Indoor Air Quality: The Measurements That Matter at Home

Six indoor measurements answer nearly every air quality question a household has: fine particulate matter (PM2.5), carbon dioxide, volatile organic compounds, radon, carbon monoxide, and relative humidity. Each needs a different type of sensor and has its own healthy range, and EPA, WHO, and ASHRAE guidance gives real numbers for reading all six together as one picture.

By Olivia Bennett

Who This Is For

This is for anyone who has bought, or is thinking about buying, a home air quality monitor and isn’t sure what the numbers on it actually mean. It’s for parents wondering whether a nursery reading is fine or worth acting on, for people with asthma or allergies trying to figure out which measurement to watch most closely, and for homeowners who have never tested for radon and don’t know where to start.

It’s also for anyone who already reads this site’s articles on outdoor air quality alerts, indoor CO2 and focus, or cooking ventilation and wants to see how those individual topics fit into a bigger picture. Those pieces each go deep on one measurement. This one is the map that shows how all of them connect, plus two measurements, radon and carbon monoxide, that carry real safety weight and deserve their own attention here.

If you’re renting and can’t install anything permanent, most of what follows still applies. Testing kits, portable monitors, and habit changes make up the bulk of the practical advice, not renovations.

A note on scope: This article explains what to measure and how to interpret common healthy-range guidance from agencies like the EPA, WHO, and ASHRAE. It is educational information about general household air quality, not medical advice, and it is not a substitute for a licensed radon mitigation contractor, HVAC technician, or physician. If you suspect a carbon monoxide emergency, treat it as urgent and act immediately, as described in the safety section below.

Why “Indoor Air Quality” Isn’t One Number

Walk into most conversations about indoor air quality and someone will eventually ask, “so is my air good or bad?” as if a single score could answer that. It can’t, not honestly. A bedroom can have excellent particulate matter levels and terrible ventilation. A kitchen can have safe carbon monoxide readings and a radon problem two floors down that has nothing to do with anything happening in that room. Multi-sensor monitors compress several unrelated measurements into one glowing color, which is convenient for a glance but genuinely misleading if that’s the only information you use.

The six measurements in this guide, fine particulate matter, carbon dioxide, volatile organic compounds, radon, carbon monoxide, and relative humidity, come from different sources, need different sensors to detect accurately, and call for different responses when they’re off. Treating them as one undifferentiated “air quality score” is how people end up ignoring a genuine radon risk while stressing over a CO2 reading that mostly reflects a closed bedroom door overnight.

This guide treats each one on its own terms: what it is, what a healthy range looks like according to the agencies that publish that guidance, what kind of sensor actually measures it well, and how to read it alongside the other five.

The Six Measurements That Actually Matter

Before going deep on each one, here is the full picture in a single reference table: what target range agencies actually recommend, what a typical home tends to show, and which sensor type reliably catches it. Bookmark this table; nearly every other section in this guide expands on one row of it.

PM2.5: The Measurement Most Monitors Lead With

PM2.5 refers to particles 2.5 micrometers or smaller, small enough to travel deep into the lungs and, to a lesser degree, the bloodstream. Indoors, the biggest sources are usually cooking (especially frying and searing), candles, incense, and outdoor air infiltrating through open windows or a leaky building envelope during smoke events or high-pollution days.

In February 2024, the EPA lowered the annual health-based PM2.5 standard from 12.0 to 9.0 micrograms per cubic meter (µg/m³), while keeping the 24-hour standard at 35 µg/m³. Those are outdoor regulatory standards, not indoor rules, but most consumer monitors and air quality apps use them as the reference point for what “good” looks like indoors too. The World Health Organization’s 2021 global air quality guidelines set a considerably tighter bar: 5 µg/m³ annual and 15 µg/m³ for a 24-hour average, reflecting evidence that health effects appear at exposures below the EPA’s regulatory threshold. There isn’t a contradiction here so much as two different jobs: the EPA number is a legally enforceable outdoor standard, while WHO’s is a health-based aspirational target.

Most consumer PM2.5 monitors use an optical particle counter, a small laser that estimates particle mass from how light scatters off it. This approach is inexpensive and works well for everyday tracking, but it isn’t inherently precise. A 2023 study in Atmospheric Measurement Techniques evaluating the correction equations used on PurpleAir sensor data (including the one developed by Barkjohn and colleagues in 2021, now used on the EPA’s AirNow Fire and Smoke Map) found that corrected readings tracked reference-grade monitors closely under normal urban conditions and wildfire smoke, with agreement close to a 1:1 slope. The same study found a real weak spot: during mineral dust events, these sensors underestimated true PM2.5 by a factor of five to six, and at very high concentrations above roughly 600 µg/m³, even the best correction still underestimated by about 20%. The practical takeaway isn’t that budget monitors are useless; it’s that their accuracy depends on conditions, and a reading during a dust storm or extreme smoke event should be treated as a floor, not a precise number.

This site covers outdoor PM2.5 spikes and AQI alert systems in more depth elsewhere. Indoors, the same particle matters just as much, and it responds directly to habits: a range hood used consistently, fewer candles burned in small closed rooms, and a portable HEPA air cleaner in spaces without good ventilation options all measurably move the number.

Carbon Dioxide: A Ventilation Gauge, Not a Toxin at Home Levels

CO2 gets a full explainer elsewhere on this site, since the research behind its link to cognitive performance deserves its own careful treatment. The short version that matters here: indoor CO2 is a proxy for how much fresh outdoor air is reaching a room, not a direct health hazard at the concentrations most homes ever see.

ASHRAE, the professional body behind U.S. ventilation standards, is explicit that its own Standard 62.1 does not set a CO2 concentration limit. The commonly repeated “1,000 ppm” figure traces back to a 19th-century threshold for perceived body odor, not a modern health finding, and ASHRAE’s 2023 position document describes current evidence on CO2’s cognitive effects as inconsistent across studies. For a rougher, engineering-grade signal, the European standard CEN 16798-1 (2019) defines ventilation adequacy by how far indoor CO2 sits above the outdoor baseline: roughly 550 ppm above outdoor for the highest category of ventilation, up to about 1,350 ppm above outdoor for the lowest acceptable category.

Actual toxicity is a separate, much higher-dose story. NIOSH’s occupational exposure limits for CO2 include a recommended exposure limit of 5,000 ppm as a time-weighted average, a short-term ceiling of 30,000 ppm, and an immediately-dangerous-to-life-or-health level of 40,000 ppm, all encountered in industrial or confined spaces, not a bedroom. If your monitor’s CO2 sensor is an NDIR type and it climbs past 1,500 ppm in a closed room, that’s a ventilation cue worth acting on, not a toxicology emergency.

Volatile Organic Compounds: The Hardest Number to Interpret

Volatile organic compounds are chemicals that evaporate readily at room temperature: solvents in paint and varnish, fragrance compounds in air fresheners and cleaning products, formaldehyde off-gassing from pressed wood furniture, and residue from dry cleaning. The EPA’s own research, including its long-running TEAM Study, found that concentrations of many VOCs run consistently higher indoors than outdoors, commonly two to five times higher and, for a number of specific compounds, up to ten times higher, regardless of whether the home sits in a rural or heavily industrialized area. During and immediately after certain activities, particularly paint stripping, the EPA has measured levels up to 1,000 times typical outdoor background.

Here’s the part that trips people up: unlike PM2.5, there is no federally enforceable indoor VOC standard, and the EPA states plainly that not much is known about the health effects of the levels typically found in homes, as opposed to occupational exposure. Most consumer monitors don’t identify which specific VOC they’re detecting either. They report a single “TVOC index” from a metal-oxide or photoionization sensor, a relative number that goes up when something evaporates nearby, not a calibrated concentration you can compare against a published health guideline.

The practical use of a VOC reading, then, isn’t comparing it to a target number the way you would with PM2.5. It’s watching for spikes and figuring out their source: a VOC index that jumps right after painting, cleaning, or unpacking new furniture and then settles back down over hours to days is doing exactly what it should, flagging an event so you know to ventilate. A baseline that stays elevated with no obvious trigger is worth investigating, often by process of elimination: air out the room, remove candles or air fresheners, and see if the number drops.

Radon: The Measurement Most Homes Skip

Radon is a naturally occurring radioactive gas that seeps up from soil and rock through foundation cracks, sump pits, and gaps around pipes. It’s colorless, odorless, and completely undetectable without a test, which is exactly why it’s the measurement most households never think to check, despite carrying more documented health weight than anything else on this list. The EPA describes radon as the leading cause of lung cancer among people who have never smoked and the second-leading cause of lung cancer overall, behind smoking itself, attributing roughly 21,000 lung cancer deaths a year in the United States to radon exposure, with about 2,900 of those among lifelong non-smokers.

The EPA’s action level is 4 picocuries per liter (pCi/L): if a test comes back at or above that, the agency recommends fixing the home. Between 2 and 4 pCi/L, the EPA still suggests considering mitigation, noting there’s no fully safe level of radon exposure, just a lower one. For context, the national average indoor level is about 1.3 pCi/L, and average outdoor air runs around 0.4 pCi/L, but radon concentration depends heavily on local geology and how a specific foundation is built, so a neighbor’s low result tells you very little about your own home.

Testing comes in two basic flavors. Short-term tests, using a charcoal canister or electret device, stay in place for two to 90 days depending on the specific kit. Long-term tests run more than 90 days and give a more representative year-round picture, since radon levels shift with season, soil moisture, and how much a home is sealed up for heating or cooling. A growing option is the continuous radon monitor (CRM), a plug-in electronic device that gives ongoing readings rather than a single lab result, useful for households that want to watch trends the way they would with a CO2 or PM2.5 monitor. For placement, the EPA recommends testing on the lowest lived-in level of the home, in a regularly used room such as a living room, den, or bedroom rather than a kitchen or bathroom, positioned at least 20 inches above the floor and away from drafts, exterior walls, high heat, and high humidity.

If an initial short-term test comes back at or above 4 pCi/L, EPA guidance is to follow up with either a second short-term test or a long-term test before committing to mitigation, since a single reading can be skewed by unusual weather or a temporarily closed-up house. A confirmed elevated result typically leads to sub-slab depressurization, a mitigation system installed by a certified contractor that vents radon from beneath the foundation to the outdoors before it enters living space. It’s a well-established, relatively routine fix, not a major renovation.

Carbon Monoxide: The One With a Genuine Emergency Threshold

Carbon monoxide is produced by incomplete combustion: a gas furnace, water heater, stove, fireplace, generator, or attached garage with a running engine. Like radon, it’s colorless and odorless, but unlike radon, it can become a same-day emergency. The EPA notes that average CO levels in homes without gas stoves run 0.5 to 5 parts per million (ppm), rising to roughly 5 to 15 ppm near a properly adjusted gas stove, and 30 ppm or higher near a poorly adjusted one.

This is where the distinction between an “air quality monitor” and a dedicated CO alarm matters most. Many multi-sensor monitors include a CO reading, and it’s a genuinely useful number for everyday tracking. But a certified home safety alarm has to meet a specific, tested response standard: UL 2034, the Underwriters Laboratories standard for CO alarms, requires devices to sound within a defined window at defined concentrations, roughly 60 to 240 minutes at 70 ppm, 10 to 50 minutes at 150 ppm, and 4 to 15 minutes at 400 ppm, with faster response required as concentration rises. Not every general-purpose air quality monitor is certified to this standard or tested at these response speeds, which is why the Consumer Product Safety Commission recommends every home have dedicated CO alarms on every level, outside each sleeping area, in addition to any multi-sensor monitor a household also runs. CPSC further recommends annual professional inspection of all fuel-burning appliances, since a failing furnace or water heater is one of the most common sources of a dangerous CO buildup.

Symptoms at rising concentrations follow a predictable pattern, according to EPA’s summary of the health evidence: fatigue in healthy people and chest pain in those with heart conditions at low levels, then headache, dizziness, and impaired vision as concentrations climb, followed by confusion, nausea, and loss of coordination. Because these symptoms overlap with flu or simple tiredness, a CO alarm is not optional safety equipment; it’s the only reliable way to catch a rising concentration before symptoms make it hard to think clearly enough to leave.

Relative Humidity: The Quiet One That Controls Mold Risk

Relative humidity doesn’t get the same attention as PM2.5 or CO2 on most monitor dashboards, but it quietly governs one of the most common indoor air complaints: mold. The EPA recommends keeping indoor humidity below 60%, and ideally between 30% and 50%, stating directly that moisture control is the key to mold control. Above that range, especially sustained over days in a bathroom, basement, or laundry area, mold and dust mites both find the conditions they need to establish themselves.

The EPA’s practical guidance centers on speed and prevention: dry any water damage, spill, or leak within 24 to 48 hours, since that window is roughly how long it takes for mold to begin colonizing a damp surface. Beyond emergency drying, the recommended habits are ordinary maintenance: run bathroom and kitchen exhaust fans during and after showers or cooking, vent clothes dryers to the outside rather than indoors, keep gutters clear so water doesn’t pool near the foundation, and use a dehumidifier or air conditioner in persistently damp spaces. Very low humidity, well under 30%, isn’t part of EPA’s mold guidance specifically, but it’s worth watching too, since dry indoor air in winter is commonly associated with dry skin, irritated sinuses, and static, even though the agency’s core recommendation focuses on the upper bound that drives mold risk.

Reading a Home Monitor as One System, Not Six Separate Alarms

The real skill in home air quality monitoring isn’t memorizing six target numbers. It’s noticing which measurements move together and using that pattern to find the actual cause, rather than reacting to each number in isolation. A spike in CO2 alongside a spike in TVOC during a dinner party points to occupancy and cooking, not a hidden chemical problem. A PM2.5 spike with flat CO2 and flat VOC usually means an outdoor smoke event or a specific point source like a candle, not a ventilation failure. A creeping humidity number with no other change is a maintenance issue, not an emergency.

This is also why radon and carbon monoxide sit apart from the other four. They don’t follow the same “watch the trend” logic. Radon requires a dedicated test because it has no correlation with anything a multi-sensor monitor typically tracks, and carbon monoxide requires a certified alarm because the response window at dangerous concentrations is measured in minutes, not days. Treat those two as standalone safety checks, and treat PM2.5, CO2, VOCs, and humidity as one connected story you read together.

A Worked Example: One Household’s First Two Weeks With a Monitor

Common Measurement Mistakes

What Indoor Air Quality Professionals Do Differently

People who assess indoor air quality for a living rarely chase a single perfect reading. They look at trends across days, not one snapshot, because occupancy, cooking, and weather all create normal, temporary swings that don’t mean anything is wrong. A CO2 spike during a packed dinner party and a CO2 spike during an empty house at 3 a.m. get read completely differently, even at the identical ppm number.

They also test for radon as a matter of routine rather than a reaction to symptoms, because radon produces no symptoms at all until decades of cumulative exposure have already done their damage. A professional home inspector treats a radon test the way a dentist treats an annual X-ray: not because anything seems wrong, but because it’s the one thing you genuinely cannot assess by how a room feels.

Perhaps most importantly, they keep safety equipment and comfort monitoring in separate categories. A CO alarm and a smoke detector are non-negotiable, certified, life-safety devices. A multi-sensor air quality monitor is a genuinely useful comfort and habit-tracking tool, but it is not held to the same testing standard, and professionals don’t treat it as a replacement for either.

Copy-Ready Home Air Quality Checklist

Safety Limits and When to Take Action

Most of the measurements in this guide call for gradual habit changes. Two of them can call for immediate action, and it’s worth being explicit about where that line sits.

Carbon monoxide: If a UL 2034 alarm sounds, treat it as real. Get everyone outside into fresh air immediately, and do not re-enter until emergency services or a qualified technician confirms it’s safe. If anyone in the household shows symptoms consistent with CO exposure, headache, dizziness, confusion, nausea, or chest pain, especially if multiple people are affected at once or symptoms improve after leaving the house, seek emergency medical care and mention possible CO exposure specifically.

Radon: A confirmed result at or above 4 pCi/L calls for professional mitigation, not just ventilation habits. This isn’t an emergency in the same acute sense as CO, since radon’s harm accumulates over years, but it isn’t optional either. A result between 2 and 4 pCi/L is a reasonable trigger for a longer confirmatory test rather than immediate alarm.

PM2.5: During heavy outdoor smoke events, indoor levels can climb quickly even with windows closed. If you or a household member has asthma, COPD, heart disease, or is pregnant, treat a sustained indoor PM2.5 reading above the WHO’s 15 µg/m³ 24-hour guideline as a cue to run a HEPA air cleaner and minimize additional indoor particle sources like candles or frying until it settles.

None of this replaces professional judgment. A radon mitigation contractor, HVAC technician, or physician can assess your specific situation in ways a general guide cannot.

Key Takeaways

• Six measurements cover nearly everything worth knowing about home air: PM2.5, CO2, VOCs, radon, carbon monoxide, and relative humidity.
• Each needs a different sensor: optical for PM2.5, NDIR for CO2, PID or metal-oxide for VOCs, electrochemical for CO, and a hygrometer for humidity. Radon needs a dedicated test kit or continuous monitor, not a general air quality sensor.
• EPA’s 2024 PM2.5 standard is 9.0 µg/m³ annual; WHO’s tighter 2021 guideline is 5 µg/m³ annual.
• Radon’s action level is 4 pCi/L, and it’s the leading cause of lung cancer among non-smokers, responsible for an estimated 21,000 U.S. deaths a year.
• A dedicated UL 2034-listed CO alarm is not optional; a general air quality monitor’s CO reading is not a certified substitute.
• Keep indoor humidity between 30% and 50% to control mold risk, and dry any water damage within 24 to 48 hours.
• Read the measurements together, not in isolation. Matching patterns across sensors identifies the actual cause far better than any single number.
• Budget PM2.5 sensors are reasonably accurate under normal conditions but can meaningfully underestimate during dust storms or extreme smoke.

Frequently Asked Questions

Do I really need six different devices to track all of this?

No. A single multi-sensor monitor typically covers PM2.5, CO2, VOCs, and humidity in one unit. Radon needs a separate dedicated test kit or continuous radon monitor, and carbon monoxide needs its own UL 2034-listed alarm. That’s really only three purchases: one air quality monitor, one radon test, and CO alarms on each level of the home.

Which single measurement matters most if I can only check one?

Radon, if you’ve never tested. It carries the largest documented health burden of anything on this list and produces zero everyday symptoms, so it’s the one measurement you genuinely cannot estimate by how a room feels. Carbon monoxide alarms are equally essential but are already standard safety equipment in most homes.

How accurate are the budget multi-sensor monitors sold online?

Reasonably accurate for PM2.5 and CO2 under normal conditions when the manufacturer publishes a correction method, based on research evaluating PurpleAir-style sensors against reference monitors. Accuracy drops meaningfully during dust events or extreme smoke, and VOC readings are relative index numbers, not calibrated concentrations, on nearly all consumer devices.

Can houseplants meaningfully improve any of these six measurements?

Not at a scale that moves a monitor’s reading. Plants contribute negligibly to CO2 removal or VOC filtration compared with actual ventilation or source control. They can be a nice addition to a room, but they shouldn’t replace an exhaust fan, a cracked window, or a HEPA air cleaner in your actual air quality plan.

Should I test for radon even if my neighbor’s home tested low?

Yes. Radon concentration depends on local soil geology, foundation type, and construction details that can vary significantly between adjacent homes. The EPA’s guidance is that every home should be tested on its own, regardless of what a nearby house showed, since radon has been found in every state.

What’s the real difference between an air quality monitor and a CO alarm?

A dedicated CO alarm is certified to UL 2034, a tested standard requiring the alarm to sound within specific time windows at specific concentrations, such as within 4 to 15 minutes at 400 ppm. A general air quality monitor’s CO sensor is useful for everyday tracking but isn’t necessarily held to that same certified emergency-response standard, so it shouldn’t replace a dedicated alarm.

How often should I retest radon or recalibrate my monitors?

The EPA recommends retesting radon after any major renovation, foundation work, or HVAC change, and periodically even without one, since levels can shift over time. For sensor-based monitors, follow the manufacturer’s calibration guidance, typically every one to two years, since optical and electrochemical sensors can drift gradually with age and dust buildup.

References

• U.S. Environmental Protection Agency, “Final Reconsideration of the National Ambient Air Quality Standards for Particulate Matter,” February 7, 2024.
• World Health Organization, “WHO Global Air Quality Guidelines: Particulate Matter, Ozone, Nitrogen Dioxide, Sulfur Dioxide and Carbon Monoxide,” September 22, 2021.
• U.S. Environmental Protection Agency, “A Citizen’s Guide to Radon,” EPA 402/K-12/002.
• U.S. Environmental Protection Agency, “Health Risk of Radon.”
• U.S. Environmental Protection Agency, “Carbon Monoxide’s Impact on Indoor Air Quality.”
• U.S. Consumer Product Safety Commission, “Carbon Monoxide Alarm Conformance Testing to UL 2034: Standard for Safety.”
• U.S. Environmental Protection Agency, “A Brief Guide to Mold, Moisture and Your Home.”
• U.S. Environmental Protection Agency, “Volatile Organic Compounds’ Impact on Indoor Air Quality.”
• ASHRAE, “Indoor Carbon Dioxide,” public policy position document, 2023.
• National Institute for Occupational Safety and Health, “Carbon Dioxide,” Immediately Dangerous to Life or Health (IDLH) documentation.
• “An Evaluation of the U.S. EPA’s Correction Equation for PurpleAir Sensor Data in Smoke, Dust, and Wintertime Urban Pollution Events,” Atmospheric Measurement Techniques, March 13, 2023.
• U.S. Environmental Protection Agency, “Guide to Air Cleaners in the Home,” 2nd edition, August 2018.
• U.S. Environmental Protection Agency, “Introduction to Indoor Air Quality.”