Is marine polyurethane safe to use on indoor hardwood floors? Might it have too high a level of mildacides for safe indoor use?

Marine, or Spar Varnish, is NOT good to use indoors. It can/may be used but it is NOT very hard or durable. The properties that make it ideal for outdoor use make it impractical for indoor use mostly. Spar varnishes are softer and more elastic to be able to withstand the the expansion and contraction of wood outside. They also contain UV inhibitors to help them hold up better to exposure to sunlight. Some may contain mildecides but that should no matter for interior use. You are best off using a finish specifically designed for wood floors.

Thank you to Gary for responding. The can said Marine Polyurethane not varnish. I don't know the brand as he is finished. I'm concerned because my hardware store paint specialist said outdoor finishes have 10 times the allowed limit of indoor products of mildacides and they emit over time. But some polys listed as marine are for in or outdoor use.

If I were you, I would contact the person who did the work and ask them EXACTLY which product was used in the home. Then you would be able to see IF it was harmful or not by contacting the manufacturer.