In the words of musician Dave Matthews, "Don't drink the water..."
That's basically what Toyota is stating in regards to the exhaust water vapor coming from its fuel cell Mirai:
"We tested the health impact of drinking the water in a special lab. They said that compared to drinking milk, this drainage water has much fewer organic impurities."
That's according to Seiji Mizuno, the man in charge of designing Mirai's fuel cell stack. So, less impurities than milk in a controlled setting, but the environment in which Mirai operates determines what comes out:
"Depending on the place you are driving, some parts of the world might have certain issues, such as organisms like E. coli, which could be hazardous to your health. You never know what the quality of the air intake is."
The exit water is less acidic than beer, but it's not sterile, so Toyota strongly advises against drinking it. In fact, nobody at Toyota has ever drunk the water.
So, please..."don't drink the water."
Source: Automotive News