Selenium is indeed found in many dandruff shampoos and also in dietary supplements. It is not all that toxic, but I would never use it in the kitchen under any circumstances, nor would I put my hand in a solution containing it without rubber gloves.
The dandruff shampoo has a regulated level, but the concentrated and diluted toner may exceed levels recommended by the various governments.
Anchell and Troop say that Pyro compounds used in toning developers are the most poisonous compounds you can use. Selenium would be second on that list IMHO.