"Dirty brushes can be a breeding ground for bugs including fungi, bacteria and viruses, which can result in unwanted breakouts and skin infections," says Rishika Sinha, consultant dermatologist at The Private Clinic. Or — if you're a makeup artist and using the same brush from client to client — risk passing other skin conditions from person to person. Conditions "such as staphyloccus aureus (a common bacteria found on the skin), can cause skin infections if they are given the right opportunity, such as dry, cracked skin, or eczema — any skin condition where the barrier is compromised," she explained.
"Cream-based products in synthetic fibres makes for the perfect breeding ground for bacteria," Martin said. Using these over time can lead to the product accumulating in the bristles and the ferrule (the metal part holding the bristles and the handle together), making it hard to remove.
But it's not just avoiding the spread and growth of bacteria; cleaning your brushes also keeps them working as well as they did right out of the box. The cumulative buildup of product can affect the way the formula is applied and subsequently how it sits on the skin. The same goes for eyeliner: you're unlikely to get a sharp, pristine cat eye with leftover, dried formula caked on and in the bristles.