The Merrick Fire Department and the Nassau County Hazardous Materials Response Division were called to the Merrick Post Office on Merrick Road after a light, odorless haze filled the building around 6:30 a.m. on Thursday.
Merrick firefighters were first on the scene. Because the nature of the haze was unknown, the hazardous materials division was called in, said Ron Luparello, the department spokesman.
"We took all necessary precautions," he said.
Officials searched for the source of the haze for almost two hours. As it turned it, it was a freon leak in an air-conditioning unit in a mechanical room, Luparello said.
Traffic was tied up on Merrick Road during the morning rush. Firefighters started cleaning up around 8:30 a.m.
The North Merrick Fire Department and Nassau County Police were called for assistance.