Barbara P. voiced concern over hazards of nickel when heated. A
review of nickel's physical constants and toxicity provides some
useful Nickel itself vaporizes at such a high
temperature (over 5000 deg. F.) that any chance of inhaling nickel
vapor is remote. A more toxic nickel compound that does vaporize at
low temperatures is nickel carbonyl, but even that is unlikely since
a controlled temperature much lower than your torch flame is required
to create it, and the nickel has to be in a carbon monoxide
atmosphere. So that scenario is unlikely unless your heating
processes contain a lot of carbon monoxide gas, which is itself toxic
and more likely to be of harm than any nickel present. The more
likely problem with nickel is ionic nickel created by galvanic
processes when nickel or its alloys are in contact with sweat or
acids. That is what causes an allergic reaction over time.
I hope this helps.