We measured the levels of airborne endotoxins in South Korea and compared them to PM10, PM2.5, and O3 levels in ambient environments; environmental factors affecting these levels were also analyzed. A total of 81 air samples were collected and analyzed using the kinetic Limulus Amebocyte Lysate (LAL) assay. The geometric mean was determined for the levels of endotoxin (0.132 EU m–3), PM10 (51.9 µg m–3), PM2.5 (22.6 µg m–3), and O3 (0.018 ppm). The endotoxin levels were significantly higher in fall and winter than in summer. The levels of PM10 and PM2.5 were significantly higher, and the level of O3 was by far its highest, in spring. Negative correlations were found between the endotoxin and O3 levels (r = –0.491) and between the endotoxin levels and temperature (r = –0.302). The PM10 levels were also negatively associated with the O3 levels and temperature but positively associated with the PM2.5 levels. Given the negative relationship between airborne endotoxins and O3 determined here, further studies with larger sample sizes are needed to identify the responsible mechanisms.