An estimated 1 in 5 U.S. households owe student loans, according to a study released Wednesday by the Pew Research Center. It's the largest number on record as college enrollment numbers grow during the economic downtown and tuition costs soar.

The analysis found that 22.4 million households, or 19 percent, had college debt in 2010. That is double the share in 1989, and up from 15 percent in 2007, just before the recession - representing the biggest three-year increase in student debt in more than two decades.

The debt burden as a share of household income was the greatest for the poorest 20 percent of households, or those making less than $21,044. The poorest 20 percent of households saw their debt grow, to 13 percent from 11 percent.

In all, 40 percent of households headed by someone younger than age 35 owed college debt, the highest share of any age group.

“Comparing the debt to their economic resources, the lowest-income fifth of households are the ones experiencing the greatest stresses,” said Richard Fry, a senior economist at Pew who analyzed the numbers.

More well-off families are digging deeper into their pockets to pay for costly private colleges, while lower-income people in search of higher-wage jobs are enrolling in community colleges, public universities and other schools as a way to boost their resumes, the analysis found. The Pew report found that the richest 20 percent of households, or those with annual income of $97,586 or higher, owed the biggest share of outstanding student debt - 31 percent, up from 28 percent in 2007.

The study is based on the Survey of Consumer Finances, conducted every three years and sponsored by the Federal Reserve. The numbers are as of 2010, the latest available for that survey. Separate Fed data have pointed to subsequent increases in student loans since 2010 that totaled $914 billion in the April-June quarter, but don't provide demographic breakdowns.

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