Unlike electrical or plumbing work, air-duct cleaning is not a licensed trade at the federal level (and in most states). That means your protection comes from consumer law rather than a licensing board — and the relevant federal authority is the Federal Trade Commission.
The FTC's consumer guidance specifically flags the bait-and-switch pattern that defines the low end of this industry: a rock-bottom 'whole-home' price in an ad or robocall that, once the crew arrives, balloons with per-vent fees, 'hook-up' charges, or a sudden 'mold problem' requiring expensive treatment. Deceptive pricing and false claims of this kind can be unfair or deceptive acts under Section 5 of the FTC Act.
For a homeowner, the practical protections are simple: get the full scope and total price in writing before any work starts, be skeptical of unsolicited cold-call and ultra-cheap offers, and verify a voluntary credential like NADCA membership rather than assuming a license exists. If you're misled, you can report the company to the FTC and your state attorney general.