Condensation? If the line is a radon vent it may be cooled enough by the air it is moving to cause condensation if it is exposed inside the house.

Thanks for the input, but now I'm confused. The schedule 40 PVC pipe I'm using is stamped DWV. But it's pressure rated to 280 psi, and its NSF pw certified (potable water). It's also ASTM D1785, which is the standard for pressure rated PVC pipe. I'm not clear on what is wrong with this. Is it just that I didn't buy it from a pool place? I know the there is a big difference with the fittings, DWV vs. pressure. I have the pressure fittings. Spa flex is rated at 55 psi. Obviously easier, but junk by my standards. I can't get myself to use that, I want to use rigid pipe.

Most people who decide to install security cameras at home now are using the preassembled kits made in China and marketed everywhere. The power wiring is 12vdc, and can be run just about anywhere - a dead short will result only in a camera not working, so the wire itself doesn't pose a hazard. That said, running ANYTHING through a dryer vent is a fail idea. The wire (or other obstruction) gives lint something to cling to, and the resulting buildup/clog is a serious fire hazard. If you run your camera wire through the dryer vent, I can rather guarantee you won't have an electrical fire because of it - but you WILL likely have a NON-electrical fire.

I suspect that the vent screens were placed first, simply for ventilation, and then they were tempting/convenient access points over the years to bring in gas/electrical with the least amount of work. It looks to me like the large holes, like the one where the gas line enters, were simply sloppy work when the line was put through, rather than the work of a rat. There is nothing wrong with having these services come through wire screens like this, but it will be a lot of work to get these off, replace them, and have it all repaired and looking neat. For example, how would you intend to deal with the gas line that goes through the center of the screen, meaning...how would you get a new, intact piece of screen on without undoing the gas pipes? Here is my suggestion: leave the screens in place and buy some matching wire to make neat patches. You can simply cut the patching pieces so that there are stubs of wire around the edges, and then these can be bent over the existing wire with a flat blade screwdriver and/or some needle nose pliers to hold them in place. If you don't like the idea of small patches, get pieces of wire mesh that are as large as the entire openings and then fix them in place over the wire in the existing openings. When you're finished patching, get some Rustoleum brand spray paint to make everything blend in and to deal with the existing rust.