Glass fiber - that's it! I couldn't remember the words (thus the "" around paper.)

Our scout troop used half a dozen of these filters, and never had any problem with an element tearing. We finally threw them all away because the filter element would clog easily (high sand and debris content in the water); of course, we rarely clogged an element, because the pump handles broke off before the first element could be used up. We finally trash-canned all 8, and switched to the MSR Miniworks - and I have yet to ever have a problem with that filter. Of course, the problem with the PUR handles may have been as much a function of 12-year-old boys as any manufacturing defect.