One of the most discussed strategies for accelerating hydrogen energy deployment is blending hydrogen into existing natural gas pipelines. The logic is straightforward: the natural gas pipeline network covers approximately 3 million miles across the United States; hydrogen pipelines total roughly 2,011 miles. Rather than building new dedicated hydrogen infrastructure from scratch, blend hydrogen into the existing network and use it where it can substitute for natural gas in heating, industrial processes, or power generation.
The strategy has real appeal, and pilot projects have demonstrated it is technically feasible at low blend percentages. But the GAO’s April 2026 technology assessment details a set of material challenges that substantially complicate the picture — and that have been noted in engineering literature for decades without being fully resolved.
What Embrittlement Is
Hydrogen embrittlement is the process by which hydrogen atoms penetrate into metal structures and degrade their mechanical properties. When hydrogen diffuses into steel, it reduces the metal’s ductility and fracture resistance, making it more susceptible to cracking under stress. The effect is more pronounced under higher pressure, in higher-strength steels, and in the presence of existing flaws or stress concentrations — all of which are common in long-operating pipeline infrastructure.
The consequences are concrete: acute pipeline failure risk, reduced service life, and compromised structural integrity at joints, valves, welds, and fittings. The GAO report notes that the construction and operational history of aging natural gas pipelines is often incomplete or poorly documented, which makes systematic risk assessment for hydrogen blending difficult. You cannot evaluate embrittlement risk for a pipeline section if you do not know what steel grade was used, what welding practices were followed, or what stress history the pipe has accumulated over decades of service.
Which Pipes Are Most at Risk
The embrittlement risk is not uniform across the pipeline network. High-pressure transmission lines — typically built from higher-strength steels — are most vulnerable. Higher-strength steels are generally more susceptible to hydrogen-induced cracking than lower-strength materials. This is the opposite of the intuitive expectation: stronger steel is more brittle in the presence of hydrogen, not less.
Distribution lines, which operate at lower pressure and are typically built from lower-strength steels or plastic pipe, carry significantly lower embrittlement risk. Some plastic distribution pipes are essentially impermeable to hydrogen. Hawaii’s gas utility has used a synthetic natural gas blend containing up to 15 percent hydrogen in distribution and transmission pipelines continuously since 1970 — but its system characteristics and operating conditions differ from the majority of continental U.S. infrastructure.
Dedicated hydrogen pipelines are designed specifically to manage embrittlement risk: they use selected steel grades, apply rigorous quality controls on welding and inspection, and are operated within design parameters that account for hydrogen’s behavior. The existing natural gas pipeline system was built to none of those specifications, because it was built for natural gas.
Blending Projects and Regulatory Status
Hydrogen-natural gas blending projects have been undertaken or announced in California, Colorado, Hawaii, and New Jersey, as well as across the U.S.-Canada border. Results are mixed. California conducted a pilot delivering a 20 percent hydrogen-natural gas blend in a closed-loop residential system in 2021 and commissioned a comprehensive safety and operational study. Colorado announced plans for up to 10 percent blending in 2023, then delayed and subsequently cancelled the project following a state utilities commission decision. New Jersey has been researching injection of less than 1 percent hydrogen into a distribution pipeline since 2021.
Federal regulatory jurisdiction over blending is unclear. The Federal Energy Regulatory Commission has authority over certain natural gas pipeline operations but has not determined whether that authority extends to hydrogen-natural gas blends. The Pipeline and Hazardous Materials Safety Administration published a request for public comment on blending reporting requirements in March 2024 and is analyzing responses. No comprehensive federal blending framework exists.
The lack of standards extends to measurement and detection. Leak detection technologies designed for natural gas may not function correctly if hydrogen is introduced into the pipeline at significant concentrations. Metering systems designed to measure natural gas by volume or energy content need recalibration for blended streams. Compressor stations that use pipeline gas for their own operation face combustion challenges because hydrogen and natural gas have different flame speeds and combustion characteristics.
The Microbe Problem in Underground Storage
A less-discussed but significant challenge applies to underground hydrogen storage in depleted natural gas reservoirs. Various microorganisms — bacteria that consume hydrogen and produce methane or hydrogen sulfide — are naturally present in underground geological formations. They can reduce the purity of stored hydrogen, alter its chemical composition, and affect how it can subsequently be used. The specific microbial communities in candidate storage sites are difficult to characterize because many underground microorganisms cannot be cultivated in laboratory conditions.
This problem does not affect salt cavern storage, which is the only method demonstrated at commercial scale for hydrogen. Salt caverns are inhospitable to most microbial life. But proposals to expand underground storage into depleted oil and gas wells — a potentially vast resource — face this biological complication in addition to the geological and sealing uncertainties inherent in any repurposed well system.
None of these problems is presented in the GAO report as insurmountable. The report documents active research programs at national laboratories addressing embrittlement, storage integrity, and detection technology. NASA has safely managed hydrogen infrastructure for decades. Industrial users have developed handling protocols that make large-scale hydrogen operations viable. The engineering community understands the challenges. What does not yet exist is the sustained investment and regulatory framework needed to translate that understanding into safe, large-scale deployment of blended or pure hydrogen through existing infrastructure. That gap is where the policy work has to happen.