Why are humans only able to use a limited about of the world's water?

Answer 1

we cant because they is so many chemicals in it. sometimes if u drink dirty water u will get really sick or even die. or even salt water like at beaches and stuff if u only drink that you'll get dehydrated because of the salt.

hope this helped

Answer 2

If by "use" this question means "have access to drink" the answer is simple. The majority of the worlds water is salt water which we cannot drink.

Of the remainder a large part is frozen, underground or too remote to access. That leaves the fish water easily available in streams and lakes or in relatively shallow wells.A portion of this is polluted or has taste and odour problems.

Well water is limited resource and once we empty the underground reservoir it is gone.