I really like button down shirts and would like to add a few short sleeve or sleeveless for summer. I usually don't iron, so I'd like to find some iron-free versions. But I've noticed most of them are "treated". Can anyone give me an idea of what that means?

I definitely don't want questionable chemicals on my clothes. I don't even dry clean for that reason (we have a green dry cleaner in town, but it's not convenient). Any thoughts on no-iron shirts and if they're safe? Or maybe I should just stick with poly or rayon blends.