As the clean-up work continues at the site of the massive explosion in Tianjin, rain has brought a new challenge.The fear has been that it could activate some of the chemicals still scattered around the site,leading to new explosions and more environmental issues.Environmental officials estimated the volume of polluted water in the crater left by the blasts as "tens of thousands of tonnes."Phenol, a volatile, toxic chemical, has been detected in water samples collected near the Port of Tianjin for the first time.But at the same time, levels of the dangerous chemical sodium cyanide have been decreasing.When it mixes with water it can create a fatal gas.Authorities are also closely monitoring the air quality as they fear rain may set off reactions with the scattered chemicals and release toxic gases into the air.So how big is the problem of hazardous chemicals left at the site?And what can be done to prevent further harm to residents and the environment?