The best thing to breathe is fresh air.

Volvo does not want you to perish from your own carbon monoxide, of more likely carbon dioxide. You probably would not die before you lost enough fine motor control and got into an accident.

I actually think that most other cars do not really have a true recirculation setting, and that is why there is no limitation on using that mode in their cars.

As a Haz Mat team member, I have seen several articles that mention putting a switch in the fan circuit of cruisers. This is so as not to take toxic vapors into the car because many cars use a constantly running fan to pressurize the cockpit with fresh air to keep exhaust fumes out.
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2 8s & 2 7s 600,000 miles total