700 tonnes of sodium cyanide were stored at the warehouse that exploded

 If a teeny tiny capsule of Sodium Cyanide will kill anyone in about 30 seconds you can see why people are worried. And likely this was in powdered form so when it was blown up it was and possibly is in the air, the wind, and if it has or will rain there or firemen used water to extinguish flames there it likely might also be already in the water table there as well. People have good reason to be scared. It is therefore likely when 700 tonnes of Sodium Cyanide explode that literally thousands of people over time might get sick and die throughout this now contaminated area. However, it is unlikely that this story will be properly reported because it would make the government look bad.

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Reports that 700 tonnes of sodium cyanide were stored at the warehouse that exploded sparked concerns that it could contaminate the water.
"If this issue isn't addressed soon, it could be even worse than the blast itself," said Weibo user Tao Weng WT, in a censored post.
Officials have denied reports that there were increased levels of sodium cyanide detected in nearby drains.
They have also said the area is sealed off and hazardous material was contained at the site, in response to journalists' queries on possible leakages.

Is there a risk of airborne pollution?

Worried residents have been wearing face masks

 to guard against air pollution

Reporters on Thursday asked if pollutants released into the air from the explosion posed a serious health risk.
They also asked if the pollutants would drift towards other cities such as the capital Beijing, just 100km (62 miles) from Tianjin.
But government officials visibly struggled to answer the queries. Mr Wen attempted to leave several times, but was stopped by reporters who continued peppering him with questions.
He eventually said that a south-westerly wind was blowing pollutants into the sea away from populated areas, and while the particles in the air were dangerous, "so far we have observed [the pollution] has not reached a very high level".
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China explosions: The fears and questions after Tianjin blasts

Even the statement about the stuff "blowing into the sea" is not reassuring, especially if your livelihood is making a living fishing in the ocean, because if you ate fish contaminated by the powder you likely might get sick or die too. However, it would be more likely the fish would die first and be floating on top of the ocean if they weren't first eaten by other fish. 

Once again the State of California (for good reasons) doesn't allow ANY Sodium Cyanide to be in California and hasn't allowed this for likely at least 50 years now because it is so very dangerous. In the 1970s I knew that only in Nevada was Sodium Cyanide allowed and even then it is a very controlled substance only allowed in mineral refining areas of the state.