John,

That crud is a mixture of soot, carbon, atmospheric gook from pollution, dust, grinding swarf, and condensates from the smoke caused by the flux on rods burning, among other things. In other words, it is generic, mixed crud. SOme of it, like the grinding swarf, is definitely toxic and harmful to the lungs (small particles of glass fibers and ceramic), and some of it is pretty benign (the carbon from lighting an O/A torch).

You could pressure wash it with a really long wand, perhaps, but then you're faced with the issue of properly disposing of the runoff water which is a haz-mat.

The simplest thing to do is probably to paint over it, the same way that asbestos is dealt with. A coat or two of good paint will seal the crud in place, ending any issuesof it becoming airborne.

It seems to me that these days a lot of people are complaining about lung problems that never would have arisen in the "less-enlightened" times a couple of decades and a thousand lawyers ago. :-)

Rich