Also worth noting another key issue with car exhaust in a confined space is carbon monoxide, you’ll feel the CO2 build up and make it difficult to breath in your environment before it does any damage, the CO on the other hand will kill you quietly. CO breaks down relatively quickly in the environment by reacting with other substances in the air, so it’s not really a long term pollutant concern.

There’s also other chemicals and particulates, but they’re mostly going to be at lower concentrations that aren’t going to kill you in a hurry, but may contribute to longer term cancer risks and such, but that’s a little harder for people to wrap their heads around. You won’t immediately die of cancer in your garage from breathing exhaust but it might give you cancer years or decades down the line.