indoor particles are mainly of outdoor origin. Effects of ventilation rates on indoor air particle concentrations and several inorganic gases were studied. We found that ventilation had a strong influence on indoor particle and gas concentrations. I/O (indoor/outdoor) ratio in different in different particle size classes.