[ohthanks]

I sell residential RO systems. Industry wide the tubing is typically LDPE and the housings/fittings are mostly polypropylene. There is no real practical way to build a residential RO system without using these materials.

It's a trade off between different types of plastic / chemical exposure. Not really ideal but I suspect nearly all sources of treated water will have traveled through a similar amount of plastic to get to you.

Copper is avoided on RO systems because the post membrane water typically drops 1-3 points in PH and will slowly corrode and leech copper into the treated water. All post membrane fittings and tubing should be plastic/stainless, cheap brass fittings with high lead content are the biggest concern. This can be resolved with post membrane PH amendment but you then create a failure point that can cause heavy metal exposure or leaks if it isn't maintained.

You could build a system with steel housings and braided stainless hose but the size/weight and cost are not practical.