28 We measured levels of airborne endotoxins in South Korea and compared these to PM 10 , 29 PM 2.5 , and O 3 levels in ambient environments; environmental factors affecting these levels 30 were also analyzed. A total of 81 air samples were collected and analyzed using the kinetic 31 Limulus Amebocyte Lysate (LAL) assay. The geometric mean was determined for the levels 32 of endotoxins (0.132 EU/m 3 ), PM 10 (51.9 µg/m 3 ), PM 2.5 (22.6 µg/m 3 ), and O 3 (0.018 ppm). 33 Endotoxin levels were

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... nificantly higher in the fall and winter as compared to summer. 34 PM 10 and PM 2.5 were significantly higher, and O 3 was by far the highest, in spring. Negative 35 correlations were found between endotoxin and O 3 levels (r = -0.491) and between endotoxin 36 levels and temperature (r = -0.302). PM 10 levels were also negatively associated with O 3 37 levels and temperature but positively associated with PM 2.5 levels. Given the negative 38 relationship between airborne endotoxins and O 3 determined here, further studies at larger 39 sample sizes are needed to identify the mechanisms responsible. 40 41