A lot of outdoor gear has warnings like the following,
WARNING: This product contains chemicals known to the State of California to cause cancer and birth defects or other reproductive harm.
while sometimes it says
may contain chemicals
To keep this questions from being too broad, I am going to limit it to just fuel canisters. While there are carcinogens produced by operating a canister stove, a campfire will also produce them and I am unaware of any canister fuels that lack the warning.
So how much should I worry/care about these warnings?