Paint fumes, and more specifically, thinner fumes, and more specifically-er, laquer thinner fumes. I know, it's a problem of my own making, and I could easily solve it by going to acrylic (which I think I'm going to have to) but the nausea from huffing fumes really makes me want to do a poor job of brush cleaning. The thought of wearing a face mask with an organics filter just doesn't conjure up images of relaxing hobbies to me.