Consistent with Jocelyn Kaiser's News & Comment article “Showdown over clean air science” ([25 July, p. 466][1]), airborne particulate matter (PM) has been repeatedly associated with morbidity and mortality, even at concentrations well within the Environmental Protection Agency's (EPA's) 150-microgram per cubic meter (μg/m3) upper acceptability limit on 24-hour average PM of 10 micrometers or less (PM-10). Failure to identify plausible mechanisms by which PM-10 (or PM-2.5, or both) might cause such effects at these low concentrations suggests to some that stressors associated with PM, rather than PM itself, might be causal. Attributing PM effects to 24-hour averages reported under the National Ambient Air Quality Standard (NAAQS) is like attributing daily mortality reported in a war zone to 24-hour airborne lead concentrations instead of bullets. Real-time PM monitoring has revealed significant variability during 24-hour periods of low PM ([1][2]). Brief PM excursions have reached twice the estimated concentration prevailing during the 1952 London fog. Effects that EPA attributes to 24-hour average PM seem equally consistent with causation by excursions to high PM concentrations, whose health significance is becoming increasingly evident. Excursions also could explain why a 24-hour PM effect threshold has been undiscernible, even though noncancerous effects typically exhibit thresholds. Effect thresholds can exist for PM too, but if they are threshold excursions embedded in 24-hour averages, their contribution to the 24-hour averages might be imperceptibly small, suggesting absence of a threshold. This approach represents a more economical challenge for industry, whose compliance with the NAAQS could then focus on a small fraction of daily operations when PM control is least effective. 1. [↵][3]R. A. Michaels. Aerosol Sci. Technol. 25 , 437 (1996). [1]: /lookup/doi/10.1126/science.277.5325.466 [2]: #ref-1 [3]: #xref-ref-1-1 "View reference 1 in text"