ENR has an article on this today. Here's a paragraph from it:
The board also called for removing exemptions in 31 states—including Alaska, Arizona, Connecticut, Maine, and New York—so that all future natural gas infrastructure projects require licensed professional engineer approval and stamping. Massachusetts only adopted such a requirement after the disaster. “It’s important to note that in all 50 states you have to have a license to cut hair, “said Bruce Landsberg, NTSB vice chairman, “but you can design a gas system that affects public safety and 31 states exempt you from having a license or even an engineering degree.”