Infiltration of ambient PM _2.5 and levels of indoor generated non-ETS PM _2.5 in residences of four European cities

Ambient fine particle (PM _2.5) concentrations are associated with premature mortality and other health effects. Urban populations spend a majority of their time in indoor environments, and thus exposures are modified by building envelopes. Ambient particles have been found to penetrate indoors very efficiently (penetration efficiency P≈1.0), where they are slowly removed by deposition, adsorption, and other mechanisms. Other particles are generated indoors, even in buildings with no obvious sources like combustion devices, cooking, use of aerosol products, etc.. The health effects of indoor generated particles are currently not well understood, and require information on concentrations and exposure levels. The current work apportions residential PM _2.5 concentrations measured in the EXPOLIS study to ambient and non-ambient fractions. The results show that the mean infiltration efficiency of PM _2.5 particles is similar in all four cities included in the analysis, ranging from 0.59 in Helsinki to 0.70 in Athens, with Basle and Prague in between. Mean residential indoor concentrations of ambient particles range from 7 (Helsinki) to 21 μg m ^-3 (Athens). Based on PM _2.5 decay rates estimated in the US, estimates of air exchange rates and indoor source strengths were calculated. The mean air exchange rate was highest in Athens and lowest in Prague. Indoor source strengths were similar in Athens, Basle and Prague, but lower in Helsinki. Some suggestions of possible determinants of indoor generated non-ETS PM _2.5 were acquired using regression analysis. Building materials and other building and family characteristics were associated with the indoor generated particle levels. A significant fraction of the indoor concentrations remained unexplained.