Once I knew what to look for, the same three things showed up everywhere.
One. More than one stage.
A single screen has to do every job at once. Catch the mud, catch the grit, catch the small stuff. So it plugs up fast.
Stages split the work. The rough stuff gets caught early, so the fine membrane at the end stays clear and keeps working.
Two. How fine the membrane actually is.
This is where the specs get boring, so I'll keep it short.
Most hiking straws filter at 0.2 or 0.1 microns.
A human hair is about 70 microns across. So we're already talking about gaps you can't see.
The finer membranes go down to 0.01 microns. That's the gap that catches bacteria and protozoa, the bugs that put people in the hospital after a flood.
Worth saying plainly: no straw like this filters out viruses or chemicals. More on that in a second.
Three. Whether it still works after sitting for years.
This one I hadn't thought about at all.
Anything with a chemical in it degrades. Anything with a battery in it dies.
Pure mechanical filtration has neither. Nothing to expire on a shelf.
You store it, you forget it, and it's the same tool in ten years that it was the day you bought it.