The fire at the Williamsburg warehouse ignited Saturday. It was still smoldering in spots Thursday morning when de Blasio visited.
Fire officials have received several complaints about a chemical smell in the area but so far an evacuation order has not been issued.
A stubborn warehouse fire made it a tough Saturday for emergency crews in New Jersey.
Albany officials say a big warehouse fire that has been billowing smoke across Interstate 787 and the Hudson River since Friday is too dangerous to fight, and will be allowed to burn itself out over several days.