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

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