I've always thought that is a hazard of welding galvanized metal. Check this out:
http://www.osha.gov/doc/outreachtraining/htmlfiles/weldhlth.html
"Inhalation of zinc oxide fumes can occur when welding or cutting on zinc-coated metals. Exposure to these fumes is known to cause metal fume fever."
Bending is done at around 300F, and both gas and electric welding are done at several thousand degrees.
What wattage is the bulb?
Out of curiosity, what diameter pipe did you guys use?