The Communist Party newspaper the People's Daily also reported that rescuers were attempting to remove 700 tonnes of sodium cyanide from the area late on Thursday.
However, Wen Wurui, head of Tianjin's environment protection bureau, told a televised briefing that harmful chemicals detected in the air were not at "excessively high" levels.
A team of 217 nuclear and biochemical materials specialists from the Chinese military began work at the site yesterday, the official Xinhua news agency said.
Thursday's detonation (11.30pm on Wednesday local time) at a chemical warehouse in the major Chinese port city left a devastated landscape of incinerated cars, toppled shipping containers and burned-out buildings.
The military specialists tested the air around the site for toxic gases, with rescue teams ordered to wear protective clothing in the vicinity due to the ongoing risk of leaking poisonous chemicals, Xinhua reported.
Environmental campaign group Greenpeace warned that substances from the site could be dangerous, saying it was "critical" that the potential toxins in the air were monitored closely.
A lack of answers as to what caused the blast has reinforced questions about standards in the country, where campaigners say lives are sacrificed amid a lack of respect for safety and poor implementation.
A panel of officials at a Thursday press conference were peppered with questions about what chemicals were in the tanks that exploded, but they refused to provide details, and the briefing ended abruptly with officials rushing off stage.
"Clearly there is no real culture of safety in the workplace in China," said Geoffrey Crothall, spokesman for Hong Kong-based China Labour Bulletin, which promotes worker rights.
Citing rescue headquarters, Xinhua said of the 50 people killed, 12 were firefighters.
An AFP reporter in Tianjin in the hours after the blast saw shattered glass up to 3km from the site of the blast, which unleashed a vast fireball that dwarfed towers in the area, lit up the night sky and rained debris on the city.
The blast site sits in a giant logistics hub more than twice the size of Hong Kong.
It hosts car plants, aircraft assembly lines, oil refineries and other service and production facilities.
Xinhua said 701 people were hospitalised, 71 of them in critical condition.
The blaze that followed the blast was brought "under initial control" late on Thursday, Xinhua cited the Public Security Ministry as saying, after 1000 firefighters and 143 fire engines had been deployed to the site.
Xinhua described the facility as a storage and distribution centre of containers of dangerous goods, including chemicals.
Executives from the storage centre's owner, Tianjin Dongjiang Port Rui Hai International Logistics, were taken into custody by police, it said.
China has a dismal industrial safety record as some factory and warehouse owners evade regulations to save money and pay off corrupt officials to look the other way.
Pollution killing 1.6m a year
Air pollution is killing about 4000 people in China a day, accounting for 1 in 6 premature deaths in the world's most populous country, a new study finds.
Physicists at the University of California, Berkeley, calculated that about 1.6 million people in China die each year from heart, lung and stroke problems because of incredibly polluted air, especially small particles of haze. Earlier studies put the annual Chinese air pollution death toll at 1 to 2 million, but this is the first to use newly released Chinese air monitoring figures.
The study blamed emissions from the burning of coal, both for electricity and heating homes. The study, to be published in the journal PLOS One, uses air measurements and then computer model calculations that estimate heart, lung and stroke deaths for different types of pollutants.
Study lead author Robert Rohde said that 38 per cent of the Chinese population lives in an area with a long-term air quality average that the United States Environmental Protection Agency calls "unhealthy".
"It's a very big number," Rohde said. "It's a little hard to wrap your mind around the numbers. Some of the worst in China is to the southwest of Beijing."
- AFP, AP
