I know here in Canada, products can only contain a certain percentage of deet. Not sure on the US, but I think you can get stronger concentrations there. I know it is nasty stuff. I had a bottle from when I was in the military, which was over 99% deet, and I had it on my face when I took a phone call. It ate the numbers off the phone and the phone stuck to my ear where the deet had dissolved into the plastic (picture a wall phone from early 90's, not a current cell phone).

Most products I see now are something like 5% deet. Used to be able to get 40-60% products.