Lung-depositing surface area (LDSA) of particles in office spaces around Europe: Size distributions, I/O-ratios and infiltration

Air pollution, and specifically particulate matter pollution, is one of the greatest dangers to human health. Outdoor air pollution ranks third in causes for premature death. Improving indoor air quality is of immense importance, as the time spent indoors is often much greater than the time spent outdoors. In this experimental study, we evaluate the levels of particle pollution in indoor air in four offices across Europe, compare the indoor particles to outdoor particles and assess where the particles originate from. The measurements were conducted with an Electrical Low-Pressure Impactor (ELPI+) for particles between 6 nm and 1 µm. The chosen metric, lung-deposited particle surface area (LDSA), targets the health impacts of particle pollution. Based on the measurements, we determined that most of the indoor air particles infiltrated from outdoor air, although two of the offices had very limited indoor activity during the measurement campaigns and may not represent typical use. The highest median indoor LDSA concentration during daytime hours was 27.2 µm²/cm³, whereas the lowest was 2.8 µm²/cm³. Indoor air in general had lower LDSA concentrations than outdoor air, the corresponding outdoor LDSA concentrations being 35.8 µm²/cm³ and 9.8 µm²/cm³. The particle size ranges which contributed to the highest concentrations were 50–100 nm and 300–500 nm. These size ranges correspond to soot mode and accumulation mode particles, which represent local and regional sources, respectively. Based on this study, limiting particle infiltration is the key factor in keeping indoor air in offices free of lung-depositing particles.