I am a nurse and this is what I know:
I have treated people with lion fish stings. Dependsing on where you are stung, how badly you are stung, how many soines you are stung by, and if you are allergic to bee stings... they could potentially be deadly. Most people are ok if they are stung by a lion. It just huts. If in the case you were stung, you run it under hot water. Then you could call poison control and ask there opinion if you need medical attention. I would assume the bigger the fish the worse the sting. I don't know that for a fact. If you are careful, you shouldn't have to worry about getting stung. Basically, don't pet your lionfish. I would not be worried about being stung. Just be cautious with one.
HTH
Leopard