Here's my experience with CO2. The largest single
user of CO2 in the US is the oil
patch. They inject 65,000,000 tons of the stuff into the ground every year (with expectations that number will grow to 20 billion
tons within a decade). There are hundreds (if not thousands) of miles of major pipeline devoted to moving that CO2 around. And most of those pipelines are old oil
pipelines put into new service
The oil industry only cares about one contaminant in their CO2 - oxygen. They're injecting fully oxidized carbon into the ground in order to extract non-oxidized hydrocarbons. The last thing they want is to inject high pressure oxygen as part of the process, it ruins their product. Any other contaminant (benzene, sulfur, acetaldehyde) they could not care less, it makes no difference to them. The industrial CO2 you can get in the oil patch you do not want to drink. If they don't care about the contaminants that could kill you do you think their suppliers do?
As with any product (including compressed air) you get it from a reputable supplier and you're fine. But everyone here is recommending that you go down to the corner and get it from Joe, who gets it from who knows where. I know that in the patch it frequently ends up in tanks
on the back of welding rigs because it's free. And welders only really have one concern about contaminants too - oxygen, because they are using it for inerting.
You say no one has ever gotten sick from brewing with industrial CO2. Well, benzene is a chronic exposure carcinogen. Are you saying you know someone (actually a large sample size of someones) that has been exposed chronically in this manner for 30 or 40 years? Get back to me when you do. When I was a kid I used to mix raw asbestos fibers into epoxy
as a thickener (and a damn good one). Everyone said it was wonderful stuff.
I said it the first time around, I can and do use industrial CO2 and don't have any problem with that. What I have a problem with is sweeping the risks under the rug and pretending they're not there. Have an honest discussion acknowledge that they're one-in-a-million, maybe less, maybe more, and then let people decide for themselves.