Traditional environmental public health surveillance consists of separately measuring hazards, exposures, and health outcomes. The Environmental Public Health Tracking (EPHT) Network seeks to accrue additional information by linking hazard or exposure data to health outcomes data. A natural progression is to consider tracking the “link” itself, that is, to track the association between an environmental hazard and a health outcome. The Maine EPHT Program conducted a case-crossover analysis to measure associations between daily estimated ambient ozone and particulate matter (PM2.5) and asthma-related emergency department (ED) visits for 2000–2003. We found an overall association of 7 percent (95% confidence interval, 4–11) excess asthma-related ED visits per 10-ppb increase in ozone averaged over 4 days (lag 0–3). The association was positive in the first 3 years and negative in the last. The excess risk was concentrated among females aged 15 to 34 and males younger than 15. The methodology for tracking associations between ambient air quality and acute morbidity is not generalizable to most other EPHT topic areas, but there are ample reasons to pursue this activity. The analysis can potentially help evaluate the effectiveness of regulatory and intervention programs, as well as inform us about trends, sensitive subpopulations, and changing potency of air constituents.