Background: Air pollution is a serious threat to children health. Given that children spend over 80% of their time indoors, understanding transport of pollutants from outdoor to indoor environments is important for assessing the impact of exposure to outdoor pollution on children health. The most common advice given during a smoke pollution episode is to stay indoors. How well this works depends on how clean the indoor air is and how pollutants from outdoor air contribute to pollutants load in indoor air. Objective: To assess the amount of outdoor air pollution coming indoors threatening children health. Methods: A Medline/EMBASE search of scientific articles was performed to evaluate the indoor-to-outdoor (I/O) concentration ratios of two main pollutants: ultrafine particles (UFP) and ozone (O3). Result: Under infiltration condition, the highest I/O ratios (0.6 - 0.9) were usually observed for larger UFP (70 - 100 nm), while the lowest I/O ratios (0.1 - 0.4) occurred typically around 10 - 20 nm. O3 I/O ratios vary according to air exchange and may be 0.6 - 0.8 for interiors having a large volume exchange with outdoor air (i.e. open windows) and 0.3 - 0.4 with conventional air conditioning systems. Conclusions: In the absence of indoor sources or activities, indoor UFP particles originate from outdoors. O3 concentration indoors may reach concentration similar to outdoors. Environmental and energy policies must also explicitly account for all the impacts of fossil fuel combustion on child health and development.
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