Factors controlling the lung dose of road traffic-generated sub-micrometre aerosols from outdoor to indoor environments

Estimates of lung dose of submicron particles in the human respiratory system play an essential role in assessing health outcomes of aerosol exposure. The objective of this study is to calculate the regional lung dose of traffic-generated particles by different metrics from exposure in outdoor and indoor environments, and to identify main factors determining the lung dose. Particle number size distributions were collected in both indoor and outdoor environments in two unoccupied apartments from 22nd February to 30th April 2012 in Bologna, Italy. The whole lung dose of outdoor aerosols by number, surface area and mass at a traffic site was 1.0 x 1010 particle/hour, 130 mm2/hour, and 1.9 µg/hour, respectively. A majority of particles by number and surface area was found to deposit in the alveolar region (65%). The physical properties of particles such as shape, hygroscopicity and density play an important role in the calculation of surface area and mass dose due to shifting the lung deposition curve. Particle number can predict well the regional dose by number, while PM2.5 and PM10 are good metrics for the prediction of surface area and mass dose. Good correlations between NOx and the surface areas and mass dose (r2 ~0.8) and number dose (r2 ~0.7) of submicron aerosols suggest that NOx may be a good indicator for predicting the health outcomes of traffic-generated aerosols. The doses of indoor submicrometre aerosols are less than those of outdoor aerosols by factors of 4.1 (for number), 2.7 (for surface area) and 2.1 (for mass). Due to traffic emissions, the lung dose of outdoor aerosols in the traffic area was much higher than that in the residential area by 5 times for number and surface area and 2 times for mass. A different exercise level (standing, walking, running, and cycling) has only a slight influence on the whole lung deposition fraction of submicron aerosols, but has a large effect on the dose due to the differences in ventilation rate.