No problem! Glad it helped out with the peace of mind if nothing else.
I have no idea why he couldn't have taken 2 seconds to tell you "Hey, dry ice off puts CO2 as it melts. If that builds up in the car, it may kill your animals, so be careful how close they are in enclosed spaces if you plan on any long stops."
Though, I can't imagine a fist-sized chunk (about as much as I would expect to be provided, bigger would be unnecessary) being enough to build up to even make several inches of CO2 gas along the floorboards of a car, but I don't know what the volume it offputs really is.
If there really were something dangerous about the rats themselves, I would immediately think they were not going to be fit for animal consumption without risk of harm. Definitely nothing wrong with the rats in this case.
Your method works out well, as the gas would have settled in the trunk away from the animals and would have taken substantial amounts of melted dry ice to get much into the passenger area of the car.
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