CONYERS, Ga. — More than 90,000 residents east of Atlanta were told to keep sheltering in place Monday a day after a chemical plant fire sent a massive plume of dark smoke high into the sky that could be seen for miles.

The haze and chemical smell had spread to Atlanta by Monday morning, prompting firefighters to use detectors to check the quality of air in various parts of the city, Mayor Andre Dickens said.

Closer to the source of the fire, officials said chlorine, a harmful irritant, had been detected in the air from the fire at the BioLab plant in Conyers, the Rockdale County government said in statement early Monday. The plant is about 25 miles southeast of downtown Atlanta.

“For everyone sheltering in place, the best practice is to turn the air conditioning off and keep windows and doors shut,” the statement said.

In Atlanta, officials said they believe the hazy conditions and chemical smell is “related to the BioLab fire, but why we are seeing the change in conditions is what we are attempting to figure out.”

“Latest plume modeling indicates it moving to the northeast, which it is clearly not,” the Atlanta-Fulton County Emergency Management Agency said in a statement.

The fire was brought under control around 4 p.m. Sunday, officials said.

The fire ignited when a sprinkler head malfunctioned around 5 a.m. Sunday, Rockdale County Fire Chief Marian McDaniel told reporters. The malfunction caused water to mix with a water-reactive chemical, creating the plume.

BioLab’s website says it is the pool and spa water care division of Lawrenceville, Georgia-based KIK Consumer Products.