More than 140 Colorado schools in the state’s 20 largest districts have enough students opting out of immunizations to potentially compromise herd immunity and hasten the spread of communicable diseases.

The Boulder Valley School District has the most schools–three-dozen–with exemption rates of 10 percent or higher, but several other districts, including Academy 20, Jeffco, Poudre, Pueblo and Thompson have a dozen or more schools in that category. Meanwhile, Denver Public Schools, the state’s largest district, has just three schools with exemption rates higher than 10 percent.

Hover your cursor over dots on the map below to see school names and exemption rates.

In Colorado, parents may legally excuse their children from immunizations by claiming religious, medical, or personal belief exemptions. Personal belief exemptions are by far the most common kind of exemption in the state.

Exemption rates of 10 percent or more are significant because herd immunity usually requires immunization rates of 90-95 percent. If too few students are immunized to achieve herd immunity thresholds, communicable diseases like measles and whooping cough spread more easily.

To learn more about immunization compliance and exemption rates for individual schools in the state’s 20 largest school districts, go to Chalkbeat’s first-of-its-kind database with school-by-school data.