Have you ever tried taking one of these filters apart? I know it seems kooky, but hey, if it's made in Mexico, but I'm paying $35 for 2 filter replacements every two months, I figured I'd better see what I am getting! So I industriously hammered apart my filter because I was intrigued about just what quality filter material there was (remember, I'm an engineer). And guess what? It doesn't look like that glossy diagram at all! The diagram Stage 3 as a 1/3 of the cross-sectional width, but it is only about 15 mm or .30 width. Stage 2 is labeled as a microfilter activated carbon, which in ordinary language translates into a solid charcoal core about 50mm wide. But the big surprise was Stage 1. They don't tell you what this "added layer" is anywhere, but it looks like fiberglass paper. I don't know what it was, but it was definitely papery and easily melted upon rubbing between fingers. At purwaterfilter.com, under the FAQs it states:

"The faucet mount, countertop and undersink cartridges are made of carbon and an active agent to remove lead. The carbon and active agent are bound in a block form."

That is pretty ambiguous for an explanation--even for nonscientists.