DEET can cause tumours in the parotid glands although I have no reference to cite... only my experience with a possible connection.

The best repellant for blackflies is a trick... let the little buggers bite you behind the ears and vigorously scratch behind the ears where the skin is tight to the skull until it bleeds. Scratch every so often to keep it bleeding. It's a good idea to swipe the bleeding area with an alcohol wipe every now and then.

This will allow you to continue fishing until you catch your limit and head for the truck. I discovered this in Labrador by mistake one day when I ran out of Muskol. Labrador, where the blackflies have learned to seek out the smell of bug repellants. They like their DEET neat.

I tried sticking a fish hook with bloody gills in the back of the brim of my hat but that seemed seemed to attract the critters in larger numbers.

Safety warning... if you use a bug jacket and you smoke, never try to get the last few drags of a cigarette through the mesh unless the jacket is certified non-flammble. The fire does not last long enough to keep flies away for any length of time. And you look silly when you explain why you have no eyebrows.