Aranet4 Home – a CO2 monitor that tells you to open a window

You know that meeting room feeling. Two hours in, ten people, door closed, and everybody is suddenly very sleepy. Or the bedroom in winter where you wake up with a heavy head even after eight hours.

Sometimes that’s the air. The Aranet4 Home is a CO2 monitor: a small battery-powered box with an e-ink screen that shows how much carbon dioxide is in the room, so you know when to open a window.

It’s a very nerdy gift. I’d be thrilled to unwrap it.

What is the Aranet4 Home?

It’s a 7 x 7 cm (2.76 inch) square with a big number in the middle: the CO2 level in ppm (parts per million). Smaller numbers show temperature and humidity, and it also measures air pressure. A green, yellow or red bar under the number tells you at a glance whether the air is fine or stale.

It’s made by Aranet, a brand of SAF Tehnika, a company radio engineers started in 1999. The monitors are made in the EU. The Aranet4 has been on Amazon since 2019 and is still in the top 10 of Amazon’s indoor air quality meters.

Aranet says a well-ventilated room sits around 400 to 800 ppm. When the bar turns yellow or red, it’s time for fresh air.

How does it work?

Inside is an NDIR (non-dispersive infrared) sensor. It measures CO2 by how much infrared light the air absorbs. That’s the type of sensor used in professional gear, and a big part of why the Aranet4 costs more than many cheap air quality meters. Aranet lists the accuracy as ±(30 ppm + 3% of the reading).

The screen is e-ink, like a Kindle, so it barely uses power. Two AA batteries (included) last up to 4 years, says Aranet. There’s an optional buzzer that beeps when CO2 gets too high, and a free app over Bluetooth that keeps 90 days of history in graphs.

What I like / What could be better

What I like:

• One number, one color. You don’t need to understand air quality to know what “red” means. It means: window.
• No cable, no charging, no subscription. Put it on a shelf and forget about it for years.
• BreatheSafeAir’s reviewer has used one for about four years and still calls it the best all-round consumer CO2 monitor they’ve tried. Amazon buyers give it 4.7 out of 5 from about 1,900 ratings.

What could be better:

• It’s pricey for a box that measures one gas. Cheaper monitors exist. The sensor is where the money goes.
• Bluetooth only. No Wi-Fi, no Alexa or Google Home, so you can’t check your home from the office.
• It measures CO2, not carbon monoxide (CO). Aranet says this clearly. It’s not a safety alarm, so keep your CO detector.
• It also doesn’t measure dust, smoke or pollen. If that’s your worry, look for a different device.

Who is the Aranet4 Home for?

Home office workers who hit a wall every afternoon. Parents who want to know if the kids’ bedroom gets stuffy at night. Anyone who likes data about their home. It’s also small enough to take along to a hotel room or office.

Skip it if the person you’re buying for wants smart home automations or a Wi-Fi dashboard. For more bedroom gear, see our sleep gadgets roundup. For more numbers about yourself, see our health gadgets that are actually useful.

Where can I buy the Aranet4 Home?

SAF sells it through its own store on Amazon US. It was about 189 bucks at the time of writing (October 2026).

The cheapest fix for stuffy air is still free: open a window. This just tells you when.