The current device_class for CO (carbon monoxide) only supports ppm as the unit of measurement. However, another common unit of measurement is µg/m³. For example, the OpenWeatherMap integration for example, provides data using µg/m³ which I do not want to convert to ppm for consistency with all other values reported. Also, the World Health Organization uses µg/m³ in its “WHO global air quality guidelines” so that is what I would prefer to keep using 
From OpenWeatherMap:
From “WHO global air quality guidelines”
Current carbon_monoxide device_class:
Adding µg/m³ would make it consistent with other gasses that already support it (those might need adding ppm if applicable):
