How the ‘Quirky Geography’ of ZIP Codes Obscured Flint’s Lead Problem

Caitlin Dempsey


All spatial data is not created equal.  Richard Casey Sadler, an assistant professor at Michigan State University discusses how the Michigan Department of Health and Human Services’ reliance on using ZIP code level data obscured the link between Flint residents and lead exposure.  By using ZIP codes, officials at the state agency ended up summarizing blood lead data for areas that covered not just the city of Flint but also adjacent county areas.  Sadler geocoded the data at the parcel level which clearly showed that Flint children “had become far more likely than out-county children to experience elevated blood lead when compared to two years prior.”

Dr. Tony Grubesic, an Arizona State University professor, has called them “one of the quirkier ‘geographies’ in the world.” Dr. Nancy Krieger, a Harvard University professor, and colleagues have called out their unacceptability for small-area analyses.

The article is a valuable case study in why scale appropriate data is critical for teasing out spatial patterns. While ZIP codes do have a place in analytics, the use of this level of data to look at Flint’s lead contamination issue was not.

This map produced by Sadler shows how ZIP codes don’t align with the boundaries of the city of Flint or its municipal water system. As Sadler notes:

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One-third of all homes with a Flint ZIP code lie outside the city. Thus, the state’s numbers for Flint were watered down by an additional 50 percent of addresses that weren’t in the city and weren’t using Flint water. This is referred to in geography as the modifiable areal unit problem.

ZIP Code boundaries overlayed with the City of Flint's boundary. Map: Richard Casey Sadler.
ZIP Code boundaries overlayed with the City of Flint’s boundary. Map: Richard Casey Sadler.


(via Rob Simmon)


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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.