PVC tubing & its various connectors are widely available, and very useful for supporting row covers. You can literally just bend 10' sections into hoops & stick both ends in the ground, to support hoop tunnels. I use PVC cages over pepper plants, to exclude pollinators when growing a seed crop. The current PVC schedules are highly UV resistant, don't rot or degrade in moist conditions, and have smooth edges that won't tear the covering fabric. The cages, when pressure fitted into the connectors, are easily disassembled for storage. I've been using the same cages since 2005, with no sign of degradation.
PVC is also widely used for running permanent, low-cost water lines for irrigation. The community garden I once belonged to ran all of its buried water lines using PVC. If I have one reservation about that, it is with the adhesive & primer used for securing connections, not the PVC itself... I would flush those lines well before watering vegetables.
PVC produces toxic byproducts when burned or exposed to high heat, but those conditions are not commonly found in gardens. It is not toxic in its stable form. I have seen organic farmers using it for the hoop tunnels mentioned above.