The blue silicone disk on the filter is an essential part of the dual valve ventilation system.
When exhaling, the blue silicone disk opens like in the picture below, allowing air that is warm, moist and filled with carbon dioxide to be ventilated out of the space between the face and the mask.
When inhaling, the blue silicone disk is closed, blocking airflow through the valve holes.
If one of the blue silicone discs would pop out of the valves on the filter during assembly, use, or when cleaning the product, It can be reattached. Put the small shaft on the blue silicone disc at the hole in the middle of the filter valve. Push and wiggle the blue disc until the small shaft has gone through the hole on the filter valve. Make sure it is attached by checking that the small dot can be seen from inside the filter valve.