Typical solid-core doors are built over a chipboard core that is often bound by formaldehyde adhesive. Now, there's an alternative: Humacore Wheatcore Doors.
Even better? They're made out of wheat!
Bonus: wheat chaff is considered industrial waste from the mass production of grain. It is typically burned. More resources: • This blog has more on wheatboard. Note that Dow no longer manufactures wheatboard in sheets: the company now making the doors bought the factory from Dow. • The greenest solid core door? A salvaged one. Here's how to find one. • The door we spied at Portland's Environmental Building Supplies (soon to be ecohaus) was finished with OSMO Polyx Oil... read about that and other natural finishes here.