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We have added a CO2 monitor, the SAF Aranet 4, as a pick. It can help you identify and mitigate high levels of CO2 that can build up in indoor spaces.
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Home air quality monitors are limited in their abilities—as well as their reliability—so if you’re concerned about the air quality inside your home, the first thing we suggest you do is measure what’s in the air outside, because outdoor conditions often determine indoor conditions.
There’s no simpler way to do that than to use the Environmental Protection Agency’s AirNow mobile app, available free on iOS and Android.
Searchable by zip code or place name, and presented on a map, AirNow’s air quality readings come from direct measurements by hundreds of monitors around the US, offering an air quality forecast in addition to real-time data.
Outdoor air quality predicts indoor air quality. Before buying a monitor, try an air quality index (AQI) app.
An accurate, reliable indoor air quality monitor would be great, but the models we’ve considered have flaws.
To test accuracy, we compared home air quality monitors’ measurements against data gathered from a pro-grade particle counter.
If data (or a hunch) suggests that you have poor air quality, improve it by circulating air through purifiers or HVAC filtration.
The type of home-focused indoor air-monitoring devices we evaluated for this guide offer less information by comparison, as they typically measure only the local airborne particle levels and sometimes volatile organic compounds—gases like the vapors from paint, cleaners, and glue. They rarely measure carbon monoxide and radon, two potentially deadly gases. Many smoke detectors also detect carbon monoxide; see our guide to smoke alarms for more about them. The EPA has a guide to radon, including state-by-state resources for knowing whether your region is at risk and how to get your home tested. And whether you have measured proof or just a hunch that your indoor air quality is poor, there are simple, consistently effective ways to clear the air as much as possible: Run an air purifier or upgrade your HVAC filters to capture fine airborne particles, and open your windows on nice days to vent any volatile organic compounds.
For 2022, we have added a recommendation for a CO2 (carbon dioxide) monitor. Researchers at the University of Colorado in Boulder found that CO2 levels could be used as a proxy for coronavirus risk in crowded public indoor spaces because we exhale both CO2 and, if we’re sick, viruses in the same breath. And even absent the virus, both public spaces and homes can accumulate high levels of CO2 from everyday activities—not just breathing, but from using gas stoves and water heaters. We contacted a researcher to talk about using CO2 monitors for air quality measurements, and together we came up with a product recommendation, as well as some advice on how to interpret and react to its information when monitoring CO2 at home.
The EPA’s free AirNow app is easily searchable, delivers air quality readings in almost every corner of the US, and lets you know in advance of upcoming air quality problems.
We recommend using the AirNow app as your first step because outdoor conditions usually cause any indoor air quality issues you may experience. Although AirNow isn’t the only app of its kind, we found in our research that many other air quality apps simply repackage the data that the EPA stations gather, often without adding much clarity, new information, or interface improvements. Compared with AirNow, some apps make it harder to pinpoint your location, whereas others use a less complete network of monitoring stations to gather the data. AirNow offers you the simplest way to get the info and interpret what it means. And—unlike other apps we tested—it doesn’t bombard you with ads the whole time.
The SAF Aranet 4 is one of the most accurate CO2 monitors available, and its measurements can act as an indicator of your indoor air quality.
The concentration of carbon dioxide (CO2) in indoor spaces is usually higher than in natural outdoor air, and it can rise to levels that impair cognitive function. The main reasons are the combustion of natural gas and of simply exhaling CO2. If you’d like to know how much CO2 is in your home’s or office’s air, the Aranet 4 is our pick. Its measurements are easy to read onscreen, as are its alerts when the level is getting too high. Its app also sends alerts, works with both iOS and Android, and adds seven-day trend monitoring to help you identify patterns of high CO2. And, helping to offset the cost, the high-quality sensor should last for up to seven years.
This device sits on a desk or table and presents indoor air quality readings of particulates, but we’re concerned about its VOC measures and its long-term durability.
We understand that many people reading this guide really do want to take indoor measurements, and for that we suggest the Temtop M10 Air Quality Monitor—with caveats. A lot of the air quality monitors we’ve considered over more than two years of research for this guide have too many credible reports of faulty sensors and connectivity problems for us to strongly recommend them. The M10 is distinguished by its simplicity, though, with a bright display showing particulate measurements (that we confirmed to be accurate), a tiny size, and a lack of Wi-Fi connectivity (a good thing in this case). It’s also affordable enough for you to keep your expectations relatively low—and there’s good reason for you to approach it that way. As with other inexpensive air quality monitors, its VOC measurement is dubious, and some buyers have reported that their M10 arrived damaged or failed within a few weeks.
The EPA’s free AirNow app is easily searchable, delivers air quality readings in almost every corner of the US, and lets you know in advance of upcoming air quality problems.
The SAF Aranet 4 is one of the most accurate CO2 monitors available, and its measurements can act as an indicator of your indoor air quality.
This device sits on a desk or table and presents indoor air quality readings of particulates, but we’re concerned about its VOC measures and its long-term durability.
Tim Heffernan has overseen Wirecutter’s air quality coverage since 2015, with that time period spanning five separate tests of air purifiers. In addition to directly testing air quality monitors, Tim interviewed representatives from five manufacturers—Airthings, Awair, Dylos, Kaiterra, and Temtop—to gain a better understanding of how the devices take their measurements and what they can be used for. We also spoke to Kathleen Owen, an air quality research engineer and a member of air quality–focused committees organized by the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE). John Holecek, a lab scientist and former NOAA researcher who specializes in aerosol and particulate pollution, has also contributed his professional expertise to our work on air quality devices and tools since 2015.
The nationwide impact of smoke from increasingly intense wildfires has led to recent spikes in poor air quality (even as the overall US air quality trend in recent decades shows significant improvement). Between these worsening fires and the increased general awareness of air quality in response to the pandemic, you might think there would be plenty of great home air quality monitors to choose from. Yet that isn’t the case. In our research, we’ve found that most monitors of this class aren’t that great, with many giving unreliable (if not false) readings, especially of volatile organic compounds (VOCs) such as chemical vapors and formaldehyde. Some monitors can accurately measure fine particulates, typically PM2.5 (particulate matter of 2.5 microns in diameter, which can penetrate the lungs). But the number of credible complaints about obviously false readings is alarming.
We believe that an app is the better first stop for most people because it could save you from buying an indoor air quality monitor you may not really need. We arrived at our position independently, and we ran it past Kathleen Owen, an expert air quality engineer and ASHRAE committee member. She agreed with our assessment, she said, based on the inaccuracies and cost of the analyzers versus the benefit.
Whether you use an app, a monitor, or nothing at all, the things you can do to improve your home’s air quality are similar. To address particulate pollutants from outdoors, such as smoke, pollen, or exhaust on a heavily trafficked road, first close your windows and doors, then run an air purifier and/or install medium-MERV HVAC filters and run the system frequently. If your outdoor air quality index (AQI) reading is good and you’re concerned about flushing out VOCs or particulates from within your home, you might ventilate the space by briefly opening windows and doors as a supplement to ongoing filtration from purifiers or HVAC filters.
Our testing proves air purifiers can clean air of pollen, smoke, viruses, and other particulates.
When we decided to recommend an app as the best air quality monitor for most people, we established a few simple criteria: We wanted an app that had a clear, easily searchable interface, that drew on a large number of widely distributed sensors to make localized readings more likely for users in all corners of the US, that didn’t have a lot of bells and whistles, and that, ideally, didn’t require a subscription or purchase for an ad-free version. Many public agencies, including the Environmental Protection Agency and its state-level equivalents, take air quality readings, and the data they gather is public. In fact, most apps use that data, not their own, as it would be prohibitively expensive for other organizations to set up a private nationwide network of air monitors. So what you get on most air quality apps is just a unique design, not different or more accurate information.
We then spent time in the Apple App Store and the Google Play Store, identifying the apps we would be most likely to consider first based on a combination of the authority of the app’s data sources, the app’s popularity among users, and our assessment of the likelihood that the app would be updated regularly and remain satisfying to use. We ultimately downloaded four apps—AirCare, Airly, AirNow, and IQAir AirVisual (iOS) (Android)—and spent several hours (across multiple days and locations) using them on iOS and Android devices, learning their quirks and comparing their features.
We never gave up on our original goal of recommending an indoor air quality monitor, yet when revisiting this guide in autumn 2021, we were alarmed by the number of serious, credible complaints stating that our 2019 air quality monitor picks delivered obviously inaccurate readings, especially for volatile organic compounds (VOCs). We had found them to be reliable on particulate readings in our testing, but clearly something was going wrong once the devices had run for a while in many people’s homes. Many reviews we read also complained of glitchy Wi-Fi connections and poor customer support. When we looked back at the competitors we had originally tested but not picked, we found a similar pattern of complaints on some of them. And in digging further, we discovered that the Environmental Protection Agency has extensively studied (PDF) home air quality monitors (part of a class of monitors it refers to as low-cost sensors) and found their accuracy and longevity wanting, with their VOC sensors lasting for just six months to a year before starting to fail.
All of this led us to revise our criteria and recommendations. For the original version of this guide, we insisted that all of our selections be capable of measuring one specific form of air pollution: particulate matter of 2.5 microns or smaller, better known simply as PM2.5. And we favored those that also measured volatile organic compounds, better known as VOCs (in practice, gases and odors). PM2.5 is of special concern because such particles can deeply penetrate the lungs and are known to cause health problems. VOCs include a huge range of compounds, the most well known of which is probably formaldehyde. A precursor chemical used in many glues, foams, plastics, and building materials found in homes, formaldehyde is a known carcinogen.
But in light of the pattern of failing VOC sensors we’ve since observed, we no longer trust those devices. We also came to doubt the value of smart air quality monitors, which typically use your location to pull in local AQI readings such as those from the EPA. Many apps, including AirNow, do the same without costing you anything.
We did not look at “personal air quality monitors,” a niche class of devices that are designed to be carried on a keychain or necklace to measure the air quality wherever you go. Based on common owner complaints—bad apps, connectivity issues, poor customer service—and what we’ve learned about home air quality monitors, we don’t think these portable devices are worth considering. Besides, an AQI app on your phone can tell you about local conditions, and if you keep an N95 respirator mask on hand when you’re out, it will protect you from particulate pollutants like smoke and dust if their levels are high.
The EPA’s free AirNow app is easily searchable, delivers air quality readings in almost every corner of the US, and lets you know in advance of upcoming air quality problems.
Outdoor conditions often determine indoor air quality, and the first tool we suggest using to monitor air quality in your area is the Environmental Protection Agency’s AirNow mobile app. Available for free on iOS and Android, AirNow offers the best combination of usability, breadth of coverage, and clarity of information among all the apps (and dedicated hardware monitors) we’ve tested.
The AirNow app makes it easy for you to find the current air quality index (AQI) in the place you’re interested in. It can use your own location to find the nearest monitor and report the current readings. You can also type in a place name or zip code if you want to know what’s going on somewhere else, or you can zoom in on a US map to get an idea of the AQI in a broader region. (For example, as I’m writing this, I can see that a large portion of California is reporting terrible air quality due to the fires burning in Sequoia National Park.)
The other apps we tried—AirCare, Airly, and IQAir AirVisual (iOS) (Android)—are less adept. AirCare, for example, lets you search by zip code, but if that location doesn’t have a monitor, the app doesn’t suggest the nearest one to it, leaving you to hunt around on the app’s rather sparse map or guess at other nearby zip codes until you find a reading. Airly doesn’t seem to recognize zip codes at all, and it doesn’t differentiate place names by state. (I typed in Moorefield, wanting to see what the conditions were in the West Virginia town I spent summers in as a kid, and Airly produced a list of five “Moorefield, United States of America” entries for me to pick through until I found the correct one.) AirVisual doesn’t let you do zip code or place-name searches, period. To find the place you’re interested in, you have to go through a tedious series of steps, first picking your region of the planet, then your country, then (in the US) your state—and then you have to scroll through all the city and town names to find yours. All of these apps use your location to find the nearest direct or estimated AQI, as AirNow does, but it’s frustrating not to be able to quickly check on the air quality elsewhere, such as you might want to do before a trip, or if you’re concerned about family or friends who are far away.
Among the apps we tested, AirNow also collects data from the widest network of monitors. It uses EPA and state and municipal monitoring stations (and occasionally those used at industrial sites, such as mines, where air quality is a pressing concern), and it offers direct AQI readings from multiple monitors in every state. You’re rarely very far from one unless you’re well out into the sparsely populated parts of the country, and if you’re in a city, you’re likely not more than a few miles from the nearest monitor. Airly and IQAir AirVisual simply take AirNow’s data and repackage it in clunkier apps. (IQAir AirVisual also occasionally gives readings that are “modelled using satellite data.”) AirCare, in contrast, employs data from a network of inexpensive PurpleAir sensors, which businesses and individuals use as an alternative to the professional-grade monitors that the EPA and other agencies use. It’s a great concept—the aim is to have so many PurpleAir sensors in use that highly localized AQI will be available for almost every corner of the world—and PurpleAir sensors proved quite accurate when California’s South Coast Air Quality Management District tested them against professional equipment (PDF). But for now, the network is nowhere near as robust as AirNow’s.
Airly and IQAir AirVisual simply take AirNow’s data and repackage it in clunkier apps.
Finally, AirNow makes it easier for you to understand your local AQI than the other apps do. The opening screen presents the current AQI in your area. This number, between 0 (perfect) and 500 (hellish), is calculated based on readings of five categories of air pollutant: ozone, particle pollution (PM2.5 and PM10, the numbers being the particles’ size in microns), carbon monoxide, sulfur dioxide, and nitrogen dioxide. The app also color-codes the AQI so you can tell at a glance if there’s anything to be concerned about. Green, for AQIs below 50, means the air is considered healthy; yellow, from 51 to 100, indicates moderate but acceptable levels of pollution; and orange, red, and purple indicate increasingly poor and unhealthy air quality. If you’d like to know more, tapping the Details icon opens a window where the measurements are broken down by category. Currently AirNow reports only two categories—particle pollution and ozone—but it’s helpful to know whether one or the other is especially high. That window also offers simple advice for how to respond to your local AQI, suggesting “Enjoy your outdoor activities” for green AQIs, for example. The other apps also use a color-coded AQI display to quickly indicate the current air quality, but they clutter up the screen with less-useful data and other visual material, such as historic charts. AirCare and Airly also serve ads in their free versions, whereas AirNow is free to download and ad-free.
The SAF Aranet 4 is one of the most accurate CO2 monitors available, and its measurements can act as an indicator of your indoor air quality.
The air in most homes contains more CO2 than outdoor air does—sometimes a lot more, especially in modern, tightly sealed houses. The elevated levels are primarily the result of our own respiration and of natural-gas stoves and ovens, and CO2 can build up in enclosed spaces to concentrations that impair cognitive function. It’s important to note that a CO2 monitor does not give a complete picture of your indoor air quality. A home or office could have low CO2 levels but high particulate levels, for example. The free AirNow app can give you a sense of local particulate levels. And if you’d like to supplement that information with a measure of CO2 in your indoor air, the SAF Aranet 4 CO2 monitor is our pick.
The Aranet 4’s non-dispersive infrared (NDIR) sensor is considered the most accurate and durable type outside of laboratory-grade equipment. Its screen displays the CO2 reading and any “unhealthy level” alerts clearly. And its app—compatible with iOS and Android—can also send alerts and show longer trends in your CO2 levels, helping you take proactive steps to keep them in the healthy range. (Thankfully, the solution is usually dead-simple: Open a few windows and let fresh air in until the level drops to a good number, especially if you’ve got a lot of guests or when you’re cooking with natural gas. HEPA air purifiers and HVAC filters make no difference because they capture particles, not gases.)
In choosing the Aranet 4, we spoke with Alex Huffman, an atmospheric chemist at the University of Denver. During the pandemic, he coauthored a study on CO2 and its relation to risk of coronavirus transmission, and some of his colleagues have been testing CO2 monitors that are aimed at homeowners. “There’s no question that there are consumer CO2 sensors out there that suck, which is not too surprising,” he said. “You’ve probably heard that the first thing you want to do is look for an NDIR sensor, non-dispersive infrared, and that’s absolutely true, but—and I don’t have a solution to this problem—what I’ve heard from colleagues is that there are increasing numbers of these sensors that purport or claim to be NDIR but aren’t. They’re just straight-up lies.”
The Aranet 4 uses a genuine NDIR sensor. Inside the device, infrared light is directed through air-sampling tube. Because CO2 molecules absorb a specific infrared wavelength, a detector at the far end of the tube simply measures how much of it gets eliminated from the light sample, thus giving an accurate measure of how much CO2 is in the air. NDIR sensors are durable, too, with an expected lifespan of seven years, helping offset their relatively high cost. And the Aranet 4 also measures temperature and relative humidity, adding a bit more utility.
CO2 monitors saw a surge in use in 2021 after researchers at the University of Colorado Boulder demonstrated that CO2 levels could be used as a proxy for the concentration of coronavirus, and therefore the risk of transmission, in an indoor public space. Concerns over the combustion byproducts, including CO2, from gas stoves have also risen. And growing awareness that high CO2 levels can impair brain function has also been driving interest in the use of CO2 monitors recently, to help ensure that crowded spaces like classrooms and offices have enough ventilation. It makes sense that CO2 would build up in places where many people congregate. But—and I may just be showing my own naiveté here—it had never occurred to me that CO2 could be a concern in an uncrowded home. So I didn’t expect to learn much when I set up the Aranet in my apartment a few months ago.
In fact I learned a lot, and my experience may help you decide whether the Aranet is worth your investment. I set it up on a hot summer day—one of the few weather conditions when my wife and I keep all the windows closed and turn off our apartment’s fresh-air ventilation. And we discovered that our air contained almost 900 parts-per-million of CO2, more than twice that of fresh outdoor air. In the course of an hour, it rose above 1,000 ppm, triggering a yellow “cause for concern” alert from the Aranet—solely the result of my breathing in the same large (25-by-30-foot) room as the monitor.
Another day, I baked bread in our gas oven, and in the course of 45 minutes the CO2 level rose from a healthy sub-500 ppm to over 1,600, well past the Aranet’s red-alert level of 1,400. A quick stir-fry on our stove’s power burner sent it from 600 to 1,550. Just making my morning single-serving pot of stovetop espresso, which takes about 7 minutes on the smallest burner, can raise the CO2 level by a hundred points.
On the other hand, I found that by simply keeping one of our two fresh-air vents slightly open and a single window cracked by an eighth of an inch, we can maintain a healthy level of about 600 ppm (if the stove or oven isn’t on); the trickle of fresh air is enough to keep the CO2 under control. And when I’m cooking or baking, fully opening one window and the vents for 20 minutes drops the indoor CO2 level close to that of the fresh air outside, which is currently hovering around 400 parts per million here in Queens. (The global average is 420, but regional CO2 levels vary for many reasons. We live in a particularly leafy—by New York City standards—neighborhood, and with all the trees in their full summer greenery, CO2-absorbing photosynthesis is probably helping knock our levels down slightly.)
In short: The Aranet surprised me by revealing just how quickly, and how high, the CO2 level in a home can rise as a result of routine daily behaviors. But almost as quickly, I learned simple counter-behaviors that keep our home’s CO2 level healthy without my needing to constantly monitor it.
This device sits on a desk or table and presents indoor air quality readings of particulates, but we’re concerned about its VOC measures and its long-term durability.
If you really want an indoor air quality monitor, we cautiously recommend the Temtop M10 Air Quality Monitor for now and will soon test a a similar model, the P10 Air Quality Monitor, the latest version of which claims to have a superior PM2.5 sensor and improved software. The M10 measures PM2.5, VOC, and formaldehyde (HCHO) to calculate an air quality index (AQI). Its bright display is readable from across a room, and its tiny size and simple form allow it to fit nicely on a desk or bedside table. We like its simple, intuitive interface, too: A single button turns the device on, and additional presses cycle you through the readings. Lacking Wi-Fi connectivity, the M10 has no associated app, let alone smart functionality. But for many people, that works just fine.
In our test of the M10’s PM2.5 sensor, it gave almost the exact same readings as other monitors we tested, and it tracked with the readings we got from the professional-grade particle counter that we used as a control. This consistency gave us confidence in the M10’s accuracy on PM2.5. The M10 also responded instantly with higher numbers when we introduced a VOC (in the form of a mist of rubbing alcohol), indicating that the VOC sensor was working as it should. We did not test its formaldehyde sensor.
With no Wi-Fi connectivity and no app, the M10 doesn’t track its readings over time, it doesn’t offer outdoor air quality data from nearby weather stations, it doesn’t give you suggestions for how to improve your air quality, and, of course, it doesn’t let you check your air quality on your phone. If that sounds like blessed simplicity, you’ll probably like it.
All that said, we have some fairly strong reservations. Although the M10 gets many positive, detailed reviews from owners who have found their units to be reliable and accurate, since late 2020 it has received a spate of one-star reviews from people whose units arrived broken or defective, and Temtop’s customer service has not been responsive. (Many of the positive reviews are from this same period, too, so it’s not an across-the-board problem, but it is a risk to weigh when you’re deciding whether to buy one.) And, as we explain in How we picked and tested, VOC sensors in low-cost air quality monitors are known to have short working lives—six months to a year, the EPA says—so we doubt their value. For issues that arise, Temtop offers a 30-day return period and a limited one-year warranty.
The Kaiterra Laser Egg+ Chemical, our former top pick among home air quality monitors, has suffered from stock issues and, since we reviewed it in 2019, has been hit by waves of complaints about failed VOC sensors, abandoned smart features, and poor customer service. We no longer recommend it.
The Temtop M10i WiFi Air Quality Monitor is functionally identical to the M10 but adds Wi-Fi connectivity (but only on 2.4 GHz networks) and a simple app that displays the device’s readings, tracks them over time, and lets you turn the device on and off. If you want those features, you have to pay about $40 to $80 more for this model over the basic M10.
In 2019 we tested the Awair 2nd Edition, which Awair has since replaced with the similar Awair Element. Both are beautiful-looking devices, but in our tests, we found the 2nd Edition frustrating to use due to poor connectivity, an unclear user interface, app-reliability issues, and other problems echoed in owner reviews of the Element.
The IQAir AirVisual Pro monitor easily connected to our Wi-Fi network. Its app is clearly laid out, and both the device and the app offer simple, practical advice on how to address indoor air quality issues (for example, they suggest opening a window if the outdoor air quality is good). But the device’s physical interface is a clunky, frustrating experience, and at this writing the machine is simply far too expensive: For $270, you should get more than just PM2.5 and CO2 measurements (plus humidity and temperature, which aren’t strictly air quality measurements).
The Dylos DC1100 is a simple, accurate particle counter that measures down to 1 micron—all the other devices we tested go to only 2.5 microns (PM2.5). But it’s a specialist’s device, generating reams of data but leaving the analysis up to the user. For the small subset of people who want raw data above anything else, the DC1100 may be attractive. For most people, however, our picks’ simpler displays and easy-to-digest alerts are much more useful.
The Igeress Indoor Air Quality Monitor is one of a number of similar, relatively inexpensive monitors that we encountered in our search. We tested it because it is reasonably popular, but it delivered wildly different readings from all the other monitors we tried, leading us to doubt its accuracy.
Airthings is a well-regarded manufacturer of air quality monitors, specializing in radon detection. When we tested monitors in 2019, none of the company’s many devices measured particulates, a requirement for inclusion in our tests. A new device that measures PM2.5, the View Plus, is available, but we have not tested it.
The Eve Room does not measure particulates, a requirement for our testing.
Tim Heffernan is a senior staff writer focusing on air and water quality and home energy efficiency. A former writer for The Atlantic, Popular Mechanics, and other national magazines, he joined Wirecutter in 2015. He owns three bikes and zero derailleurs.
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