Does experiencing pollution mobilize or dampen political participation? Extant theory and historical evidence offer competing answers, and empirically resolving this tension is difficult because pollution and politics are endogenous. We partner with a prominent political action organization in the United States to collect data on commitments to participate in their events. We then use wind speed as an instrument to estimate the effect of surface-level particulate matter concentrations on participation across 414 counties in the Western United States. Our results show that exposure to air pollution undermines political participation, likely through people experiencing and anticipating pollution's health effects. This negative relationship is strongest among relatively high-income and pro-Democratic counties -- the counties which, theoretically, face the lowest barriers to collective action. Our study yields important insights for scholars of political participation, policy feedback, and environmental justice. By undermining the prospects mass mobilization and systemic change, pollution may beget more pollution.