The fire broke out at a multi-level house at 67 Union Avenue just before 3:30 p.m. and quickly escalated to a five-alarm blaze.
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Passaic Mayor Hector Lora said four firefighters suffered from heat exhaustion.
A mayday call was sent out after one of them went down on the second floor of the two and a half story home.
All four of the injured firefighters were taken to the hospital.
There's no word yet on their conditions.
Officials say one resident was rescued. They believe about 10 residents live in the home and have all been accounted for. The Red Cross is assisting the displaced residents.
Officials believe the cause of the fire was likely an air conditioning unit that overheated inside the home, between the first and second floor.
"In extreme conditions of high temperatures when we go beyond 90 degrees, it is often the case where families may leave their air conditioners on for extended periods of time," Lora said. "I would advise residents who are listening that it's important to give these resources a break."
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Fire officials say the humidity and extreme heat made it hard to fight the fire, and they had to constantly switch crews.
"The outside temperature is in the 90s. Humidity is at 80, 90%," Passaic Fire Chief Patrick Trentacost said. "It's probably the worst scenario you can put us in."
Two other concerns were that the adjacent house would go up in flames and power lines could catch on fire.
Firefighters were able to contain it though. It took more than 100 first responders to get the fire under control.
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