Mental health problems, addiction, childhood trauma, interaction with the criminal justice system and poverty all play significant roles in whether someone becomes homeless. But the primary reason? They can no longer afford rent.

For rent to be considered affordable, a person has to spend less than 30% of their income on housing costs. By that definition, none of the 1.09 million extremely low-income people in California, or those who earn less than 30% of the median income in their area, can afford to live anywhere in the state, according to a new study by the California Housing Partnership. Predictably, these financially strapped households can barely afford the state’s escalating rents, and are most at risk of falling into homelessness.

According to the study, those who make at least 50% of the median income can afford rent in three of California’s 58 counties, and those making at least 80% of the median can afford rent in 42 of 58 counties. Only those making the median and above can have their pick of any county.

Due to lack of production, California’s stock of “naturally occurring” affordable housing is dwindling, as shoddy, older apartments that used to house lower-income families are increasingly taken by higher-income tenants.

Government-subsidized housing has not filled the gap. That’s partly because doing so would be insanely expensive. The nonpartisan Legislative Analyst’s Office estimates that building new housing for every low-income Californian who needs it would cost $15 billion to $30 billion annually, That’s what the state currently spends on Medi-Cal, its massive program to provide health care to the poor, which covers a third of the state’s residents.