Dallas-Fort Worth’s apartment market is cooling off, as occupancy rates fall for the third straight quarter to 93.6 percent, but the market is still in healthy shape, one expert says. The reason behind lower occupancy: A ramp-up in apartment construction and home sales. “Dallas-Fort Worth is one of only a handful of markets nationally where apartment development has already topped historic norms,” said Jay Parsons, the National Market Analysis Manager at Carrollton-based MPF Research, a division of RealPage Inc., wrote in an email Monday. At the end of the first quarter, Dallas-Fort Worth led the nation with 22,837 apartment units under construction, which exceeds the North Texas area’s average by more than 5,000 apartments, Parsons said. One of the most active submarkets is the downtown and Uptown submarket, where 3,518 apartments were under construction during the first quarter of 2013. That is the highest level of construction activity seen in this submarket since 1999, he said. North Texas’ rebounding single-family housing market is leading the country in home sales.