**6. Conclusions**

Premise plumbing is a complex, temporally dynamic, and spatially diverse environment that is strongly influenced by pipe materials. Virtually all pipe materials have known benefits and/or detriments for OP growth. Plumbing materials are an important driver of the chemical and biological water quality parameters that influence the control of OPs and there are no silver (copper or plastic) bullets that will uniformly inhibit the growth of *Legionella* and other OPs under all circumstances.

Synthetic plastic pipe materials vary between type and manufacturer. They can act as a supply of organic carbon for the growth of microorganisms, but exert a lower chlorine demand and tend to form fewer scales that could provide more surface area for biofilm growth. Iron pipes supply nutrients for growth, exhibit a high disinfectant demand, produce hydrogen and other nutrients through corrosion, and tend to form thick scales with extremely high surface areas. While they may no longer be used in new construction, even short sections of pipe can a ffect an entire downstream premise plumbing distribution system. Stainless steel has few known e ffects on water quality, and correspondingly, OP control, perhaps because it is the least studied and is less commonly used as a result of its high cost. Copper pipes are known for their antimicrobial ability, but this is inconsistently realized in practice, and in some cases they seem to encourage OP growth relative to other pipes. Premise plumbing materials have a role to play in preventing OP infections and, at a minimum, should be examined more

closely for their propensity to inhibit or stimulate OP proliferation during outbreak investigations. Research is needed to better define:


An improved understanding will provide actionable advice for multiple stakeholders. In addition to the obvious direct use of the results in the construction industry and by building water quality managers, water utilities can benefit from improved understanding of how the interplay of premise plumbing pipe materials with disinfectants, nutrients and corrosion control can be harnessed to reduce disease incidence.

**Author Contributions:** A.C.C., R.L.M., and Y.S. conceived the outline of review, performed literature review, and prepared the manuscript with equal contribution. K.W. and A.W. provided initial ideas on the key sections and feedbacks on review paper. M.A.E. and A.P. supervised the project and assisted in the manuscript preparation. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by NSF RAPID Grant No. 1556258, NSF SusChEM GOALI No. 170673, NSF CBET 1336650, the Alfred P. Sloan Foundation Microbiology of the Built Environment program, an NSF Graduate Research Fellowship to Abraham Cullom (Grant No. 1840995), and funding from the Copper Development Association Inc. (CDA), McLean, Virginia, USA. Part of the article processing charge was subsidized by Virginia Tech Libraries.

**Conflicts of Interest:** The authors of this research have research support from the Copper Development Association Inc. There is no other conflict of interest to disclose.
