I didn't mean for that smiley face to show up at the beginning.

The skin is a barrier to most chemicals but not all. The compound of selenium that one finds in scalp treatments is not very good, if at all, as a toner for silver prints. The same OH part of NaOH that can turn your body oil to soap makes MgOH an antacid or a laxative if you take enough. Selenium can be on either end of a compound, as can its neighbor sulfur. I would not let the one, I forget which, come in contact with my skin but have probably used the other come in contact with my scalp. Elemental sulfur is not particularly harmful, but sulfur dioxide in water forms sulfurous acid, which can eat steel. Years ago, in Morgantown WV, we had a chemical plant that liberated large quantities of sulfur dioxide. Rain puddles became sulfurous acid baths and cars (this was before undercoating) had lace fenders.

Any compound that you worry about should be investigated, but by its trade name or a generic name, not by the individual ingredients or elements. If I mix sodium hydroxide and hydrochloric acid in the right proportions, I will get salt water.

Excuse my wanderings.