This is normal, but it is not a problem. According to the description, they should be changed regularly.

Hello Shadowhunter. Thank you for your comment. Discolouration that is slightly yellowish or brownish is normal with filters. The cause of the discolouration is the substances in the tap water or from the air. Depending on the water quality, the humidifier filter discolours more quickly. Although this does not look nice, it is harmless as long as it is not mould. The discolouration also has no effect on the performance of the humidifier. If the humidifier filters have not been in use for long, they can be rinsed well, turned over and used again. Kind regards from the Stadler Form team