Since last Wednesday’s still-unexplained accident, which killed at least 114 people and injured more than 700, the Chinese government has repeatedly insisted that effective measures are being taken to ensure that the air in Tianjin remains safe. But when rain fell on Tuesday, the city’s streets began to foam, and people reported burning sensations on their lips and elbows. An environmental monitoring official denied that those phenomena had anything to do with the explosions, Chinese news outlets reported.