From the EPA: “The national percentile tells you what percent of the US population has an equal or lower value, meaning less potential for exposure/ risk/ proximity to certain facilities, or a lower percent minority“. This means that if an area has a demographic index national percentile of 70, it is an equal or higher % or “potentially susceptible individuals”  than where 70% of the US population lives. The number of “potentially susceptible individuals” is found using the average of % minority populations and % low-income populations times the population count. This number is then calculated with the national average for relative comparison. This data is shown in the map by census block group, and is an estimate backed by data and research thoroughly gathered by the EPA. The American Community Survey (ACS) does not provide demographic data at the block group resolution.