Investing in an indoor CO2 monitor is probably a smart move. We reviewed one of the best

We hear a lot about outdoor air pollution, but what about the air inside our homes? If you’re indoors right now, there’s a good chance you’re breathing something that could be quietly affecting your brain: carbon dioxide.

Research shows that elevated CO₂ levels can impair cognitive function—and chances are, the concentration in your living room or bedroom is higher than you think. Fortunately, there’s a growing range of CO₂ monitors that can help you see the invisible. We tested one of the best: the SAF Aranet4. It showed us exactly which rooms needed better ventilation and just how much of a difference fresh air can make.

Overall, the Aranet4 is a standout CO₂ monitor that combines scientific accuracy with user-friendly design. It’s super simple to install and use and has intuitive color-coded alerts revealing when your indoor air needs freshening up to protect your health and cognitive performance.

You have more CO2 in your house than you think, and it’s a problem

You spend most of your life indoors—nearly 90% of it, in fact. From bedrooms and classrooms to offices and gyms, indoor spaces accumulate more CO2 than the outside, and they can have a big influence on your health, comfort, and even how well you think.

Scientific research over the last decade has shown a troubling pattern: when CO₂ builds up in indoor spaces, your ability to think clearly, make decisions, and solve problems starts to decline.

In a landmark series of studies, researchers at Harvard put people in simulated office environments and tested their mental performance under different air quality conditions. The results were staggering. Participants working in well-ventilated, low-CO₂ conditions performed up to two times better on cognitive tasks compared to those in typical office air. Tasks involving strategy, information use, and responding to crises were especially affected.

By the time levels hit 1,400 ppm, your brain’s ability to function efficiently can be cut in half.

And it wasn’t just a fluke. A follow-up study spanning six countries and dozens of real-world buildings found the same pattern: when indoor CO₂ and fine particle pollution increased, people’s response times slowed, and accuracy dropped. In short, their brains lagged—just like yours does when you’re breathing stale air.

Other experts point to a broader issue: CO₂ is a proxy. When it’s high, it signals poor ventilation. And poor ventilation means other pollutants are probably building up too—things like volatile organic compounds (VOCs), body odors, humidity, even airborne viruses. These combined factors can leave your brain overstimulated, inflamed, or just plain overwhelmed.

You probably need a CO2 monitor

Unlike more obvious issues like mold or visible dust, CO₂ is odorless, colorless, and completely undetectable without proper tools. I would have sworn my working room, at least, is well ventilated. I open the window at least once or twice a day and it’s a big room. But it wasn’t.

That’s where CO₂ monitors come in. These small devices make the invisible visible, giving you critical insight into the health of your environment. I could see, based on my monitor, when I was in the room or not. I can see when my window needs opening. I can even see when my cats came in.

It wasn’t just my workplace, either. I mapped the CO2 around my house; from my bedroom and bathroom to the kitchen and living room. The air quality was always almost good, but not quite enough — and I honestly thought I was taking good care.

I couldn’t find a large-scale study estimating how many houses have too much CO2 inside, but based on my own experience, it’s probably higher than you think.

CO2 monitor review: SAF Aranet4 (the unboxing)

The Aranet 4’s non-dispersive infrared (NDIR) sensor is accurate, precise, and durable. A nondispersive infrared sensor (or NDIR sensor) is a simple spectroscopic sensor often used as a gas detector. It’s a laboratory-grade sensor.

The device itself looks simple enough.

The Aranet4 arrives in a tidy little box with no unnecessary frills. Open it up, and you’re greeted with a sleek, minimalist device about the size of a deck of cards. It’s lightweight (just 130 grams), battery-powered, and portable enough to slip into your bag or even your pocket. You can mount it on a wall or just put it somewhere. That’s good news for me—no cords and no mounting brackets are always an appreciated feature.

The setup is simple. Pop in the required AA batteries, and the e-ink screen springs to life. The screen looks pretty similar to that of a Kindle. Within minutes, you’re reading real-time CO₂ levels, room temperature, relative humidity, and atmospheric pressure.

There’s no calibration and no app required to get started (there is an app you can get, we’ll get to that in a bit). But I like the fact that you can get the numbers right away, literally within a couple of minutes after opening the box. It also has a simple display that can help you see whether the air quality is good (green), moderate (yellow), or bad (red).

Because it uses e-ink (similar to a Kindle), the Aranet4’s display consumes almost no power. Combined with adjustable sampling intervals, this means battery life is exceptional—up to two years on a pair of standard AA batteries if set to a 5-minute measurement interval.

SAF Aranet4 review: what it measures

The Aranet4 tracks four key environmental parameters:

• CO₂ concentration (in parts per million or ppm)
• Temperature
• Relative humidity
• Atmospheric pressure

While the real star here is CO₂, the supporting cast matters. Humidity affects comfort and mold growth. Temperature plays a role in both health and virus transmission. Atmospheric pressure isn’t always crucial, but it’s useful in certain settings like labs or high-altitude environments. They’re all useful info. It’s a good check for your thermostat or to see what the temperature is in different corners of your home.

But let’s focus on the CO2.

This color system is simple, intuitive, and immediately actionable. You don’t need to be a data analyst to get the point: if we’re at green, all is good. If it’s red, you really should open a window.

The real-time readings are very useful. I opened a window and within a couple minutes I already noticed a clear difference. But I really liked using the app for detailed data readings.

A CO2 monitor that can be as simple or as complex as you want it

The Aranet4 app (available on iOS and Android) connects via Bluetooth and allows you to:

• View detailed graphs of CO₂, temperature, humidity, and pressure
• Customize alarm thresholds
• Adjust measurement frequency (from once every minute to once every 10 minutes to save battery)
• Export your data for further analysis

Firstly, the app shows you the current CO2 level in your room on your smartphone. You can look at the short-term or the long-term and if you’re a data geek like me, you’ll probably spend a bit of time rummaging through your own data. I was curious, for instance, to see how CO2 levels vary from room to room; how they vary when I go in and out of the room; how long it takes to get to a good level when I open the window; even how much my CO2 level increases if I do some push-ups (spoiler alert, it’s more than I thought).

I do wish the app had a few more advanced features. I wish it would let you add markers (ie “I came in the room here) and that there would be a Windows app to see it on my laptop. But for the vast majority of users, it’s more than enough. You can go back and see older data, the graphs are clearly labeled, and mosst importantly, it’s all very easy to use. As I mentioned, you don’t even need the app; you can use the CO2 monitor without.

Because it doesn’t rely on Wi-Fi or cloud servers, it’s also a favorite among privacy-conscious users and professionals working in sensitive environments.

SAF Aranet 4: The verdict

I’ve tested it in offices, classrooms, bedrooms, and kitchens. And here’s the verdict: it’s one of the best CO₂ monitors on the market—accurate, intuitive, and genuinely empowering. It’s become such a reliable feature that every time I feel a bit fatigued while working I check if the CO2 level is elevated.

You can use it at home or in common spaces like school and offices and set up custom alerts (if you have a baby, for instance).

The Aranet4 is ideal for:

• Homeowners: Want to know why you feel groggy in the morning? Check your bedroom’s overnight CO₂ levels.
• Teachers and school admins: Use it in classrooms to verify ventilation and make decisions based on real data.
• Office workers: Gauge when meeting rooms need a fresh air break.
• Parents: Monitor nursery air quality or use it during sick days to reduce infection risk.
• Travelers: Take it to hotels or Airbnb rentals—anywhere ventilation might be suspect.

The only significant downside I’d mention is the price. You get a premium product, you pay a premium price. In my view. it’s worth it.

The Aranet4 isn’t flashy. It doesn’t have color touchscreens, AI assistants, or endless bells and whistles. But it does exactly what it promises to do—and it does it better than almost anything else in its class. If you care about your well-being, your focus, your productivity—or your kids’ learning—the Aranet4 is a very smart buy.

It’s easy to forget CO₂ is more than just a greenhouse gas associated with climate change. Indoors, where it can accumulate rapidly due to human breathing and poor ventilation, CO₂ becomes a powerful signal: a real-time indicator of how healthy—or potentially harmful—your air really is.