Hello Andrea,
You can simply import the openLCA methods package and you will have "Cadmium oxide" characterisation factors for EF 3.0 and EF 3.1. The case before you import the openLCA methods package and cannot find Cadmium oxide/Emission to air/unspecified, simply means that this elementary flow is not used in any ecoinvent process.
The ecoinvent 3.9.1 database that we released comes with the elementary flows (and their corresponding characterisation factors) that are used inside the ecoinvent processes only (for transparency), but it can be (and should be) extended anytime with ca. 60000 elementary flows, having characterisation factors, by importing the openLCA methods package and getting compatibility to some other databases and regionalisation as well.
Having a characterisation factor for a certain elementary flow in the database does not mean that it has an influence on your impact assessment, since this elementary flow also has to be used in a process. Since ecoinvent is not using this elementary flow, I assume that you are modelling a process with direct emission of "Cadmium oxide" into air in one of your own foreground processes. It seems that ecoinvent is always modelling the metal ions and not the oxides in their processes like mining for example. "Cadmium II" is existing in the output of a lot of processes and has a slightly different characterisation factors than "Cadmium oxide" (where I don't know if it is used in any data set).
Best wishes,
Conrad
