Web-Based Access and Visualization of Hazardous Air Pollutants

This paper reports on a project to provide Web-based access to the US Environmental Protection Agency’s (EPA’s) extensive model-based summaries of hazardous air pollutants (HAPs). As part of EPA’s Cumulative Exposure Project, long-term cumulative concentrations of 148 HAPs for the 60,803 census tracts in the 48 contiguous states have been modeled for 1990. The model results include estimates and confidence bounds that assess the estimated uncertainties for each of the HAPs in each census tract. The project challenge was to concisely display 148×60,803 (8,998,844) estimates along with uncertainty bounds. The project goal was to make these statistical summaries accessible to the public as statistical tables and graphs. The Web provides an easy way to make this information electronically accessible. One big challenge is to make the summaries conceptually accessible. The most difficult part of this is to communicate an understanding of the underlying data limitations, the modeling process, and how to interpret the model results. The easier part of ensuring conceptual accessibility is facilitating navigation through the summaries and consideration of values within a large context. Our approach allows the user to select a HAP and “drill down” through the levels of a geopolitical hierarchy. The hierarchy consists of states within the United States, counties within states, and census tracts within counties. Our Web-based approach also attended to the design of tables and graphics with the intent to make them more readable and useful. For tables, our approach focused on perceptual details such as rounding and foreground-background contrast. For graphics, our approach provided spatial context through the use of recently developed templates called linked micromap plots. Both tables and micromaps provide a hierarchically clickable drill-down to finer details. This provides fast answers to questions about the air quality in any given region in the contiguous United States.