The air quality you see in the bubble on Vitesy Hub's homepage for your Shelfy is calculated based on 4 different factors:
In the home, inside the squares with the respective values, you will see a red dot which indicates that you need to work on that in order to improve the air quality (increasing the fan speed will not improve the air quality, as it's based on different factors). This could mean adjusting the temperature in the fridge thermostat, opening the door for as little time as possible (effective from the next day), cleaning the fridge or washing the filter.