The answer to 'air duct grimy' is that air ducts can become grimy and clogged over time due to dust, dirt, pet dander, and other airborne particles circulating through the HVAC system. Symptoms of grimy air ducts include reduced airflow, musty odors, and increased dust and allergens in the home.
The best solution is to have the air ducts professionally cleaned every 3-5 years, or more often if you have pets, smoke, or live in a particularly dusty environment. While some homeowners may attempt DIY duct cleaning, it's generally recommended to hire a licensed HVAC technician who has the proper equipment and expertise to thoroughly clean the ducts without causing any damage.