Yea, I don't know the purity standards of CO2 extinguishers, but C02 itself is probably the cleanest solvent, period. Hands down.
Because carbon dioxide is soluble in water, it occurs naturally in
groundwater,
rivers and
lakes,
ice caps,
glaciers and
seawater. It is present in deposits of
petroleum and
natural gas. Carbon dioxide has a sharp and acidic odor and generates the taste of
soda water in the mouth.
That's why I suggest either dry ice, or freezer method, cause you essentially not using anything. C02 is inert, chemically inactive. Especially from a smoking standpoint.
That's why they use C02 for supercritical extractions, I bet it could be done with N2O (laughing gas) also.