Does Recycled Air in Car Build-up Harmful CO2?

Many airconditioning systems in vehicles automatically select “recycle mode” to maximise efficiency of the temperature control, but does this risk a dangerous build-up of carbon dioxide (CO2) in the cabin?

Carbon dioxide is a naturally occurring colourless odourless gas most commonly produced by burning organic/carbon compounds and respiration eg we breath out CO2.

What Level of CO2 is Harmful?

Carbon dioxide is naturally present in air at 0.04% eg about 400ppm. At these normal low levels CO2 is not harmful.

Exposure to CO2 at 1,500 to 3,000ppm for longer periods of time eg an hour or more can cause drowsiness, reduced concentration and possibly headache or “metal fog”. In these circumstances the main risk is impaired driver alertness.

Workplace Exposure Standards

Safe Work Australia advise 5,000ppm as the safe 8-hour (TWA) limit for workplaces, and 15,000ppm as the short-term exposure limit eg 15 minutes where serious respiratory distress and cognitive impairment can occur.

Practical Guidance on Use of Recycle Setting

Selecting recirculated air will not normally cause “dangerous” CO2 levels in your car. However, CO2 levels will increase in these circumstances and can cause drowsiness and reduced concentration if maintained for longer periods of time.

Some models of new cars have CO2 sensors which automatically switch to fresh air mode if CO2 levels get too high.

Vehicle in-cabin studies confirm selecting air recycle is fine for short drives, hot weather, tunnels, heavy traffic or passing polluted areas.