Cabinet boxes are mostly composite wood: particleboard, MDF or hardwood plywood. Federal rules under TSCA Title VI cap formaldehyde emissions from those panels, and products sold in the US must be certified to them. That's a floor, not a ceiling. If you want lower, ask for boxes made with no-added-formaldehyde (NAF) or ultra-low-emitting (ULEF) resins, and ask to see the documentation. Our guide to cabinet materials covers the rest.
Finishes matter as much as the box. A factory-cured finish has largely off-gassed before it arrives. A finish sprayed on site in a small room hasn't, and in January with the windows shut, you'll smell it.
Plywood or particleboard
Plywood holds screws better, survives water better and weighs less for its strength. Particleboard is flatter and cheaper. In Boulder's dry air, both behave as long as they stay dry. We'd use plywood for the sink base and anywhere near water, and either material elsewhere if every edge is sealed.
Questions for any Boulder cabinet company
Ask what the boxes are made of and how they're certified. Ask if the finish is factory-applied or sprayed on site, and what glue holds the edge banding. A supplier who can't answer has answered.