The public record for each county: what the local water utilities reported, traffic emissions from the EPA National Emissions Inventory, the EPA’s radon prediction from the area’s geology, and the sites registered nearby.
Why is there no overall score for each place?
Because in a city it mostly measures how dense the place is. Traffic and registered sites run high in almost every metro, so a single number would rank nearly every city you would want to compare at the bottom. Each category is shown on its own instead.
Does this tell me what is in my home’s water or air?
No. It describes a county, not a house. The pipes between the treatment plant and one kitchen are not in any of it, and neither is the air inside a particular home. A home test measures what is actually there.
Why do two cities show the same results?
The record is kept by county, so two cities in the same county share it. The page says so when that happens.