Financial markets had a rough week again as investors grew more concerned about the United States housing market. Much of the concern is about home loans to people with poor credit or little history of borrowing money. Most homebuyers are considered "prime," or a high-quality credit risk. Yet lenders have taken the risk of subprime loans because those buyers pay higher interest rates.

The number and complexity of nontraditional home loans grew along with the housing market. Now, that market has cooled. At the same time, many buyers, and not just in the subprime market, are seeing their monthly payments go up. Nontraditional loans often start with low payments for the first year or two.

The Mortgage Bankers Association reported this week on the condition of 80% of home loans nationally. It says 5% of all mortgage payments were at least 30 days late in the final 3 months of last year. But more than 13% of all subprime loans had late payments -- the highest level in 4 years.

By the end of last year, 14% of all home loans were subprime. Total mortgage debt in the United States was 10 trillion dollars.