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Copper or brass are handy for making the smaller tubes. Since the cool "source" air blows through the venturi tube, it keeps the tube cool. And the vapor pressure of copper is quite low until you reach very high temperatures. So I doubt anyone will be getting much of a dose of copper from using a copper venturi tube.
The creosote buildup on the outside of the venturi shows that it is staying rather cool, to the point that cresote condenses onto it.
Have your neighbor look up the graphs showing vapor pressure versus temperature for common elements. Copper is lower than 10e-8 mm of Hg even at 725 degrees C. Those copper atoms are not likely going anywhere!