Comparison of indoor and outdoor oxidative potential of PM2.5: pollution levels, temporal patterns, and key constituents

Oxidative potential (OP) of PM2.5 is an emerging health indicator representing its ability to induce oxidative stress and cause adverse health effects. We examined pollution levels, temporal variations, and key constituents of PM2.5 OP by DTT assay in both indoor and outdoor environments in Nanjing, China, for over one year. Outdoor OPM (mass-normalized OP characterizes toxicity) and OPV (volume-based OP characterizes overall oxidative burden) in Nanjing were at a medium level compared to results reported for twenty-seven cities. Although PM2.5 mass concentration consistently decreased during outdoor-to-indoor transport, OPM varied by a factor of up to 2 in either direction, indicating a change of PM2.5's ability to disrupt oxidative-reductive balance. Temporally, both outdoor and indoor OPM exhibited a significant seasonality pattern (P<0.01) as autumn summer > spring > winter. Outdoor and indoor daytime-nighttime OPV and OPM are fluctuating within two-fold range. In addition, the change in water-soluble Fe had the highest correlation coefficient (P < 0.05) with Delta OPM (Delta OPM = OPM, (in)-OPM, (out)) among constituents measured here. Our results suggest that development of mitigation strategies take indoor PM2.5's OP into account, instead of outdoors only, since they differ.