Signs of Poor Indoor Air Quality (What They Usually Mean)

Quick answer: The most useful clue is a pattern: symptoms that ease when you leave the house and come back when you return. Common ones are irritated eyes, nose, or throat, headaches, tiredness, and a cough. Physical signs include a musty or stale smell, water collecting on windows, visible mold, and a dirty HVAC filter. A sudden headache with nausea, dizziness, or drowsiness can mean carbon monoxide: get everyone outside and call 911. These signs point you toward likely causes; they cannot diagnose a health problem, so see a clinician for symptoms that persist.

Emergencies first

Stop reading and act if any of these apply. The American Lung Association lists the same situations as signs that need immediate action.

Possible carbon monoxide (CO) poisoning. CO has no color or smell. The U.S. Consumer Product Safety Commission lists headache, dizziness, weakness, nausea, vomiting, sleepiness, and confusion as symptoms. If a CO alarm sounds or several people feel sick at once, get everyone (pets included) outside to fresh air, then call 911. Do not go back in until responders say it is safe.
A gas smell or suspected gas leak. Leave the house without flipping light switches, using the phone, or starting a car in an attached garage. Once outside and away from the house, call your gas utility's emergency line or 911.
A fuel-burning appliance that is backdrafting. Furnaces, water heaters, and fireplaces are supposed to send exhaust up the flue. When the draft reverses, exhaust spills back into the room. Signs include soot around the appliance, a burning or exhaust smell near it, or a CO alarm going off. Turn the appliance off if you can do so safely, leave, and call 911 or your gas utility. Have a licensed technician inspect it before you use it again.
A chemical spill, sewage backup, or flooding that has soaked carpet, drywall, or other porous materials.
Several people having trouble breathing.

If you think someone swallowed or breathed in a household chemical, call Poison Control at 1-800-222-1222. Call 911 if the person is unconscious, having a seizure, or struggling to breathe.

The pattern that points to the building

The EPA and the American Lung Association give the same first test: if symptoms fade or go away when you leave an area, look for an indoor air source. A cold follows you to the grocery store. A problem with the house usually stays at the house.

Keep a simple log for a week. Write down the room, the time, what you were cooking or cleaning, and whether the heating or cooling was running. You are looking for a room, a time of day, or an activity that lines up with how you feel.

If the pattern centers on a workplace, our guide to sick building syndrome covers offices and other shared buildings.

Signs you can see, smell, or wipe

Sign	What it often points to	First step
Musty or "old towel" smell	Damp materials, a hidden leak, or a dirty HVAC system	Find the water; see our guide to musty smell
Water on the inside of windows, clammy basement	Humidity too high	Check with a hygrometer and dry the space; see air purifier vs dehumidifier for mold
Visible mold or mildew	Moisture plus something to grow on (dust, paint, grout)	Fix the moisture; see how to prevent mold naturally. Call a professional for large areas
Stuffy air, cooking or pet smells that linger	Weak ventilation, a saturated carbon filter, odor held in fabrics	Use exhaust fans, open windows when outdoor air is good, wash fabrics
Dusty supply vents, a gray HVAC filter	Overdue filter change	Replace the filter with the rating your system is designed for. A much denser filter can strain a weak blower
A new-furniture or fresh-paint smell that won't fade	Chemicals off-gassing from new products (VOCs)	Ventilate, and wait before starting more projects
Soot near a fireplace or gas range	Combustion particles, a poorly drafting appliance	Have the appliance serviced; use the range hood every time you cook
Books, shoes, or luggage going moldy in a closet	A damp spot with little air movement	Dry that spot and improve airflow; do not cover the smell with fragrance

Radon gives no warning signs. It has no smell and causes no symptoms you would notice, but long-term exposure raises the risk of lung cancer. The only sign is not having tested. The EPA advises fixing your home if radon is at or above 4 picocuries per liter (pCi/L) and considering a fix between 2 and 4 pCi/L. Test kits are inexpensive; our guide on how to test indoor air quality walks through the options.

If you are ready to measure

Symptoms, and why they need care to interpret

The EPA lists irritated eyes, nose, and throat, headaches, dizziness, and fatigue as possible short-term effects of indoor air pollution. Coughing, sneezing, trouble concentrating, and dry or itchy skin show up on many lists too. Indoor air can contribute to all of them. So can pollen you carried in, a virus, poor sleep, or stress.

Some rules of thumb help you sort it out:

Linked to a room or a building: looking for an indoor source makes sense.
Linked to a product, such as a heavy cleaning day, a new rug, or a scented diffuser: stop using the product and see if things improve.
Linked to nighttime in one bedroom: look at bedding, dust mites, dry air, a pet sleeping on the bed, or a damp spot such as a plant saucer.
Not linked to any place: see a clinician. A purifier is unlikely to help.

No air device, including ours, treats headaches or fatigue. If symptoms continue, talk to a doctor.

A 20-minute walk through your house

Do this before you buy anything.

Reset your nose. Stand outside for two minutes, then walk back in. Note any smell you had stopped noticing.
Check the exhaust fans. In bathrooms and the kitchen, make sure fans vent outdoors rather than into the attic. Hold a square of toilet paper to the grille; it should pull toward the fan.
Look for water. Check the basement, under sinks, and around the water heater for stains, soft drywall, or a dehumidifier bucket that needs emptying.
Open the HVAC closet. Find the date of the last filter change and look for gaps around the filter slot that let dust bypass it.
Look at soft surfaces. Sofas, mattresses, rugs, and curtains hold dust, pet dander, and odors that air filters do not reach.
Check anything that burns fuel. Look at the fireplace, gas stove, water heater, and attached garage for soot or exhaust smells. Test your CO alarms.

Then measure only what the walk pointed to: humidity, CO, radon, or particles. To act on what you find, follow the steps in how to improve indoor air quality naturally: ventilate, bring humidity into range, remove sources, filter particles, and then treat surfaces.

CO alarms: where they go

CO alarms belong in every home with a fuel-burning appliance, fireplace, or attached garage. The CPSC recommends installing them on every level of the home and outside sleeping areas. Choose battery-powered alarms or plug-in alarms with battery backup so they work in a power outage. Interconnected alarms are best, because when one sounds, they all sound.

Test each alarm monthly with its test button, replace batteries as the maker directs, and replace the whole alarm when it reaches the end of its life (most show a date on the back). Follow the manufacturer's mounting instructions, and keep alarms away from places where exhaust or drafts can give false readings, such as directly above a stove.

Which tool matches which sign

Airborne dust, smoke, or pollen: a HEPA air purifier sized for the room.
Damp air, water on windows, musty basement: fix leaks first, then use a dehumidifier.
Very dry air in winter (a hygrometer reading in the 20s): a humidifier, kept clean.
A clean, dry room where fabrics still smell: a surface treatment such as a probiotic air purifier, whose beneficial bacteria settle on surfaces and consume the organic residue that odor-causing bacteria and mold feed on.

A purifier of any kind is the wrong response to a leak, a CO problem, or radon. Those need repairs, alarms, and testing.

What a low-cost monitor can and cannot tell you

A $40 to $100 particle (PM2.5) monitor is useful for seeing cooking smoke, a neighbor's fireplace, or whether closing a window helped. It cannot detect radon, CO, most VOCs, or a damp floor joist. Low-cost VOC sensors react to many things, including peeling an orange, so treat their readings as rough trends.

Use a monitor to test an idea from your walk-through. If the particle reading stays low and you still feel worse at home, the cause may be a gas, moisture, or a health issue the monitor cannot measure. Keep looking, and talk to a doctor.

CO alarms and radon tests do a different job from a particle monitor. They cover the dangerous problems you cannot see or smell.

Kids and pets

Nurseries tend to collect new products (lotions, plug-in fresheners, new foam mattresses) and spend long hours with the door closed. A baby cannot tell you they feel better elsewhere, so do the walk for them. Check the humidity, the humidifier tank, the curtains, and whether steam from a nearby shower drifts into the room.

Pets add dander, and accidents can leave odors you no longer notice. Often the sign is a guest who reacts to the smell. Take their word for it, wash pet bedding and fabrics, and then decide whether you need a tool.

After a renovation

New cabinets, fresh paint, new carpet, and foam furniture can make a house smell like a store. That smell is VOCs off-gassing, and it usually fades over weeks to months. Ventilate more than feels necessary, run bathroom and kitchen fans, and hold off on the next project. A particle monitor may read normal even while the smell persists, because it does not measure these gases. Do not use bleach or other strong cleaners to "reset" the air; they add their own fumes.

The bottom line

Poor indoor air usually shows up as a pattern: the way you feel at home, a smell, water on the windows, a filter you meant to change. Handle emergencies first. Then read the house, measure what the walk-through points to, and choose the tool that fits the cause.

Frequently asked questions

What are the signs of poor indoor air quality? Common signs are stuffy or musty air, water collecting on windows, visible mold, a dirty HVAC filter, and cooking or chemical smells that linger. Physical symptoms such as irritated eyes, nose, or throat, headaches, tiredness, or a cough that ease when you leave and return when you come back also point to the building.

How do I know if my house has bad air quality? Watch for the leave-and-return pattern, then walk through the house checking for moisture, fuel-burning appliances, and the HVAC filter. Measure what the walk points to, such as humidity, CO, radon, or particles. A stuffy feeling alone is a clue, not a full answer.

Can poor indoor air quality cause headaches? It can contribute, along with many other causes. A sudden headache with nausea, dizziness, or drowsiness may be carbon monoxide poisoning, so get outside and call 911. Headaches that keep coming back need a doctor's evaluation, not only a purifier.

Does a musty smell mean mold? Often it means dampness and microbial growth somewhere, even if you cannot see mold on a wall. Find and fix the water source first. Large areas or growth that keeps returning need a professional.

Will an air purifier fix these signs? A HEPA purifier helps with airborne particles such as dust, smoke, and pollen, if it is sized for the room and left running. It will not fix a leak, radon, carbon monoxide, or a wet basement. Those need repairs, alarms, and testing.

Should I test my air or buy a purifier? If a walk-through points to a clear source, fix that source first. If you cannot find one, test before you buy anything. Testing for radon and installing CO alarms makes sense in every home either way.

Signs of Poor Indoor Air Quality (And What They Usually Mean)