Can you help me choose between a slab on grade and pressure treated wood foundation?

Where are you building where code says such a thing? I don't think I really understand the question so I would need more information. A pressure-treated wood foundation (PWF) would be an alternative to a concrete foundation, and a poor one at that. The end result of mixing wood and moisture will always be rot. So building a house on wood buried in the ground is not something we have or ever would recommend. 

And a slab on grade is an alternative to a basement, PWF or concrete, so I'm not sure where you're being advised to mix a PWF and a slab on grade, or how to even do that. A slab on grade floor consists of concrete and insulation (radiant heating tubes, mesh and rebar of course) but not wood. A wooden structure would sit on top, above grade where it can dry properly. 

If you are chemically sensitive then there are any number of materials that may go into a home that might cause you health concerns. We built a LEED Platinum demonstration home (the Edelweiss House) a few years ago (on a slab) and we were careful to choose healthy building materials such as zero VOC paints and finishes, formaldehyde-free plywood for cabinetry, recycled quartz counters etc.

If you have chemical sensitivities then your foundation isn't likely the biggest concern (though as mentioned we don' like the idea of sticking wood in the ground), we would recommend carefully choosing all interior finishing materials to ensure healthy indoor air quality. Below are a few pages to read over, and again, please explain a bit better what you are being told about foundations and we can probably help you. We are in the process of building another demonstration house in Wakefield, Quebec, it is from a prefab slab-on-grade form kit. The manufacturer (Legalett) has over 2 million square feet installed in North America and they have never been refused a permit. 

Here are a few pages that may help you build a home with safe indoor air:

• Clean indoor air quality
• Removing dust and moisture from homes
• Choosing safe paints and finishes
• Choosing eco-friendly sustainable counter tops
• Healthy home cleaning products

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