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.
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.
Not much is both water soluble and smaller than sodium. For the most part your basically left things like individual molecules of lead or other heavy metals which you would need to drink a lot of water from vary contaminated sources before you had a problem.
Ed: Individual sodium atoms are smaller than individual lead atoms. However, under the assumption that there is some variation in filtering capability's that will let a low % of things in that are sodium sized and slightly larger through you may have to deal with some larger atoms like lead.
It would have to pass through a fairly hydrophobic (the pore) layer. The water can barely get through since it's polar but uncharged; most metals in solution are charged ions and wouldn't be able to pass through the barrier.
It sounds like it pretty much just lets H2O through, but it's possible that other small molecules could as well. It certainly wouldn't allow for any sort of organism, but harmful elements could get in.