This is usually because of the accumulation of ultrafine dust on the surface of the PECO-Filter. This fine dust layer can block some of the light, but it should not significantly impact the effectiveness of your Molekule. Once you replace your PECO-Filter with a new one, the light level will return to normal brightness. The Pre-Filter is designed to protect the PECO-Filter from dust build-up and extends the life of the PECO-Filter, so it is also important to replace your Pre-Filters regularly.
If it’s been 6 months or more since you changed your PECO-Filter, you should replace it with a new one. If you don’t have a new PECO Filter to replace the old one, please read this article about how your Filter Auto-Refill works.