Health care reform is heavily dependent on the people who use the fewest medical services: namely, young healthy adults who, until now, haven't bothered to get insurance at all. This cohort, between the ages of 19 and 34, has the highest share of uninsured in the United States:


Census Bureau

That's about 18 million people, and it makes up 40 percent of all of the uninsured in the country younger than 65. Starting next year, as these healthier adults start to buy insurance, they should theoretically help hold down the costs for everyone who heads to the hospital more often.

But as with many aspects of America's health – consider the uneven distribution of Medicaid coverage, of chronic diseases, of uninsured children – these uninsured young adults are geographically clustered, too. This map from the Census Bureau, based on data from the 2008-2012 American Community Survey, shows that geography by state:

More interestingly, here it is by metropolitan statistical area (using the same data):

The difference is particularly stark between Boston and the Miami/Fort Lauderdale region. Of the 25 largest metro areas in the country, Boston has the lowest young-adult uninsured rate, thanks to the Massachusetts health care law (7.9 percent). Around Miami, 41 percent of all adults between 19 and 34 have no insurance.

Some of the green patches in the South are notable, too – many of them are college towns, like Gainesville and Tallahassee in Florida, and College Station in Texas, where a large share of college students likely remain on their parents' coverage.

Another way to look at this map is that these are the places where the new insurance mandate in the Affordable Care Act will be felt the strongest by the cohort of young adults who've never bothered to get coverage before.