Cross-linked polyethylene, commonly known as PEX, is a popular material for residential and commercial plumbing systems due to its flexibility, cost-effectiveness, and resistance to corrosion compared to traditional metal piping. PEX is manufactured using a process that changes the molecular structure of high-density polyethylene (HDPE) to improve its durability and thermal performance. Since PEX transports drinking water, the public often has serious questions regarding its long-term safety, particularly the potential for chemicals used in manufacturing to leach into the water supply. This article examines the scientific evidence and regulatory position to provide a clear, fact-based answer to whether PEX pipe causes cancer.
Understanding the Chemical Foundation of PEX
PEX piping is created by permanently linking the polyethylene molecules together in a process called cross-linking, which forms a three-dimensional molecular matrix that enhances the material’s strength and temperature resistance. There are three main methods of cross-linking, designated PEX-A, PEX-B, and PEX-C. The PEX-A method, for instance, uses organic peroxides to generate reactive free radicals that facilitate the cross-linking reaction.
Concerns about PEX safety stem from the necessity of incorporating various chemical additives and initiators during the manufacturing process. These include peroxides, silanes, and stabilizers like antioxidants, which are required to create a robust and durable pipe. Initial worries focus on the potential for residual manufacturing byproducts and volatile organic compounds (VOCs) to migrate from the pipe material into the water over time.
Scientific Consensus Regarding Cancer Risk
Scientific evidence and regulatory oversight do not support the conclusion that PEX piping causes cancer when the product is certified and installed correctly. The primary assurance of safety for drinking water components in North America comes from the rigorous testing required under the NSF/ANSI Standard 61. This standard evaluates the health effects of materials that contact potable water, setting stringent limits on the amount of chemical contaminants allowed to migrate from the product.
The NSF/ANSI 61 standard requires a toxicological assessment to determine a safe daily dose for any chemical leaching from the pipe. This ensures that concentrations remain well below levels that could pose an adverse health risk over a lifetime. Certified PEX pipes must demonstrate that any leaching of compounds, including those potentially associated with cancer risk, falls within these established safety margins.
All plumbing materials, including copper and stainless steel, leach trace amounts of chemicals into the water, meaning the focus is on the quantity, or dosage, of the exposure. Studies have shown that newly installed PEX pipe may leach a compound like xylene, but the observed concentration is often thousands of times below the safety threshold established by the NSF/ANSI 61 standard. PEX products carrying the “NSF-61” or “NSF pw” mark have met these requirements.
Documented Effects on Water Quality
While the long-term cancer risk from certified PEX pipe is low, the material has been documented to cause non-carcinogenic changes to water quality, particularly in new installations. These effects are primarily aesthetic, involving temporary changes to the water’s taste and odor. Such issues are often linked to the initial release of residual volatile organic compounds (VOCs) from the pipe material.
Two compounds frequently detected are methyl tert-butyl ether (MTBE) and tert-butanol (TBA), which are byproducts of the cross-linking process or related to the material’s formulation. MTBE has a low odor and taste threshold, meaning small amounts can make the water smell or taste unpleasant, sometimes described as chemical or gasoline-like. In some initial testing, MTBE concentrations have been detected above the recommended taste and odor standards.
These leaching levels are temporary and decrease significantly, often falling below aesthetic thresholds within the first few weeks or months of use. Exposure to high levels of chlorine disinfectant or high temperatures can increase the rate of chemical migration. Proper flushing of newly installed PEX lines is recommended to quickly reduce the concentration of these initial leachable compounds and address temporary taste and odor concerns.