A fire that broke out at a Kearny manufacturer of swimming pool chlorine in May proved unusually difficult to extinguish, repeatedly rekindling over the course of several hours after firefighters thought it had darkened down for good.

On Wednesday, nearly two months after the fire had finally been put out, the Alden Leeds plant on Kearny’s industrial waterfront was burning once again.

The more recent blaze was quickly put out and had not flared back to life as of Thursday afternoon, but the state’s leading environmental group says this second fire since May underscores the danger the plant poses to the public.

“This terrible situation in Kearny is getting worse, with two fires in two months at a dangerous chlorine plant. There needs to be an investigation into why these fires are happening, putting people and communities at risk. The larger blaze in May was even more threatening and could have been much worse. We were somewhat lucky again that damage from the latest blaze was minimized, but these fires have to stop. Chlorine gas is poisonous and can burn the eyes, nose and skin and make it difficult to breathe,” Jeff Tittel, director of the New Jersey Sierra Club, said in a statement.

“We have seen problems with the Alden Leeds plant before when there was a huge fire there back in the 70’s. What’s even worse is that New Jersey has hundreds of facilities with even more hazardous chemicals. We need to hold the Alden Leeds plant accountable and make sure these fires don’t keep happening. It is too important for the safety and health of the community.”

Chlorine stored in solid form presents a unique challenge to firefighters. Copious amounts of water may snuff out a fire involving chlorine, but the highly reactive substance can easily reignite once it dries out.

The burning chlorine from May’s fire forced the closure of the nearby Pulaski Skyway and led to air-quality warnings for residents throughout the area, including in Bayonne, Jersey City and New York City.

Alden Leeds remained an active scene until June 11, the Kearny Fire Department said. Air-quality monitors were installed and a cleanup was finished by that date, the department also said.

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