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Paired PM 10 and fine particulate matter (PM 2.5) samples were collected indoor and outdoor at a fully mechanically ventilated high school in the ultraindustrialized ship channel region of Houston, Texas over a two-month period. The primary foci of this research are to measure the elemental composition of ambient outdoor and indoor coarse particulate matter (PM 10-2.5), characterize aerosol infiltration/ventilation, and obtain clues to their origins. Several metallic hazardous air pollutants were detected inside the school but at concentrations lower than even the most rigorous indoor air quality benchmarks. Only chromium may have potentially posed a hazard based on indoor air quality standards set by the California Environmental Protection Agency. Indoor-to-outdoor (I/O) mass abundance ratios for some metals (Li, Be, Si, Ti, Cr, Co, Ni, Zn, As, Se, Zr, Cd, Sn, Sb, Hf, and Pb) were significantly greater than 1 in PM 10-2.5 , suggesting corresponding indoor sources in the PM 10-2.5 size range. Some other metals (Al, K, Ca, V, Mn, Fe, Cu, Ga, Rb, Sr, Y, Mo, Ce, Pr, Nd, Sm, Gd, Dy, Yb, and U) in PM 10-2.5 had I/O ratios close to one suggesting they had predominantly outdoor origins. Enrichment factors of several metals including light rare earths (e.g. Cu, Sb, Pb, Mo, Cd, La, Ce, Pr, Nd, and Sm) with respect to local soil were overall the highest both indoors and outdoors, illustrating their anthropogenic origins. Several lines of evidence including light lanthanoid ratios, simultaneous La–Ce-Sm variations (ternary diagrams), and correlations with cracking catalysts indicated that petroleum refineries are principally responsible for the elevated lanthanoid concentrations. Rare earth element analysis revealed shifts between transient catalyst releases from petroleum refining operations and windblown soil resuspension as their dominant sources. • Numerous metals measured in paired coarse PM indoors and outdoors of a school. • All indoor metals were at very low levels and below health standards (except Cr). • Metals traced to petroleum refining, oil combustion, soil/road dust, and vehicles. • Most rare earths and many indoor metals infiltrated/ventilated from outside. • Li, Be, Si, Sc, Ti, Cr, Ni, Zn, As, Se, Zr, Cd, Sn, Sb, Hf, W, Pb, Co had indoor sources. [ABSTRACT FROM AUTHOR]