Exploratory assessment of indoor and outdoor particle number concentrations in Hanoi households.

No studies have been conducted in Vietnam to understand the levels of atmospheric ultrafine particles, despite having adverse health effects. Information about indoor air quality in Vietnam is also limited. Hence we aimed to conduct the first assessment of ultrafine particle concentrations in terms of particle number (PN) in Hanoi, by simultaneously measuring indoor and outdoor PN concentrations from six households at different locations across the city in January 2016. We also acquired PM data for this monitoring period from an air quality monitoring station located at the US Embassy in Hanoi, to compare the general trends between PN and PM concentrations. The mean daily indoor and outdoor PN concentrations for the monitoring period were 1.9×10p/cm and 3.3×10p/cm, respectively, with an increase during rush hour traffic. It was concluded that traffic was the main contributor to outdoor PN concentrations, with agricultural burning having a small influence at one study location. The mean ratio of indoor to outdoor PN concentrations for all six sites was 0.66±0.26, which points to outdoor air as the main driver of indoor PN concentrations, rather than indoor sources. These PN concentrations and I/O ratios are similar to those reported for a number of cities in developed countries. However, in contrast to PN, ambient mean PM concentrations in Hanoi (60-70μg/m) were significantly higher than those typically recorded in developed countries. These findings demonstrate that urban particle mass (PM) concentrations are not indicative of the PN concentrations, which can be explained by different sources contributing to PN and PM, and that direct measurements of PN are necessary to provide information about population exposure to ultrafine particles and for management of air quality.