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By Phil Helsel

A firefighter trapped for more than 31 hours was rescued from the blast site in the Chinese port city of Tianjin, according to a city official and state media.

At least 56 people — including 21 firefighters — were killed when two explosions erupted at around 11:30 p.m. Wednesday local time. Several firefighters are reportedly still missing.

The firefighter rescued Friday was identified by state media as 19-year-old Zhou Ti. China's state broadcaster CCTV said he received treatment for burns, smoke inhalation and an injured leg.

"Forces from all sides are searching for the (remaining) missing firefighters," Tianjin Fire Department head Zhou Tian said at a news conference Friday, according to The Associated Press.

State news agency Xinhua said that a total of 701 people remained hospitalized after the blast and that more than a dozen firefighters were still missing.

Following the blasts, many residents kept their windows and doors shut after officials reported dangerous toxins in the air.

The air pollutants — including methylbenzene, methane, epoxy ethane — were "basically at normal levels" by Thursday afternoon, Nankai University professor Feng Yin Chang told reporters on Friday.

However she said that it was still "not very clear" which chemicals had been involved in the initial explosion.

A 200-strong nuclear, biological and chemical team from the Chinese army was drafted in to test for toxins surrounding the site.

There were also concerns from foreign experts that the firefighters' use of water may have reacted with one of the chemicals being stored at the factory, contributing to the explosions' ferocity.

David Leggett, a chemical safety expert based in California, told Reuters that because calcium carbide reacts with water to create acetylene, a highly explosive gas, this could have caused ammonium nitrate, another chemical believed top be present, to detonate.

"In my mind, the presence of ammonium nitrate makes it easier to explain the level of devastation," he told the news agency.

Lei Jinde, the deputy propaganda department head of China's fire department, told state-backed news website that firefighters "knew there was calcium carbide inside but we didn't know whether it had already exploded," according to Reuters.

"At that point no one knew, it wasn't that the firefighters were stupid," he said, according to the news agency.

Tianjin is the world’s 10th busiest container port in the world, according to the World Shipping Council, and in 2013 handled nearly as much volume as the port of Los Angeles and the port of New York and New Jersey combined.

Reuters contributed.