[WiseWeasel]

The pore size is very slightly bigger than a single water molecule, and smaller than a single sodium ion. Heavy metal atoms are much larger than sodium atoms, and would not go through the membrane. Any kind of virus or bacteria is much, much larger than this molecular scale, and have no hope of traversing. I am not aware of any harmful water-soluble substances as small as a water molecule.

[ScottBurson]

Hydrogen fluoride comes to mind, but it's probably not an important pollutant in most of the water sources we care about.

I can't think of anything else that small that would be a problem.

[WiseWeasel]

Wouldn't hydrogen fluoride just be fluoride ions in solution? Water molecules tend to clump around charged ions, which might impede traversal of the membrane.