Exposure to atmospheric particulate matter (PM) represents one of the biggest environmental health risks. We show that the intrinsic PM toxicity at a rural site, far from traffic emissions, is comparable to that of urban areas heavily impacted by traffic. Potentially toxic, redox-active compounds in PM are efficiently scavenged in the presence of fog but are also produced in fog. These findings provide evidence that atmospheric processing can significantly alter the toxicity of airborne PM.