There are two sides to growing income inequality: stratospheric incomes among the economic elite, and wage stagnation among low and middle-income workers. Almost a third of U.S. workers (41.7 million people) now earn less than $12 an hour. Nearly half (58.3 million) earn less than $15 an hour.

A new report from Oxfam America and the Economic Policy Institute, a left-leaning Washington, D.C., think-tank, maps where these workers are concentrated and what we might do to improve people’s livelihoods. It notes, for example, that the federal minimum wage ($7.25) hasn’t increased in seven years and that the Earned Income Tax Credit–the federal government’s most important anti-poverty program–covers only a small sliver of families

Arkansas, Idaho, and New Mexico have the highest percentages of low-wage workers: in each, more than 39% of the workforce lives on less than $12 an hour. That’s about $24,960 a year–roughly the federal poverty level for a family of four. In general, there are more low-income workers in the South, and less in the Northeast (Massachusetts has the lowest concentrations).

The main map here shows the effect of raising minimum wages to at least $10.10 an hour. Overall, 25 million workers would benefit, with the greatest concentrations in places like southwest Arkansas’s Fourth Congressional District and and California’s 21st, in the San Joaquin Valley.

Several states have raised their minimum wages recently, notably California and New York, which are planning to gradually get to $15 an hour. But in 21 states, including many in the South, the federal minimum is the going rate. That may be okay for a teenager living at home, but in fact most people on low-wages don’t fit that profile. Three quarters are 25-plus and 40% are over the age of 40.

“Sadly, the U.S. is at the bottom of the heap of rich countries when it comes wages and benefits: We have the highest percentages of low-wage workers, and the lowest number of required paid sick days,” says Jeffrey Buchanan, a senior policy adviser at Oxfam America, in a news release.

“We need to do better by our low-wage workers and their families. Not only could these changes benefit millions of people, but they could bolster incomes and purchasing power to help our economy as a whole to thrive.”