2:08 p.m. on August 4, 2010 (EDT)
Kinda along the same lines as others, but you have limited options when it comes to heavy metals and chemicals. Any filter with a avtivated charcoal core will remove chemicals, and some heavy metals, but not all neccesarily. The filter I use is a MSR Miniworks EX, which uses a .2 or .3 micron ceramic filter i forget the exact size, and has a charcoal core.
Activated charcoal will remove any heavy metal in the water that is bound to an organic source. So it's a 50/50/90 situation. Could be if your area that they are all bound and you can filter them out, or it could be that they are all free due to not having been in the water a long time and you wont be able to filter them.
I only use, and have only ever used filters that in some way use activated charcoal. They remove any bad taste, odor, bacteria, chemicals, some heavy metals.
Viruses! First off, if you in the US you are about 99% likely to never encounter a virus in the water unless you are hiking near a volcanic vent or in a jungle/true swamp where there is alot of deomposting material/animals in the water.
Secondly, viruses are like heavy metals, they will often bind to other organic compounds. In this case the filter will filter out these viruses. If you do some research you will see that in a large (about 85% or more)percentage of the locations where there are viruses in the water that they are bound to other organic compounds.
Bottom line is, use a filter with at least .3 micron, .2 is better. Activated charcoal core or element will remove any chemical, and alot of heavy metals/viruses.
Water Purification: Water purification can only be done by boiling, or using a UV treatment like a steripen, or a chemical treatment like bleach or aqua mira. These methods only kill bacteria and viruses and does not remove them, if that bothers you. These methods also will not remove or otherwise inactivate any chemicals or heavy metals.
I prefer to use my MSR Miniworks EX, and if i am really worried about the water containing viruses for whatever reason I would either boil it or add a tab of aqua mira in from my survival kit.
Hope this helps some, happy trails!