The apartment-rental market cooled in the first quarter, according to reports from three research companies, suggesting a six-year boom that has pushed the cost of housing to unaffordable heights in many U.S. cities might be coming to an end.

The national vacancy rate, which has risen for three consecutive quarters, hit 4.5% in the first three months of the year, up from a recent low of 4.2% in the second quarter of 2015, according to market research firm Reis Inc.

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