Construction employers added 6,000 workers to payrolls in May as the industry’s unemployment rate dropped to 8.6%, its lowest May level in six years, according to an analysis of new government data by the Associated General Contractors of America.

“Even with five straight months of construction employment gains, the industry remains vulnerable to sudden shifts in demand,” said Ken Simonson, the association’s chief economist. “Should Washington allow federal highway funding to come to a halt as predicted later this summer, the sudden loss of billions of dollars worth of demand would cost many construction workers, as well as workers from other segments of the economy, their jobs.”

Construction employment totaled 6,004,000 in May, the highest total since June 2009, and an increase of 188,000 or 3.2% from a year earlier, Simonson noted. Residential construction employers added 3,300 jobs on May and 105,600 (4.9%) over 12 months. Nonresidential construction firms added 2,700 employees since April and 82,000 (2.2%) since May 2013.

The unemployment rate for workers actively looking for jobs and last employed in construction declined from 10.8% a year earlier to 8.6% in May—the lowest May rate since 2008. Simonson noted that the unemployment rate for construction workers had fallen by more than half since May 2010, when it reached 20.1%. During that time, the number of unemployed workers who last worked in construction declined by over one million, but industry employment increased by only 480,000.