Ok - after considering responses and some more research, here is the gist of it.
- Stainless Steel is 10 - 20% Chromium.
-
Cr(VI) - Hexavalent Chromium is something you don't want to be eating, inhaling etc. all day.
- Cr(0) and Cr(III) which is an essential nutrient,
will turn into Cr(VI) in the presence of oxygen at temperatures in the range of 200-300C (400-570F)
So working with stainless steel or chrome at temperatures over 400F in a way that produces dust or fumes will produce Cr(VI) and you should consider taking precautions to avoid it - depending on your intelligence a risk avoidance profile.
What does this mean practically for a hobby machinist?
If you take normal sane safety precautions - ventilation, dust protection, eye protection, don't worry about it.
It seems that the biggest risk is from welding and grinding. Note that wielding that uses an inert gas will produce less Cr(Vi) - because no/low oxygen environment.
Let me be perfectly clear - There are plenty of hazards that exist in our shops with varying degrees of danger, some have immediate consequences other accumulate over time. I prefer to know / learn / understand them. Being informed is not worrying.
I developed a mild skin irritation as a result of working with stainless steel. Not a big deal. I'll more be more careful next time.
Your risk profile may be different - you do you.