A doctor has warned road users about a mistake that most of us will make as we use our cars each day...
Dr Clay Moss, a U.S.-based expert, has gone viral with a video that’s already pulled in more than 6.9 million views.
His warning? It’s all about the humble recirculation button — a feature you’ve almost certainly flicked on during a drive.
"Did you know there's a single button in your car that could be causing more harm than good?" Dr Moss asked in the clip, before pointing to the dashboard icon most drivers recognize: a circular arrow symbol near the AC controls.
"You guys remember this little guy right here that we always love putting on?" he said, referring to the recirculation feature.
It’s a setting many use to keep smog out or cool down the car faster on a hot day. But, according to Dr Moss, it could be doing more than blocking outside air — it might actually be cutting off your brain’s oxygen supply.
"That little recirculation symbol stops outside air from getting in, which is great when you're trying to quickly cool off the air on a hot day, but it can have unintended consequences over time," he explained.
And the consequences aren’t just about air quality.
"You see, when you're breathing in a sealed car, carbon dioxide builds up pretty quickly. Thirty minutes of recirculation and you could easily exceed 12,000 PPM [parts per million] of CO2, where your brain literally starts to slow down."
With more people in the car, it only gets worse.
"You could easily hit 2500–3000ppm of carbon dioxide. That's when you start to get drowsy, foggy and irritable."
It’s not just theory, either.
A 2018 study focused on driving conditions in the U.S. found that CO2 levels can spike to risky levels inside a car using recirculation mode — even when only one person is inside. Longer trips or carrying passengers only increase the danger, per the Daily Mail.
According to Dr Moss, once those CO2 levels rise past certain thresholds, your mental performance drops, your reaction time slows, and that dreaded brain fog kicks in.
But there’s good news — the solution is super simple.
"Now, the fix is super easy," he said. "You just either crack the window, or switch to the fresh air mode and turn off the recirculation mode for just two minutes. It flushes the CO2 out and keeps your brain sharp."
And it’s not just about comfort, he adds.
"Your car's climate settings aren't just about comfort, they mess with your cognition too," he said. "So breathe better, drive safe."
The video has sparked a flood of reactions online, with plenty of drivers admitting they had no idea their go-to button could be linked to things like fatigue or even mood swings.
"I drive to work two hours each way every day… is this why I get so sleepy by the time I get to my destination?" one person asked in the comments.
Contrary to this, a second said: "I’ve been driving this way for decades and nothing ever happened to me or my family!"