I ran across this interesting article that welders like me that stick our face in the weld to see what's going on might want to read.
http://www.twi-global.com/technical-knowledge/faqs/health-and-safety-faqs/faq-the-use-of-thoriated-tungsten-electrodes/
http://www.twi-global.com/technical-knowledge/faqs/health-and-safety-faqs/faq-the-use-of-thoriated-tungsten-electrodes/