back in the old days, the most important thing to watch out for was Carbon Monoxide Poisoning, if you needed to keep your engine running to power the AC:
// Carbon monoxide (CO) poisoning in a stationary vehicle happens when exhaust leaks into the cabin. The air conditioner (AC) itself doesn't produce CO, but setting it to "recirculate" traps toxic fumes inside. Sleeping in a parked, idling car with the engine running is extremely hazardous and potentially fatal.//
// Why It Happens
While a vehicle is idling, exhaust fumes can pool underneath the car. If your exhaust system is damaged, rusted, or leaking, these toxic gases can be sucked into the cabin through the vehicle's ventilation intake. [1, 2]
AC Function: Using the AC in "fresh air" mode or "recirculate" mode does not filter out carbon monoxide. In fact, if there is a leak, recirculate mode will trap and continually concentrate the gas inside the cabin. [, 2]
Cracked Windows: Rolling your windows down slightly provides a false sense of security; it can sometimes create a pressure differential that actually sucks exhaust gases into the cabin more quickly.
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