The nationwide affordable housing crisis is nearing a record high: More than 8 million renters in 2015 had “worst case housing needs,” according to a report released last week by the Department of Housing and Urban Development (HUD).

Very low-income, unassisted families who pay more than half their monthly income for rent and/or live in severely substandard housing are labeled as worst case needs residents. The Worst Case Housing Needs: 2017 Report to Congress reveals that in 2015, 8.3 million households had worst case needs, a 66 percent spike since 2001 and a number approaching the record high of 8.48 million in 2011.

According to the report, cases “cut across all regions of the county and include all racial and ethnic groups, regardless of whether they live in cities, suburbs, or rural areas.”

Most of the nation’s very low-income renters—those who earn less than 50 percent of Area Median Income—reside in the South (6.7 million), followed by the West (4.5 million). The areas with the highest concentrations of worst case households among very low-income renters, however, were in major urban areas: the New York metropolitan area, the Los Angeles metropolitan area, and the Chicago metropolitan area.

HUD’s report revealed that while ongoing economic recovery will help increase incomes for very low-income renters, other factors continue to drive the affordable housing crisis. The report cites severe rent burden—those paying more than 50 percent of their income towards monthly rent—as one of the primary factors. Out of the households with worst case needs in 2015, 98.2% had severe rent burden.

The other main cause includes a scarcity of units with affordable rents. Despite an increase in overall rental units and in median renter’s income over the past two years, monthly rents also increased and the shortage of affordable and available units for this population became more severe. For the poorest renters, rent hikes outpace income increases, according to the report.

Nationwide, only 66 affordable units exist for every 100 extremely low-income renters, and of that, only 38 are available for occupancy.