@bluedolphin-- Yes, there is seamless underwear made with cotton, but you're not going to find one that is 100% cotton. There has to be some synthetic material used because that's what gives the stretch to the underwear and causes it to be invisible. But you can find seamless underwear that has up to 70% cotton. The rest is usually spandex or a similar synthetic material.
I think that 70% cotton is fairly healthy. Not many women wear this type of underwear everyday. I certainly don't. I only wear them when the outfit requires it and I suggest the same to everyone. I agree with you that wearing synthetic underwear for a long time can be problematic.