I typically use 29 but 40 was all that they had so I bought some. The skitoes were terrible. The 29 worked well so I thought the 40 would be better. ***WARNING*** 40 is too strong. On friday I put some where I typically do. On my cheeks and on the back of my neck. It stung all day but I ignored it. Friday night i had huge blotches on my face and neck where I put it and it still burned. On saturday I smartened up and put it only on my hat and on my clothing on my shoulders. I developed breathing problems later in the day. This stuff is just too dang strong. I took my cap off on the way back to the car some I could breath.