We have a fairly new poured two inch (second floor) slab of concrete that has cracks in it already.  We're going to put tile down in a few rooms and are trying to figure out what the most effective but least toxic (no voc) way to deal with the cracks would be.   There is about one crack per room that will be tiled at this point.

Should we use a ditra mat or is there a nontoxic product to use to fill the cracks? (House is for people who are chemically sensitive)

You don't want just a crack filler you want a crack mender to try to hold the different pieces of concrete together, hopefully there is steel in the floor. Most crack menders are epoxy that I think must be non-toxic when cured. Check with your local supplier.

We have a fairly new poured two inch (second floor) slab of concrete that has cracks in it already

What was the mix and what was it poured over?

Hairline cracks or BIG cracks?

hairline cracks - maybe 1/32 inch?
it's two inches of concrete that has no additives in it (healthy house construction) and radiant in it. I don't believe they used steel, and they did not put in joints (which they did in basement).

it's cured for over a year

do you have any name brands you suggest - the one the tile guy suggested was asphalt based, not epoxy

The Ditra is not going to do anything for the cracks in the slab, but use of it will prevent the cracks from being transferred to your new tile flooring. If your slab is adequately supported by structure and subfloor, then the cracks might be nothing to worry about.

What do you consider to be "nontoxic"?

I like the polyurethane self-leveling concrete sealant. It fills holes and cracks, but doesn't do much for the stability of a cracking slab other than to ensure that foreign substances don't get in the crack and make it worse. Almost everyone has a formulation.... Loctite PL, Quikrete, etc.

It's light gray and seems too thick out of the tube, but it penetrates cracks better than you think it will.

Thank you - nontoxic is basically as low voc as possible (little offgassing)

having same issues with finding good things to use for a shower pan that aren't as expensive as the Schluter Kerdi - which I love and is non-off gassing, but so expensive compared to what tile guy will do a custom pan for (basically $300 from him vs $1300 for Schluter liner/pan)

Spartan Chemical has some epoxy patching products for concrete with no VOC, probably can't find it at the big box store though.