How to Check and Monitor Air Quality and Humidity in Your Home

You can check air quality in your home with three devices: an indoor monitor for PM2.5, TVOC, CO2 and humidity, a carbon monoxide alarm on each floor, and a radon kit on the lowest floor you live on. Their readings mean the most when you compare them with EPA and WHO reference levels and follow them for several days.

In this guide, we'll cover what to measure, how to run an air quality check in your home and test humidity, how to set up monitoring you can follow from your phone, and what to do when the numbers come back high.

Why Indoor Air Is Often More Polluted Than Outdoor Air

Americans spend about 90% of their time indoors, and some pollutants reach concentrations 2 to 5 times higher inside than they typically do outdoors. EPA connects part of this to energy-efficient construction: when a tightly built home lacks mechanical ventilation, less outdoor air comes in to dilute what's released inside.

Daily life supplies the rest. Frying releases fine particles, new furniture and cleaning sprays add VOCs, and every person in the house raises CO2 by breathing. None of it is visible, so measuring is the way to find out how much of each is in your air.

What to Measure in an Indoor Air Quality Test

A home indoor air quality test looks at seven readings. Some come with firm agency numbers, such as EPA's humidity range and radon action level. TVOC, indoor CO2 and mold have no official indoor limit, and for those we list the closest reference point available.

Pollutant	Where it comes from at home	What counts as normal	How to measure it
PM2.5	Frying and searing, candles, fireplaces, smoking, outdoor smoke drifting in	No indoor standard. Outdoor references: EPA 9 µg/m³ (annual) and 35 µg/m³ (24-hour); WHO guideline 15 µg/m³ (24-hour) and 5 µg/m³ (annual)	Indoor monitor with a laser particle sensor
TVOC	Paint, new furniture, cleaning sprays, air fresheners and cooking all release volatile organic compounds	No EPA, WHO, ASHRAE or CDC limit. Each monitor uses its own scale, so watch for sudden jumps	Monitor with a VOC sensor
CO2	People breathing, gas cooking	No indoor health limit. Outdoor air runs about 300 to 500 ppm, and ASHRAE treats 1,000 to 1,200 ppm in rooms of seated people as a ventilation indicator	Monitor with an NDIR CO2 sensor for a direct reading. Some monitors estimate a "CO2 equivalent" instead
Humidity	Showers, cooking, laundry dried indoors, weather	30% to 50% relative humidity, always below 60% (EPA)	Hygrometer, or any monitor or AC controller with a humidity sensor
Carbon monoxide	Gas stoves, furnaces, fireplaces, attached garages	No agreed indoor standard. Homes without gas stoves average 0.5 to 5 ppm, and levels near a properly adjusted gas stove are often 5 to 15 ppm (EPA)	CO alarm
Radon	Soil gas entering through the foundation	EPA recommends fixing the home at 4 pCi/L or higher	Short-term or long-term radon test kit
Mold	Leaks, damp basements, poorly ventilated bathrooms	No EPA or other federal limit for mold or mold spores	Visual and smell check, humidity tracking, professional sampling for hidden mold

Sources: PM2.5 from EPA's 2024 annual PM2.5 standard and the WHO Global Air Quality Guidelines (2021). CO2 from ASHRAE Technical FAQ 35. Humidity and mold from EPA's Brief Guide to Mold, Moisture and Your Home. Carbon monoxide from EPA's page on CO levels in homes. Radon from EPA's radon mitigation page.

Both PM2.5 figures in the table come from outdoor standards. No agency has published an indoor PM2.5 limit, so monitor makers and health guides borrow the outdoor numbers.

CO2 causes a different kind of confusion. Many websites and monitors present 1,000 ppm as an ASHRAE health limit, yet ASHRAE says its standards contain no indoor CO2 limit and calls the evidence for health effects at common indoor levels inconsistent. A rising CO2 reading is best read as a sign that the room needs more fresh air.

Signs of Poor Indoor Air Quality

Your body often reacts to polluted air before any test confirms it. The most common signs of poor indoor air quality are:

• Headaches, congestion, coughing or sinus pressure that ease when you leave the house and return once you're back
• Feeling foggy or tired at home while you feel fine at work
• A musty smell, usually from mold
• A chemical smell that lingers, often VOCs from fresh paint, flooring or new furniture
• Stale, stuffy air in rooms where ventilation falls short and CO2 builds up
• Condensation on the inside of windows, which EPA links to high humidity

Burning eyes, a scratchy throat, and dry skin belong on the same list of symptoms of bad air quality. When several of them appear together, a monitor or test kit can confirm which pollutant is involved.

How to Test Air Quality in Your Home

Testing goes in three steps, and each one costs more than the one before it. Working through them in order means you pay for a professional only when the cheaper steps leave a question open.

1. Walk Through Each Room

A first air quality check in your home needs only your eyes and nose. Look for dark spots on walls and ceilings, water stains, dust collecting on vents and fog on windows in the morning, and pay attention to any smell that stays in one room.

Keep a symptom log for a week as well, noting the room and the time of day. Patterns in the log show where to put a monitor or test kit.

2. Use Kits and Detectors for Radon, CO and Mold

• Radon. Radon needs its own test kit or detector. Place a kit on the lowest floor you live on, at least 20 inches above the floor. EPA calls DIY kits easy and inexpensive, and some states give them away.
• Carbon monoxide. CPSC advises a working CO alarm on every level of the home and outside each bedroom. General air quality monitors don't measure CO.
• Mold. When mold is visible, EPA says sampling is usually unnecessary, and the next step is to clean it and fix the moisture behind it. Testing indoor air for mold becomes useful when you smell mold but can't find it.

3. Book a Professional Assessment When Needed

A professional indoor air quality test relies on lab analysis, which can identify individual VOCs, mold species and moisture hidden inside walls. That level of detail is worth paying for when:

• you suspect mold behind walls, under floors or inside ductwork
• radon levels in your county are high and you want a certified measurement
• symptoms continue after you've removed obvious sources and improved ventilation

Radon results of 4 pCi/L or higher call for a mitigation system. EPA says most homes can be fixed for about the cost of other common home repairs, with the final price set by the size and design of the house.

How to Check Humidity in Your House

A digital hygrometer is the most direct way to check humidity in your house. EPA puts the cost of these small meters at about $10 to $50 and recommends keeping indoor humidity between 30% and 50%, with every room below 60%.

Other options give either the same number or a rough hint, listed here from most to least accurate:

1. Digital hygrometer or air quality monitor. Shows relative humidity as a percentage, and many models also store the day's highs and lows.
2. Smart thermostat or AC controller with a humidity sensor. Sensibo Sky, Air and Air PRO controllers include temperature and humidity sensors, so the reading and its history appear in the same app you use to run the AC.
3. Rough checks at home. Window condensation can signal high humidity. The ice-water glass test is less useful. By the standard dew point formula, a glass at about 38°F (3°C) fogs in a 72°F (22°C) room once humidity passes roughly 30%, so it fogs in most homes. A glass that stays dry suggests dry air, and a wet one doesn't tell you much.

Where to Place a Hygrometer

To measure humidity in a room accurately, set the meter at about chest height and keep it away from showers, kettles, cooktops and outside doors. Steam and drafts shift the reading for several minutes, which makes a meter right beside them unreliable for the room as a whole. Readings also vary from room to room, so check bedrooms, bathrooms and the basement separately.

Seasonal Changes in Indoor Humidity

Heating dries indoor air, so winter readings often drift toward 30% or lower. Humid summers and damp basements push readings the other way, and mold risk climbs with them. Take readings over a few days in each season before deciding what to change.

Lowering indoor humidity starts with the largest moisture sources in the house, such as laundry dried indoors, showers and cooking without a range hood.

Monitoring Air Quality and Humidity Continuously

Most people check the air quality in their home once, usually after a symptom appears. A single indoor air quality test records the conditions of that day, while indoor pollutants move with daily routines:

• Evening cooking. PM2.5 climbs while you fry or sear food and falls after the range hood clears it.
• Overnight in a closed bedroom. CO2 builds while people sleep with the door shut.
• After a shower. Humidity jumps and stays high when the exhaust fan is off.

Features to Look For in a Home Air Quality Monitor

Feature	What it gives you
Sensors	PM2.5, TVOC, CO2 and humidity cover most homes. Radon and carbon monoxide still need dedicated devices
App with history	Patterns only show up across days of data
Push alerts	You hear about a spike while it's happening
Power	Plug-in units record around the clock. Battery units need charging or new batteries
Smart home integrations	Readings can switch on fans, purifiers or the AC automatically

Best Spot for an Air Quality Monitor

• At breathing height, in the room where you spend the most time
• Away from windows, vents and supply grilles
• Out of direct sunlight
• A few feet from the stove, candles or a diffuser

Sensibo Elements for Air Quality and Humidity

Sensibo Elements is our smart air quality monitor. It tracks TVOC, PM2.5, CO2 equivalent, ethanol, temperature and humidity, and it estimates the CO2 equivalent value from TVOC levels and overall air conditions without a dedicated CO2 sensor.

When pollution rises, the app sends an alert to your phone, and live readings and history are available from anywhere. Elements connects with Google Home, Amazon Alexa, SmartThings and IFTTT. If you also use a Sensibo AC controller, PureBoost can turn on the AC fan automatically when pollution goes up. Elements runs on a wall adapter, so it records day and night.

What to Do When Readings Come Back High

Start by removing or reducing the source, then bring in more fresh air, and use filtration for what remains.

Reduce Sources and Ventilate

Store paints, solvents and cleaners in sealed containers, ideally outside your living space, and choose low-VOC paints and finishes. A range hood that vents outside handles cooking particles, and a bathroom fan running during and after showers handles steam. Never idle a car in an attached garage. Drying a leak or spill within 24 to 48 hours prevents mold growth in most cases.

Choose the Right Filters

A true HEPA filter removes at least 99.97% of particles at 0.3 microns, the size that's hardest to catch, and traps larger and smaller particles at a higher rate. HEPA doesn't capture gases, so VOCs call for an activated carbon filter. Air purifiers with both HEPA and carbon filters handle particles and gases in one unit. For central heating and cooling, your HVAC manual lists the highest-rated filter the system can take.

Mold and Carbon Monoxide Problems

Problem	What to do
Mold covering less than about 10 square feet	You can usually clean it yourself, then fix the moisture source (EPA)
Mold covering more than 10 square feet	Follow EPA's guidance for larger cleanups or hire a contractor experienced with mold
CO alarm sounds	Get outside immediately and call 911 from fresh air, then wait for responders to say when it's safe to go back in

Monthly and Seasonal Upkeep

• Replace HVAC and purifier filters on the manufacturer's schedule.
• Review your monitor's history once a month for spikes that repeat at the same time or in the same room.
• Test CO alarms following the manufacturer's instructions.
• Retest radon after renovations or when you start using a lower floor of the home.

FAQ

How often should I check the air quality in my home?

Keep a monitor running all the time for PM2.5, TVOC, CO2 and humidity, since these change within hours. Test for mold after water damage or when a musty smell appears.

What counts as good indoor air quality?

Air is in good shape when readings stay near the reference levels for most of the day and short spikes, such as PM2.5 during cooking, clear afterward. Readings that stay high overnight or for days point to a ventilation or source problem.

What is a good humidity level for a house?

Between 30% and 50% relative humidity, with 60% as the upper limit. Above 60%, damp surfaces stay wet long enough for mold to start growing.

Can my phone check indoor air quality?

No, phones have no air quality sensors, and the AQI in a weather app describes outdoor air near you. Your phone shows indoor readings only through the app of a connected monitor.

Do air purifiers work?

For particles, yes, as long as the purifier's Clean Air Delivery Rate (CADR) matches the size of the room. They don't reduce CO2, radon or carbon monoxide.

Can houseplants clean your air?

Not enough to notice. Drexel University researchers estimated it would take 10 to 1,000 plants per square meter of floor space to match a building's normal air exchange.