More than 1,500 people were evacuated after Friday's earthshaking explosion at the Caribbean Petroleum Corp. in the suburb of Bayamon, just west of the capital of San Juan.
Only a few people were slightly injured by the blast, which broke windows, tore doors off their hinges and shook the ground. But authorities are now concerned about those downwind of the fire, with breezes pushing smoke toward more populated neighborhoods.
Authorities urged residents - especially those with respiratory problems - who live near the site to temporarily move elsewhere until the fire is extinguished and the smoke brought under control.
"To those people with respiratory problems, our recommendation is that you not wait," Gov. Luis Fortuno said.
Several people have sought medical help for respiratory distress, and one firefighter was treated Saturday for exposure to high temperatures, Fortuno said. Those who were evacuated and an additional 530 people who sought shelter away from home have not returned.
Fortuno said a large stadium has been prepared to accommodate some 30,000 people who live in the area if necessary.
Crews were monitoring the situation and anticipated more evacuations if weather worsened.
"The smoke is extremely toxic," said Jose Bartolomei, a state epidemiologist who monitors asthma. "An asthmatic patient will definitely hyper-react to this."
Adding to the danger is the presence of invisible gases such as carbon monoxide and sulfur, said Luis Antonio Ocasio, spokesman for the island's Environmental Quality Board.
Local officials, along with a New Jersey crew from the U.S. Environmental Protection Agency, are monitoring the air with handheld devices and probing whether any fuel spilled into the nearby ocean, he said.
The cause of the explosion is still under investigation. Fifteen of 40 fuel tanks caught fire, and five were still burning Saturday afternoon, Fortuno said.
He has declared a state of emergency for five municipalities, including San Juan and Bayamon, but said initial estimated damages of $6.4 million are not sufficient to receive federal funds.