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Builders have climbed on the PEXCross-linked polyethylene. Specialized type of polyethylene plastic that is strengthened by chemical bonds formed in addition to the usual bonds in the polymerization process. PEX is used primarily as tubing for hot- and cold-water distribution and radiant-floor heating. bandwagon in droves. Cross-linked polyethylene tubing  is increasingly taking the place of copper in residential plumbing systems  for a variety of reasons: ease of installation, resistance to acidic water, and the virtual elimination of leak-prone fittings.
It all adds up to a juggernaut for a building material that's only been available in the U.S. since the 1980s.
But Arlene DiMarino isn't sure about the safety of PEX.
"I am very concerned about using PEX tubing for water supply in my home," she writes in a Q&A post  at Green Building Advisor. "I was told that this plastic was BPA [bisphenol A]-free but when I did some reading I found out that there have been some reports of MTBEs and some VOCsVolatile organic compound. An organic compound that evaporates readily into the atmosphere; as defined by the U.S. Environmental Protection Agency, VOCs are organic compounds that volatize and then become involved in photochemical smog production. [volatile organic compounds] leaching from this plastic." (To read similar Q&A threads, see PEX vs. copper , Water supply: PEX?  and Should I consider PEX? )
Her plumber is pushing for PEX. Should she go along?
Q&A: PEX vs. Copper 
This forum specializes in the sometimes arcane world of wall assemblies, thermal boundaries and moisture management, not plastics, suggests J Chesnut.
"The effects of the many varieties of plastics is not a subject that GBAGreenBuildingAdvisor.com can offer much insight," he writes. "I think it is an important topic but haven't found a source for information or discourse I feel confident in.
"BPA seems better understood than many other aspects of plastics in that it does enter a body by leaching into water," he adds. "I recently worked on a house for a physician where we had these discussions. The physician decided to take his chances with the copper supply lines instead of PEX, accepting the premium in cost."
Steve El says evidence pointing to any hazards with PEX is "slim to none," but he adds the same could be said at one point of lead paint, radon , asbestos , or the radium painted on watch dials to make them glow in the dark.
"I have no reason to think PEX is hazardous," he writes. "On the other hand, I have no reason to think it is not either. The experiment (purchase by end users) is only in its earliest stages. One can say the same about a long list of stuff, a lot of which is part of my daily life... for these reasons, I share your skepticism."
"As with most building materials today, it's not easy to determine the relative merits of copper and PEX for domestic water supply," writes Robert Riversong. "There are significant benefits and liabilities, including deleterious health impacts of both."
Here's a paraphrase of Riversong's advice: Copper should be soldered with lead-free solder, but soldering flux is not only toxic but also corrosive to copper. Copper is vulnerable to corrosion and pin hole leaks. Water turbulence in copper lines can increase the amount of dissolved lead and copper in the water, especially when the pH is less than 6.5.
When acidic or soft water sits in the line for more than six hours, the line should be flushed for up to 60 seconds before the water is used for drinking or cooking. And in these conditions, hot water should never be used for cooking or drinking.
PEX, on the other hand, is resistant to acids, better at resisting freezing damage, and doesn't scale or corrode. It's been used in Europe since the 1960s, and even "plastic-phobic" California embraced PEX.
One Achilles' heel with PEX, Riversong reports, it is chlorine resistance. "Even short-term exposure to sunlight can dramatically reduce the resistance of PEX to chlorine and result in premature rupture of the pipe," he writes. "Studies show just a one-week exposure to sunlight may reduce the chlorine resistance lifetime of some PEX pipes by half; with a two week exposure completely depleting PEX of any chlorine resistance.
"California’s January 2009 approval of PEX relies upon the less-protective PEX chlorine resistance standard ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. F2023, instead of the much superior NSF P171 standard," he says. "ASTM F2023 only assures an adjusted lifetime of 25 years, while the NSF P171 standard assures a 40 year adjusted lifetime."
PEX can't be melted and resued, an environmental drawback, and it produces toxic smoke when it burns. More troubling, the California lawsuit alleged PEX could indeed leach certain chemicals, including methyl tertiary-butyl ether (the MTBE referred to in the original post) along with ETBE, a related chemical, Riversong says.
Both PEX and copper are used in the homes of friends and family, writes Steve El, adding, "I don't know anyone who has suffered from either one."
"I don't know a soul with copper toxicity despite the long use of copper in those homes. We filter our drinking water at the tap. Supposedly the filter is good for copper. That's good enough for us."
An anonymous poster seems to agree: "No pipe made today will kill most of us before something else takes you off this planet," this person writes. "Find a new worry...Have you noticed any of your neighbors dropping like flies after living in a home with indoor plumbing? The stuff we waste time worrying about is nuts. "
And John Reimers adds this: "If the water service from the meter to the house is in PVC pipe, as well as the water main in many places, how is the importance of the internal house piping affected? Whether PEX or copper is used seems somewhat mute if the main service is already carried in one of the worst types of pipe."
Then again, suggests J Chesnut, we'd do well to learn more about the effects of chemicals in the environment. He calls it an "extensive and daunting topic" that could use the attention of a regular blogger here at Green Building Advisor.
"I'd love to see that too," replies Steve El. "One MAJOR problem is that there is practically zero funding to study how synthetic chemicals interact with each other AFTER they have been 'disposed' of (often by dumping, flushing, pumping, injecting, burning etc) in the wild. The philosophic perspective used by our rule makers in defining 'safe' does not usually take this into account."
Doug writes that lack of information is a problem, yes, but that the Pharos Project  of the Healthy Building Network might be a new source of information.
"Still, the likelihood of health effects will still be something of a guess, and will change due to any number of factors, so it's still a bit of a crap shoot. No harm learning more about what's in stuff and how toxic it's thought to be though."
Along those lines, Riversong tells of his uncle, a physician and professor of medicine, who had given up hope for a medical miracle as he lay dying of cancer. "The last thing he said to me -- a man who was one of the world's most dedicated medical doctors -- was, 'We just have no idea what we're doing,' " Riversong says. "And that's the simple and awful truth."
Here's what GBA technical director Peter Yost has to say:
The question of how safe PEX tubing is has an inherent counterpart question which is, How safe are all the alternatives to PEX? And as others have made clear in this discussion, there is currently and to the best of our knowledge no clear winner. But a couple or so things to consider:
There is a great Environmental Building News feature article entitled, “Piping in Perspective: Selecting Pipe for Plumbing in Buildings.”  In it, many of the issues discussed in this blog, and more, are covered.
And in subsequent EBN content on the topic of PEX piping it should be noted that bisphenol-A (BPA), while clearly an issue for plastics such as polycarbonate, is not clearly connected to safety concerns with PEX. Probably the “winner” for domestic piping coming out of this feature article is a relatively unknown option, Fusitherm’s polypropylene piping system .
One GBAer mentioned the Pharos Project and how it might be researching and evaluating the issue of domestic water piping and PEX. So far, this is not an area of building materials that Pharos has addressed. But Pharos is becoming a bit more connected to GBA, at least indirectly, through the brand new partnership between Pharos and BuildingGreen’s GreenSpec . And a new feature of GreenSpecPharos soon to be deployed is the ability of users/members to vote/weigh-in with the GreenSpecPharos researchers on which types of building products they should evaluate next.
As a result of this GBA blog discussion, GreenSpec editors have rewritten the domestic water piping overview on GBA . It’s nothing conclusive unfortunately, but it does suggest that since there is no clear winner for piping, your selection is likely to be driven by project-specific design and construction considerations. For example, I prefer piping that best accommodates structured plumbing design and efficient hot water flow; the ability and ease with which PEX permits large radius bends reduces turbulence and means more efficient plug flow of hot water.