2017 Yes https://www.sciencedirect.com/science/article/abs/pii/S0048969716321258?via%3Dihub Oliveira, Marta Europe Exposure Study Students Elementary (K-5) Air School Polycyclic aromatic hydrocarbons (PAHS) indoor/outdoor Children Risk Total mean concentrations (ΣPAHs) ranged 2.8–54 ng m− 3 in indoor air, whereas corresponding outdoor levels were 7.1–48 ng m− 3. Indoor/outdoor ratios of lighter congeners (2–3 aromatic rings) demonstrated a contribution from indoor origin while heavier PAHs (4–6 aromatic rings) originated mostly from infiltration of ambient air indoors; traffic (both from diesel and gasoline fuelled vehicles) was the predominant source of indoor PAHs. Total cancer risk of 8–10 years old children exceeded (up to 22 times) USEPA recommended guideline of 10− 6, and 7–87 times WHO health–based threshold of 10− 5. Risk due to indoor exposure in schools was 2–10 times higher than outdoors, mainly because of the higher amount of time that students spent indoors.