Makeup brushes can cause skin inflammation, redness and even acne. How? They can harbour bacteria that can settle on the skin. As the bacteria multiplies acne can develop or rash like redness can appear. Wash your brushes in shampoo and add a few drops of tea tree oil in the hot water and rinse your brushes. Dry the brushes horizontally so you don't rot the (brush) hairs or leave any moisture residue from the drying process.