Not necessarily. Some materials like satin might indeed cause an acne breakout. But that's only if you've not washed them properly or have been using the same covers for a very long time. You could always reduce the acne breakout by choosing the right materials. For instance, fabrics like cotton, bamboo, hemp, and microfiber absorb the oils and dirt on your skin. Therefore, make sure to purchase a pillow cover that best suits your skin type.