It's not a CO2 tank, but my coworker and I almost died transporting some human tissue grafts on dry ice (frozen CO2). We were taking the grafts to a surgery about an hour away, and the doctor didn't use the grafts, so we left the grafts in her little Honda Fit while we went to lunch after the surgery. These are boxes filled with 80 lbs of dry ice, intended to keep the grafts at -40 C for about a week, so stopping for lunch wasn't an issue. On the way home we got about 15 minutes into the drive when both of use started to have a lot of trouble breathing. At first I thought I was having a heartache or something, and then I realized both of us were in trouble, and then my clogged mind suddenly realized what was happening, and I rolled down our windows. It was like someone took a clamp off my lungs and we could breathe fine. We pulled over for a few minutes to literally catch our breath, and then drove home the rest of the way with the windows cracked. But hey, now we made it into our companies safety manual on how to deal with dry ice!