Canada’s Addressing Air Pollution Horizontal Initiative will contribute to the shared ultimate outcomes of “Canadians have clean air” and “adverse impacts on human health and ecosystems are reduced.” To track the success of the activities under this Initiative, performance indicators are needed. A performance indicator to track human health risks has already been developed (the percent of the Canadian population living in areas where air quality standards are achieved), but a quantitative indicator to track long-term changes in ecosystem health is not yet established. Environment and Climate Change Canada scientists are currently developing possible ecosystem health indicators based on exceedances of critical loads of acidity. These indicators draw on the following available inputs: (1) ~10 km gridded deposition of nitrogen and sulfur based on ADAGIO (Atmospheric Deposition Analysis Generated by optimal Interpolation from Observations), a combination of measurement data and chemical model output; (2) surface water critical loads of acidity based on the steady-state water chemistry model applied to sampled lakes within a ~45 km grid cell; and (3) critical loads for terrestrial ecosystems calculated using soil data at a resolution of 250 m from SoilGrids (soilgrids.org) and scaled up to the ADAGIO grid. We discuss the options as well as how they address the challenges of developing an indicator that is representative of the state of ecosystem health, robust with respect to interannual variability and future changes in model development and measurement coverage, and that can be communicated clearly to the public and the policy community.