Like IQAir’s Outdoor monitor, the AirGradient Open Air is meant to be mounted, not placed. It comes with two steel zip ties for pole mounting or wall mounting with screws. There’s even a hole for a secure screw to prevent theft. And with its weather-resistant enclosure, the Open Air can be placed in the elements. Its three sensors measure PM 2.5, CO₂, NOx (nitrogen oxides), and TVOCs, along with temperature and humidity. I can click to share my data with AirGradient’s open-hardware community map, and on the nonprofit Open AQ. The AirGradient map also lets me see specific readings for PM 2.5 or CO₂.

At just under $200, the AirGradient Open Air is cheaper than other outdoor air monitors I’ve tested, and if you like building your own, you can purchase an Open Air Kit for $125. The Open Air doesn’t have an indicator light, and there isn’t an AirGradient app. Instead, I log onto AirGradient.com and launch the dashboard to see the graphs and numbers. The user experience isn’t as streamlined as IQAir, but once I was used to AirGradient’s color-coded graph, it was easy to understand. If you’re looking for an inexpensive outdoor air quality monitor and you don’t mind not having an app, then the Open Air might be for you.

I wish I had used the IQAir AirVisual Outdoor Monitor years ago when I first tested IQAir’s AirVisual Pro Indoor Air Quality Monitor (above). The display gives real-time indoor monitoring along with data from the closest outdoor air quality station, or your very own right-outside-the-window IQAirVisual Outdoor. There's so much more accuracy in having both a dedicated indoor and outdoor monitor. Having both of IQAir’s monitors enables the AirVisual Pro to show both side-by-side readings of indoor and outdoor on its easy-to-read screen.

Depending on the index number, the visual of the screen's two boys will be green with a smile for good air and seem disillusioned at moderate yellow air, going all the way to purple with a respirator mask for bad air. Sometimes both boys are smiling green, sometimes the indoor boy is happy and the outside one is mired in dirty air. There’s real agency in knowing both the indoor and outdoor air quality index. The monitor also pairs with IQAir’s intuitive app, which has one of the best user experiences I’ve tried.

Unlike PurpleAir’s plug-and-play crowdsourced air quality map, IQAir has a registration process for accepting data from your Outdoor monitor. Submission requirements include photos of your mounted outdoor monitor. IQAir’s Outdoor comes with a mounting plate, two UV-resistant cable ties for a pole mount, and wall plugs and screws for a wall mount. I’m unable to drill into the brick outside my building, and IQAir doesn’t have an option for placing it on a windowsill. There’s no stand to keep it upright. The power adaptor cannot get wet with rain or snow, and it’s suggested to place it at a minimum of 5 feet off the ground. IQAir requires a photo of your Outdoor placement. I took a picture of the Outdoor monitor on my windowsill and submitted it to IQAir.

My submission to be a contributor for the app was rejected. An email from IQAir detailed why: “The data reported from your monitor doesn’t correlate with surrounding stations. Please make sure the environment is well ventilated and check if there is any pollution source nearby, then resubmit your publication so that we could review it again. From the provided photos, it appears that your installation does not ensure good airflow … Make sure air inlet is completely unobstructed, and air can freely flow around the device.”

IQAir included a photo of a monitor secured above the door on a freestanding house. And while I’ve yet to resubmit, having an outdoor monitor that works with my IQAir Air Visual has been helpful. When my neighbors use their fire pit, both my PurpleAir Zen and my IQAir Outdoor registered a PM 2.5 of 160, letting me know to shut my kitchen window. And it has four sensors that can pick up PM 2.5, CO₂, temperature, humidity, and barometric pressure. I have to either look at my AirVisual monitor for a real-time reading or open the app, because the IQAir Outdoor doesn’t have an indicator light; it’s truly meant to be tucked away outside.

Airthings' solid, unassuming minimalist design has an easy-to-read display screen, and it’s the first indoor monitor I’ve tested that has a dedicated radon sensor. It also measures PM 2.5, CO₂, VOCs, temperature, humidity, and air pressure. And while the Airthings View Plus has an easy-to-use app, the monitor will not work without internet/WiFi. I’ve tested monitors in the past that will work, complete with accurate screen display, even when not paired with a device. With Airthings, you’ll need to make sure that you both have internet and can pair it with your phone. The Airthings also needs time to calibrate. I saw radon, PM 2.5, and humidity almost immediately, but the app told me I needed to wait days before the monitor calibrated the CO₂ and VOC sensors.

Aside from the fact that Airthings won’t work without internet, my other issue with the monitor is that I need to wave my hand in front of the device for an air quality check. The movement from my hand triggers the sensor to show any value above the recommended levels, along with the indicator light, which shows green for good, yellow for fair, and red for poor. I’m testing this monitor in a cabin in Maine, and the humidity is high. Each time I wave my hand in front of the device, the light glows red and the display will show the humidity at 71 percent. The more I test indoor air quality monitors, the more I appreciate those with color-coded indicator lights that I can see from across the room. I still recommend Airthings because of its ability to measure everything from radon to PM 2.5. It also has backup batteries in case of a power outage.

When that apocalyptic orange cloud set over San Francisco in 2020, the crowdsourced air quality index map PurpleAir was the place to see how bad the air really was. PurpleAir’s Real-Time Air Quality Map is created by the public using sensors that PurpleAir sells. It offers two types of outdoor air monitors: the PurpleAir Classic Air Quality Monitor and the PurpleAir Classic SD Edition, which is able to store data without Wi-Fi. The one I have is PurpleAir's new Touch Indoor Air Quality Monitor. Its simple white oval plastic orb reminds me of a decorative, color-changing night light. The PurpleAir Touch measures PM 2.5. If an air quality monitor can measure only one thing, it should be PM 2.5. PurpleAir uses the same color scale for the US Air Quality Index, or AQI Scale. The Touch’s inner LED ring glows green for good air, yellow for moderate, orange for unhealthy for sensitive groups, red for unhealthy, purple for very unhealthy, and deep violet/maroon for hazardous. Depending on the number of taps of your finger, the glow gets brighter or turns off; it’s adjustable.

PurpleAir told me an app is in the works, but for the moment it's the PurpleAir website that has its “community scientist”-sourced map. I was confused the first time I looked at PurpleAir’s map, because it included both indoor and outdoor monitors. And while there the specific model of monitor pops up when you click the number, I’m not sure if I care that Zoe’s indoor air quality in Brooklyn is awesome or that Zander must have burned his muffins in Queens. I think including the indoor monitor data on the map muddies the user experience. To find out the actual AQI number that the PurpleAir Touch is reading, you need to register your Touch with PurpleAir and view it through PurpleAir's web portal. On its Real-Time PurpleAir Map I can see the circle that’s my Touch. There I can see the numbers for my living room. You can choose to be private or public. Looking at the map, I now want to get PurpleAir's outdoor monitor. And while configuring, registering, and checking a web portal is an extra step, I appreciate the simplicity of the Touch’s color system. The PurpleAir touch also makes a useful night light, though it was too bright for me at its highest setting.

At under $40, the Govee Smart Air Quality Monitor is one of the cheapest models I’ve tested. Govee has an easy-to-use and -pair app that can be further paired with Govee air purifiers, and an easy-to-read screen displaying the PM 2.5 along with temperature and humidity. I’ve been using the Govee Smart Air Quality Monitor for the past five months, and it likes to hang out at zero PM 2.5 much more than my other monitors. That said, I’ve moved it all around my home and its sensors spike when I’m cooking or using the toaster; it is just less sensitive than other far more expensive models. And even though it isn’t reading every piece of particulate matter flowing through the air, there’s a lot to be said for an affordable air quality sensor.

The Govee Smart is the size of a hotdog on a roll and comes in white or black. It has the option of displaying a digital clock (in military time) instead of the PM 2.5 reading, but I used it exclusively as an air quality monitor. The Govee Smart also has an air quality indicator color bar on the display; green for good, a confusing blue for moderate, then yellow, and red indicates bad air. If you’re an air quality monitor rookie, the Govee Smart might be the gateway air quality tool you’ve been seeking. I suggest placing your monitor outside your kitchen door or at least near your kitchen, always at breathing height—so at least 3 feet from the floor. Monitoring your PM 2.5 will give you a good idea if your cooking fumes are being pulled outside via exhaust or if they're wafting into your living space. And once you see your air quality, you can’t unsee it.

At under $50 and the size of a juice box, Ikea’s Vindstyrka packs a lot of features in a small package at a low price. The air quality monitor can be used on its own, paired with Ikea’s Starkvind air purifier, and/or connected to Ikea’s Home smart app via the Dirigera hub. I was curious about the Vindstyrka back when I reviewed the Starkvind air purifier. And while I liked the modern look of the purifier, it was on the small side, and the only way I could connect it with my phone was to also use Ikea’s smart home hub. Still, like the Govee Smart Air Monitor above, an inexpensive air quality monitor is a great option for those who don’t want to spend hundreds of dollars.

Unlike the PurpleAir’s bold and colorful—though more expensive—monitors, the Vindstyrka has a demure, almost hard-to-read display. It’s easier to see when the button on the top turns on the light illuminating the display, but I have trouble reading the monitor if I’m across the room. It isn’t a font issue; instead, the difficulty lies with Vindstyrka’s matte gray early-version-of-Kindle screen. Ironically, the Vindstyrka is easiest to read in low light, as it auto-illuminates its display screen. Still, for the price, the Vindstyrka does more than monitor PM 2.5, temperature, and humidity. It also tracks VOCs or volatile organic compounds, those vapors and gases that can cause a host of health issues. And while the Vindstyrka display screen uses an arrow to indicate if VOCs are rising, falling, or remaining the same, it’s some indication. Though to take full advantage of the Vindstyrka, it’s best to use it with the Ikea Home smart app. And if you don’t mind being Ikea-bound, then the Vindstyrka is a solid little monitor at a low price.

When I first used the Awair five years ago, it didn’t bother me that it had its own composite scoring system. The Awair sensor uses a fan and laser sensor to detect PM 2.5; toxic chemicals known as volatile organic compounds, or VOCs; CO_2; temperature; and humidity. Those individual scores go into Awair’s special algorithm that weights each of the five sensors and its actual toxic levels to humans over time, creating its color-coded scoring system. That means a score of 1 is a health hazard while a score of 100 means you’ve reached indoor air nirvana. The issue for me and my more air-aware self is when I see a reading of 100 on an indoor air monitor I think it means the air is bad. Yes, there is a tiny green light indicating overall good air, but still, it is confusing for me.

The score is the default setting on the display and can be easily overridden through the Awair app. By clicking on the gear in the upper right corner, you can change the display from the Awair Score to either temperature, humidity, CO₂, chemicals, dust PM 2.5, or a clock. Though, even when I change the display, the indicator light will shine for the Awair score, so right now my PM 2.5 is 4—that’s good—but my indicator light is a confusing orange because of the high humidity that puts Awair’s composite score at a moderate 55. And as for the VOCs or toxic chemicals, it measures for a mixture as it cannot tease out a specific VOC like formaldehyde or benzene.

I can’t help but think the QP Pro Air Quality Monitor was designed to sit on your desk and be used by those with excellent eyesight. I needed my reading glasses to use the touchscreen display. While the QP Pro’s app is easy to read, it’s odd that for a small monitor that does so much—it reads PM 2.5, VOCs, temperature, CO₂, and humidity—requires a wee bit of squinting. The QP Pro’s bright 3-inch Retina display lets you choose the sensor you want to prioritize to a bigger font. So, for example, you can tap the CO₂ and that will be the easiest-to-read number. Scroll on the screen and it will display the time, weather, outdoor AQI, and UV reading. And, in case of power outage, its lithium-ion battery has a four-hour life.

While this little monitor does a lot, I found it something I would use on a bedside table or next to my computer. The screen, while crisp and clear, is impossible to read from across the room. Still, for the price it packs a lot in a tiny package.

I liked the QP Lite Air Quality Monitor more than the QP Pro for the simple reason it had a clear display. And while it’s smaller than the Pro, its lithium-ion battery life is slightly longer at 6.5 hours. The OLED display and color indicator light, utilizing the AQI color codes, are easy on the eyes. And while the Lite doesn’t measure VOCs, it does measure PM 10, PM 2.5, CO₂, temperature, and humidity.

The Lite’s ring-box size makes it the perfect Airbnb or hotel room accessory. I recently brought it along with me to a New England wedding and my suitemate and I woke up to CO₂ above a thousand. We felt groggy and kept yawning. I opened the windows and we felt better in minutes. This is what an air quality monitor can do. Instead of a vague feeling that you’re getting tired or don’t feel great, one glance at the Lite’s screen and I knew to let the fresh air in. The touch bar at the top of the Lite took some getting used to, but tapping it scrolls through the readings.