What’s the cleanest big city in the U.S.? How about the dirtiest? In our July issue, we decided to find out, by analyzing data to score each of the 50 largest cities on air quality, water quality, industrial pollution (toxics), Superfund sites, and sanitation. Rather than just the cities themselves, we looked at metropolitan areas, which include surrounding counties and suburbs. (This can have a real effect on a place’s score; Chicago, for example, has excellent water but its score is brought down by problems in the outlying areas). Because we only looked at the 50 largest places, there may be smaller cities that are much cleaner or dirtier than these—and because the scores represent relative rankings, that a city comes in first or last in a given category does not necessarily mean it’s perfectly pristine or dangerously filthy.

HOW WE SCORED: Metro areas earned points on a scale from 1 to 50 in each category, with the numbers reflecting a city’s ranking next to the other places. Since it’s a 50-city survey, the cleanest in a category got 50 points. Next, to get a preliminary score, we weighted the five categories in terms of the health threat each one posed relative to the others (the weighting was further adjusted for cities with no sanitation data.) Water and air scores, for example, got twice the weight of sanitation scores, since it’s of little value to have pristine streets if what you drink and breathe are killing you. To get final scores, we averaged all the points for each city.

Here, then, is our list of the 50 largest metro areas in America and how they compare. Higher total numbers mean a cleaner city. Next, check out the most surveilled cities in America.