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I thought CO2 monitors were a waste of money. Then I bought one out of curiosity and it completely changed how I think about the air in my flat.

The Thing That Made Me Buy It

March 2024. I’d been tracking PM2.5 and humidity for months with my Temtop M10, feeling pretty smug about understanding my flat’s air quality. Numbers looked good. Air purifier was running. Job done, right?

Except I kept getting this afternoon brain fog. Around 3 PM every day, I’d just hit a wall. Couldn’t think straight, felt drowsy, generally useless. Figured it was just me being rubbish at afternoons.

Then I read something about CO2 levels affecting concentration. Skeptical, but curious enough to look into it.

Most CO2 monitors are either cheap and inaccurate, or proper scientific kit that costs a fortune. But I kept seeing this thing called the Aranet4 Home mentioned in forums. People who actually knew what they were talking about seemed to rate it.

It’s not cheap. Quite expensive for what’s essentially a fancy thermometer that tells you when to open a window.

But I was annoyed enough about the brain fog to give it a go.

What Actually Is It?

The Aranet4 Home is a portable sensor that measures CO2, temperature, humidity, and air pressure. It’s about the size of a small phone but twice as thick. Has an e-ink screen (like a Kindle) that shows you the readings.

The clever bit: it uses NDIR sensor technology, which is the proper scientific way to measure CO2. Not the cheap electrochemical sensors that drift and need constant calibration. This thing is actually accurate.

Runs on two AA batteries. And here’s the mad bit: those batteries last up to four years. Not a typo. Four. Years.

First Day: The Numbers Were Worse Than I Expected

Stuck it on my desk. Within an hour: 1,247 ppm CO2.

For context:

• Outdoor air: around 400-450 ppm
• Indoor “good” level: below 800 ppm
• Indoor “acceptable”: 800-1,000 ppm
• My office at 3 PM: 1,247 ppm

No wonder I felt like my brain was full of cotton wool.

The screen goes green (good), amber (moderate), or red (bad). I was firmly in amber, heading for red. And this was just me, sitting at a desk, in a room with the door closed.

The Bedroom Test (This Got Worse)

That night, I moved it to the bedroom. Left it running while I slept.

Morning reading: 1,842 ppm CO2.

The WHO recommends keeping indoor CO2 below 1,000 ppm. I’d been sleeping in nearly double that. Every. Single. Night.

Suddenly the morning grogginess made sense. I wasn’t just “not a morning person” – I’d been breathing stale, oxygen-depleted air for 8 hours straight.

What I Changed (And What Actually Worked)

Office (where I work):

• Started opening the window for 10 minutes every hour
• Cracked the door open to get some airflow
• CO2 now stays below 900 ppm
• Afternoon brain fog: gone

Genuinely shocking how much difference this made. Same desk, same work, completely different mental clarity. Just by letting some fresh air in.

Bedroom:

• Started sleeping with the door open
• Cracked the window about 2 inches (even in winter)
• CO2 dropped to 600-800 ppm overnight
• Morning grogginess: much better

The window thing felt counterintuitive in winter. Surely I’d freeze? But the fresh air is worth the extra blanket. And the monitor doesn’t lie – the CO2 stays in the green range now.

The Battery Life Thing Is Actually True

I bought this in March 2024. It’s now November 2025. Still on the original AA batteries. The screen shows “battery good.”

This is bonkers. My Temtop M10 needs charging every few days. The Aranet4 just… keeps going. The e-ink display uses almost no power, and it only takes a reading every 5 minutes (you can change this, but 5 minutes is plenty).

For a portable device, this is genuinely impressive. I can stick it in my bag, take it to a mate’s flat, check their air quality, and not worry about it dying.

The App Is Fine (Nothing Special)

You can pair it with your phone via Bluetooth. The app shows you graphs of historical data, lets you export CSVs if you’re into that sort of thing, and set custom alarm thresholds.

It’s… fine. Nothing amazing, nothing terrible. Does the job. I mostly just glance at the device itself rather than opening the app.

The app does let you see 90 days of history, which is useful for spotting patterns. Like “oh, every Thursday afternoon my CO2 spikes because I forget to open the window during long Zoom calls.”

What It Doesn’t Do (And I Wish It Did)

No PM2.5 sensor. It’s purely for CO2, temperature, humidity, and air pressure. If you want particulate matter readings, you need a separate device like my Temtop M10.

This is fine for me – I already have PM2.5 covered. But if you’re starting from scratch, you’ll need two devices.

The battery cover pops off easily. If you’re chucking it in a bag, the cover can come loose and the batteries fall out. Minor annoyance, but worth mentioning.

It’s not waterproof. Don’t leave it in the bathroom or anywhere damp. It’s IP20 rated, which basically means “keep it dry.”

Is It Actually Worth It?

Honestly? For me, yes.

I’ve changed my ventilation habits based on what the monitor shows. I’m more productive in the afternoon. I sleep better. These aren’t placebo effects – I can literally see the CO2 numbers drop when I open a window and my concentration improve immediately after.

But here’s the thing: you don’t need an expensive CO2 monitor. You could just open windows more often and probably be fine.

The monitor is useful if:

• You work from home and want to optimize concentration
• You’re curious about actual numbers rather than guessing
• You have a bedroom that feels stuffy and want to know why
• You’re a data nerd who likes tracking things (guilty)

If you just want to “breathe better air,” opening windows more often costs £0 and solves 90% of the problem.

The Thing That Surprised Me Most

How bad the CO2 gets in normal situations.

I tested it at:

• Mate’s flat during dinner party (6 people): 2,100 ppm within an hour
• My local pub (quiet Tuesday evening): 1,400 ppm
• Train carriage (not even busy): 1,600 ppm
• Cinema during a film: 1,800 ppm

These are all places I regularly spend time. And the CO2 levels are consistently higher than recommended. Not dangerously high, but enough to affect how you feel.

The cinema one explained why I always feel tired during films. Not the film’s fault – it’s the stale air.

What I Actually Do With It Now

I keep it on my desk during the day. Glance at it every so often. If it creeps above 1,000 ppm, I open the window for a bit.

At night, it moves to the bedroom. Helps me remember to crack the window before bed.

Occasionally I’ll take it to a restaurant or pub out of curiosity. Usually regret checking the numbers (ignorance was bliss).

It’s become one of those things I didn’t know I needed until I had it. Like a good kitchen knife or a decent pillow.

The Honest Verdict

Pros:

• Actually accurate (NDIR sensor, not the cheap stuff)
• Battery life is genuinely ridiculous (1.5 years and counting)
• E-ink screen is easy to read in any light
• Portable – stick it in a bag, take it anywhere
• Simple to use – no faff, just works

Cons:

• Quite expensive for a CO2 monitor
• Doesn’t measure PM2.5 (need a separate device)
• Battery cover pops off too easily
• App is fine but nothing special

Would I buy it again? Yes. But only because I’m the sort of person who likes having actual data rather than guessing. If you’re happy just opening windows more often, you don’t need this.

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Interested in tracking CO2 levels in your home? Check out our air quality monitor guide for more options and recommendations.