Then I visited the place that wave-soldered small PC boards for us the last place I worked. It was pointed out to me that every six months the 12" to 14" stainless steel fume ducts leading away from the molten solder area had to be taken down and cleaned out
You'll notice in my comments, I noted the temperature IN NORMAL HAND SOLDERING. This is a relatively passive process where the only significant energy input to the solder is the heat applied. For example, you can vaporize water by shaking it in a bottle (energy = mechanical), by applying heat until, ultimately, it boils (energy = heat), or by spraying it through a aerosol nozzle (energy = mechanical pressure). The point being that the total energy input determines the point at which a substance vaporizes.
In the case of wave soldering, you have, not only heat, but the mechanical energy necessary to form the "waves" so the amount of heat necessary to cause at least some of the solder to vaporize is consequently less.
It is, therefore, reasonable to expect that this solder vapor will occur at a lower temperature than would be expected in hand soldering or, for that matter, in a solder pot. That being the case, exposure to the vapor will also occur at much lower temperatures.
While, not that long ago, even wave soldering in small batches was accomplished without much protection, fortunately it didn't take long to discover the hazards involved in lead poisoning. With hand soldering, however, the primary hazard was, and remains, the toxic elements of the fluxes used both internal and external to the solder itself. And, if anyone would tend to minimize this hazard in comparison to the lead issue, let them experience hour after hour of struggling for the next breath as I did for many years prior to the advent of the current "rescue inhalers" which I have had to use ever since.
So, it seems that soldering without appropriate protection has turned out to be a "double edged sword" from both the fluxes at lower hand soldering temperatures and from lead vapor in unprotected process soldering.
AND, it ain't pretty to struggle for breath either!