You WILL see minute cracks where the rails and stiles meet on an all wood door, as well as where the panel is inserted into those stiles and rails. That's just how wood behaves in the presence of moisture--including your wood floors. They too will gap in the winter and potentially cup in the summer. It can be controlled a bit more if you use AC in the summer and use a humidifier in the winter, but nothing on the planet will stop it. Choosing wood stiles and rails with MDF panel will stop most of the cracking at the joint where the panel meets the stiles and rails, but it won't stop the cracking where the stiles meet the rails. If the natural expansion and contraction of wood will drive you crazy then don't get wood. Get an all MDF door that is routed to resemble a recessed panel door or raised panel door or whatever you are looking at. And then choose thermofoil if you want a "perfect" paint job too. Because even sprayed MDF will still not be 100% perfect. It's about as good as you get for smooth paint, but thermofoil is smoother and more even still.

Ikea follows REACH guidelines which is the EU standard on the use of chemicals and I assume as they don't advertise it that their products don't qualify for LEEDS or GreenGuard Certification. Many composite board manufacturers follow ANSI standards which limit the amount of formaldehyde content (whether that is a higher or lower standard than the one achieved by Ikea I don't know) Ikea environmental statement: "Decreasing product emissions and formaldehyde use IKEA is concerned about product emissions and formaldehyde use. We have a long-term vision that emission levels in wood based products shall be at the same levels found in natural wood. For instance; we have investigated methods in order to replace formaldehyde in almost all glue applications. IKEA has decided on a new formaldehyde emission standard level, which will be implemented during 2008. The new emission standard means that we will decrease current limit levels by half in all wood based board. The new level will be comparable to half of E1 (the European Formaldehyde standard level, which is considered to not be hazardous to health)." Attached link indicates that paint finishes are much more effective at ceiling in emissions than other applied finishes but I don't know the science behind the statement. In part I think it is the complexity of the machining. I think even in wood to get square inside corners you need pass each routered edge separately (don't quote me). For those rounded edges the routered edge is probably done in one continuous pass (much quicker and cheaper). Here is a link that might be useful: info on sealing MDF

My hubby designed and built all of our cabinetry for our kitchen remodel. The cabinet doors are actually solid maple for the shaker style frame part and the inside of the panel is actually maple plywood. The plywood is still a floating panel inside the door part... that gets kind of technical about the building procedure, but it was cheaper than using solid maple that he would have to join... He sprayed the paint on after the doors were fully assembled. You can see in the closeup photo that there are some hairline cracks in some of the seams where the shaker frames meet. I can honestly say they don't bother me a bit... they're not at every seam, but they are here and there. To me it is just a reminder that we have real wood under that paint and it's part of the character of the kitchen. But you would have to judge that for yourself, what bothers you is likely different than what would bother me. Our kitchen is a mix of solid wood and plywood, but my hubby refused to work with MDF. (The cabinet boxes are also maple plywood, the insides of my cabinets and doors are left a natural color with just a clear shellac coat on the inside.)

MDF is fine depending on the brand of cabinets. I'm assuming they mean just the center panels are MDF? I'm in SW Florida and went with all wood because that's what the Amish cabinet maker I used uses. I'm not quite in the house yet, but I knew going in that there would at some point be some hairline "cracking" of the joints. I'm fine with it as to me it's part of the beauty of wood. YMMV.