Home-price growth continues to surge in 2021.

In June, home prices rose 18.6% on an annual basis, compared to 16.8% in May, according to the S&P CoreLogic Case-Shiller national home price index.

The figures mark the largest gain in the history of the index, which began in 1987. The high demand for homes has led to the historic increase in prices largely due to low interest rates. Home prices are 41% higher than at the height of the housing boom in 2006.

The strongest increases were in Phoenix (29.3%), San Diego (27.1%), and Seattle (25%). Only Chicago did not report higher prices in June compared to May.

Craig Lazzara, managing director and global head of index investment strategy at S&P DJI, said the price gains and the consistency of the price gains have been “extraordinary.”

“In June all 20 cities rose, and all 20 gained more in the 12 months ended in June than they had gained in the 12 months ended in May,” Lazzara said.

Home prices have continued to rise amid strong demand and low supply, which remains down 12% year-to-year in July, according to the National Association of Realtors.

Peter Boockvar, chief investment officer at Bleakly Advisory Group, has called the price increases “out of control, unsustainable, and unhealthy.”

Home sales have started to slow which could be a sign of price gains beginning to slow as a result.

Ally Home President Glenn Brunker told CNBC that data shows 45% of home buyers are delaying buying a home due to market conditions, with 29% of them citing the price and 20% saying homes are selling too fast.

Low rates are expected to keep prices strong and are not expected to rise in the near term, according to the Federal Reserve.