The K&N Drycharger Sock for PN E990009 is a specially designed, durable polyester wrap that fits over your K&N air filter to provide an extra layer of protection. It helps repel water, dirt, and debris while maintaining high airflow. The Drycharger is treated with a silicone-based water-repellent finish, which helps extend the life of the filter and improves performance in dusty or wet conditions. It is washable and reusable, designed specifically to fit the K&N PN E990009 filter.