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The exposure to airborne particulate matter (PM) and its constituents is an important factor to be considered when evaluating their potential health risk. Transition metals found in PM are known to contribute significantly to the exacerbation of respiratory ailments. Exposure to these constituents results in the induction of oxidative stress in the bronchial epithelium, thus promoting the secretion of inflammatory mediators. Therefore, it is important to know the contributions of PM2.5 constituents to further investigate their relationship with toxic responses and associated health risks. PM2.5 samples from three rural (Humacao, Guayama, and Guayanilla) and two urban (more populated) sites (Bayamón and Ponce) from Puerto Rico were analyzed for various inorganic constituents. A total of 59 trace elements were analyzed, of which eight were considered with the greatest toxic potential. The highest annual average concentration of PM2.5 was reported at the urban site of Ponce (5.82 ± 1.40 μg m−3), while Bayamón's average concentration was not as high (4.69 ± 1.30 μg m−3) compared to concentrations at the rural sites Humacao, Guayama, and Guayanilla (4.33 ± 1.20 μg m−3, 4.93 ± 1.50 μg m−3, and 4.88 ± 1.20 μg m−3 respectively. The concentration at the Ponce site exhibited the highest summer value (7.57 μg m−3) compared to that of all the rural sites (~ 6.40 μg m−3). The lowest summer PM2.5 values were obtained at the Humacao site with an average of 5.76 μg m−3. Average Cu and Zn concentrations were 3- and 2-fold higher at the urban sites (0.68 ng m−3 and 6.74 ng m−3 respectively) compared to the rural sites (0.17 ng m−3 and 4.11 ng m−3). Relative toxicity of inorganic PM extract indicates Bayamón (urban) and Guayama with similar low LC50 followed by Humacao, Guayanilla, and finally Ponce (urban) with the highest LC50. Of the eight potential toxic metals considered, only Fe was found to be higher at the rural sites. To our understanding, there are different sources of emission for these metals which potentially indicate main anthropogenic sources, together with the trade winds adding periodically volcanic and African Dust Storm particulates that affect Puerto Rico. These results are the first of their kind to be reported in Puerto Rico. [ABSTRACT FROM AUTHOR]