Pretty close to all of it. There are some chemicals that are tough to find better versions of, such as the ones that we use to bond water pipes, and some of the caulking and construction adhesives. Luckily, these all cure in short amounts of time, and are typically used in small amounts, thus making their health dangers very limited in duration and location. That being said, the vast majority of a house can be built using safe, healthy materials; materials that have no negative health consequences. I would think of it like this: let’s start building our homes in such a way that you wouldn’t think twice about your kids being around the construction process, and wouldn’t hesitate to have them put their mouths on anything that was finished. If you’re not willing to let your child touch something being used to build your most important environment, what does that say about whether you should use it?