I am going to use MonsterMaker's foam latex to cast characters this time. I used GM's before. The quality is good but the smell of ammonium (or some other chemicals) made my throat painfull each time. I think MM's has the smiliar smell when working and baking on it. I think a respirator may work. I checked in hardware store and found they all for welding or painting purpose. Can somebody make a suggestion of a respirator?
What you need is a fume mask, which filters out the nasties.
But you can go a long way to making life better for yourself by opening a window and positioning a fan so it blows the fumes away from you.
You should probably get that checked out. You could be allergic. The first time I used MM's I took a nice deep whiff and almost died, but it cleared my congestion!