Lyric discussion by Bonnemort 

Cadmium (Cd) is an element. It is used in a pigment [from wikipedia:]

Cadmium forms various salts, with cadmium sulfide being the most common. This sulfide is used as a yellow pigment. Cadmium selenide can be used as red pigment, commonly called cadmium red.

It is also an ingredient in cigarette smoke, which is said to cause lung damage and kidney disease.

To me though, this song seems to parallel the self-destrucitve nature of smoking cigarettes with a doomed relationship.

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