Grade 1 is the least.
This means that the content of substances that will cause harm if ingested in water is the lowest (of course, if you go into detail, the story is a bit different).
That is
Level 1 water is rather dangerous –
Even though grade 1 water is safe, it is relative and can be dangerous, so you should drink it mixed or filtered (whether natural or commercially available)! -ㅇ
You can think of it like this.
For reference, mineral water or natural mineral water can be thought of as being naturally filtered by passing through stones and sand underground.