Our skin is designed to fight infections and environmental stresses. It is important to try and maintain a natural pH level on your skin for this reason.
The pH level of the skin is referring to how acidic or alkaline it is. Ph is on a scale of 1-14, with 1 being the most acidic to 14 being the most alkaline. Your skin’s pH should stand somewhere around 4.5/5.5.
Our skin has a thin, protective layer on its surface, referred to as the acid mantle. This acid mantle is made up of sebum (oil) excreted from the skin's sweat glands, lipids and of course some bacteria. This mixes with lactic and amino acids from sweat to create the skin's pH.
All this can be stabilized with some toner, like the Obagi Toner or SkinMedica Toner. Plus, toner also acts as an antiseptic so it will kill any bacteria left on the skin after cleansing. Keeping your pH level normal will help with product penetration! That’s why I always recommending cleansing your skin and then applying a toner.