Allergens can absolutely be absorbed through the skin. Beyond the obvious topical dermatitis you might get from touching poison ivy, for example, irritants that come into contact with your skin can give you effects that are more than skin deep. Pain, inflammation, fatigue, migraines, nervous system issues… These are all examples of how our bodies may be affected by what we put on our skin.
Don’t forget that our skin is our largest organ and provides a semipermeable barrier between us and everything else. If you are sensitive to something ingested, you’ll want to avoid touching it or smelling it, too. After all, aromas are literally molecules with mass and weight, and even something this small can cause some people to react.
I always discuss personal care and cleaning products with my food sensitivity clients. I had one client with persistent, unexplained shoulder pain who swore by a certain blue topical salve that she thought was giving her relief. She used it 4-5x/day. She finally switched brands after blue dye showed up reactive on her Mediator Release Test (MRT) and the pain went away.
Another example is a client who tested sensitive to citrus fruits on the MRT. When she uses cosmetics or cleansers that contain citric acid, her skin will flare up and react for several days.
I talk to my clients about avoiding aloe in skin care products if they are sensitive to the Lily family and coconut derivatives like sodium lauryl sulfate if they are sensitive to coconut. We look at scents like vanilla and lemon in soaps, etc., and chemical preservatives like sorbates in shampoos and cleansers. When items like these foods or chemicals turn up high in the MRT, I encourage them to avoid the allergen in every way.
Beyond that, we talk in general about using clean, non-toxic products to lower their overall toxic load.