2017 No https://pubmed.ncbi.nlm.nih.gov/28375618/ Bozlaker, Ayse North America Observational Epidemiology Study High School (9-12) Average indoor-to-outdoor (I/O) abundance ratios for the majority of elements were close to unity providing evidence that indoor metal-bearing PM2.5 had predominantly outdoor origins. Only Co had an I/O abundance ratio >1 but its indoor sources could not be explicitly identified. La and 17 other elements (Na, K, V, Ni, Co, Cu, Zn, Ga, As, Se, Mo, Cd, Sn, Sb, Ba, W, and Pb), including air toxics were enriched relative to the local soil both in indoor and outdoor PM2.5 demonstrating their noncrustal origins. Several lines of evidence including receptor modeling, lanthanoid ratios, and La–Ce–Sm ternary diagrams pointed to petroleum refineries as being largely responsible for enhanced La and total lanthanoid concentrations in the majority of paired indoor and outdoor PM2.5.