Ozone (O3) is implicated in the natural source inventory of ClO4-, a hydrophilic salt that migrates to groundwater and interferes with the uptake of iodide in mammals, including humans. Tropospheric O3 is elevated in many urban and some rural areas in the United States and globally. We previously showed that controlled O3 exposure at near-ambient concentrations (up to 114 nL L-1, 12-h mean) did not increase foliar ClO4-. Under laboratory conditions, O3 has been shown to oxidize Cl- to ClO4-. Plant tissues contain Cl- and exhibit responses to O3 invoking redox reactions. As higher levels of O3 are associated with stratospheric incursion and with developing megacities, we have hypothesized that exposure of vegetation to such elevated O3 may increase foliar ClO4-. This would contribute to ClO4- in environments without obvious point sources. At these high O3 concentrations (up to 204 nL L-1, 12-h mean; 320 nL L-1 maximum), we demonstrated an increase in the ClO4- concentration in surface soil that was linearly related to the O3 concentration. There was no relationship of foliar ClO4- with O3 exposure or dose (stomatal uptake). Accumulation of ClO4- varied among species at low O3, but this was not related to soil surface ClO4- or to foliar ClO4- concentrations following exposure to O3. These data extend our previous conclusions to the highest levels of plausible O3 exposure, that tropospheric O3 contributes to environmental ClO4- through interaction with the soil but not through increased foliar ClO4-.