Air pollution abatement strategies for controlling nitrogen dioxide, sulfur dioxide, and ozone emissions in the United States focus on a 'Standards-based' approach. This approach places limits on air pollution by maintaining a baseline value for air quality, no matter what the ecosystem can or cannot withstand. In the paper, the authors present example critical levels maps for the conterminous U.S. developed using the 'effects-based' mapping approach as defined by the United Nations Economic Commission for Europe's Convention on Long-Range Transboundary Air Pollution, Task Force on Mapping. The approach emphasizes the pollution level or load capacity an ecosystem can accommodate before degradation occurs, and allows for analysis of cumulative effects. They present the first stage of an analysis that reports the distribution of exceedances of critical levels for NO2, SO3, and O3 in sensitive forest, crop, and natural vegetation ecosystems in the contiguous United States. They conclude that extrapolation to surrounding geographic areas requires the analysis of diverse and compounding factors that preclude simple extrapolation methods. (Copyright (c) 1994 Kluwer Academic Publishers.)