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Just over 13 million U.S. mortgage loans are currently underwater, meaning that homeowners owe more on the mortgages than the property is worth. The number is shrinking, but homeowners with negative home equity face a tough choice: stick it out or sell the house for less than they owe on it.

According to the latest data on negative equity from Zillow Inc. (NASDAQ: Z), more than 43% — some 22.3 million households — of U.S. homeowners are either underwater or they do not have enough equity in their homes to allow them to move. Home equity of at least 20% is typically needed in order for a homeowner to meet the costs of selling a home without a loss.

Zillow’s chief economist said:

Looking at the effective negative equity rate could explain why recent, healthy declines in the number of underwater borrowers haven’t yet translated into more homes for sale.

Rising home values eventually may overcome the negative equity for these homeowners, but waiting for home prices to rise can be enervating and risky.

Effective negative home equity rates (that is, underwater mortgages plus those with less than 20% positive equity) are highest in Las Vegas (71.5%), Atlanta (64.1%) and Riverside, Calif. (59.7%).

Zillow expects the number of underwater mortgages to fall from a current level of 25.4% to 23.5% by the first quarter of next year. That means another 1.4 million homeowners will once more get their heads above water. It is, indeed, a slow process.