Quick answer
Indoor air quality (IAQ) is the condition of the air inside and around a building, especially how it affects the health and comfort of the people in it.
It depends on what is released indoors, how much outdoor air comes in, humidity, and filtration. EPA ranks the fixes in order: control sources first, then ventilate, then clean the air.
Quick facts
The key facts about Indoor Air Quality, with sources:
- EPA definition
- Air quality in and around buildings as it relates to occupants' health and comfort 1
- Time spent indoors
- About 90% of the time for most people, mostly at home 3
- EPA's three strategies
- Source control, ventilation, air cleaning 2
- Humidity target
- Below 60% RH, ideally 30–50% 4
- Radon action level
- Fix the home at 4 pCi/L or above 5
- CO alarms
- Every level and outside sleeping areas 6
- Duct cleaning
- Not recommended routinely, only as needed 7
Key takeaways
- Most people spend about 90% of their time indoors, mostly at home 3.
- EPA's first fix is source control; ventilation and air cleaning come after 2.
- Your HVAC system helps through filtration, humidity control and ventilation.
- Two tests matter more than any gadget: a radon test and working CO alarms 5 6.
- EPA does not recommend routine duct cleaning 7.
What indoor air quality means
EPA defines indoor air quality as the air quality within and around buildings "as it relates to the health and comfort of building occupants" 1. In a home, that covers particles like dust and smoke, gases like carbon monoxide and VOCs, biological growth like mold, radon from the soil, and humidity.
It matters because of time. EPA notes most people spend about 90% of their time indoors, much of it at home, so much of their exposure to air pollution happens there 3.
EPA lists the main causes: indoor sources that release gases or particles, and too little ventilation to dilute and carry them away. High temperature and humidity can raise levels of some pollutants too 1.
EPA's order of fixes
- 1. Source control: remove or reduce what makes the pollution (usually most effective) 2
- 2. Ventilation: bring in outdoor air and exhaust stale air (see ventilation rate)
- 3. Air cleaning: filters and air cleaners (see MERV ratings)
How your HVAC system affects IAQ
- Filtration. The furnace or air handler filter catches particles when the fan runs. EPA suggests MERV 13 or as high as the system allows 3.
- Humidity. Air conditioning removes moisture; a dehumidifier or humidifier can fine-tune it. EPA advises below 60%, ideally 30–50% 4.
- Ventilation. Some systems bring in outdoor air through a fresh-air duct, ERV or HRV.
- Combustion safety. Gas furnaces must vent exhaust outdoors. A cracked heat exchanger or blocked vent can release CO.
- Drains and coils. Wet coils and pans can grow mold if drains clog 4.
Common indoor pollutants and the usual fix
| Pollutant | Common sources | First fix |
|---|---|---|
| Fine particles (PM2.5) | Cooking, smoke, candles, wildfire smoke | Range hood, MERV 13 filter, portable HEPA |
| Carbon monoxide (CO) | Faulty gas appliances, cars in attached garages, generators | CO alarms, annual furnace inspection 6 |
| Radon | Soil gas entering the foundation | Test; mitigate at 4 pCi/L or more 5 |
| VOCs | Paints, cleaners, new furniture | Source control and ventilation 2 |
| Mold | Excess moisture, leaks, clogged AC drains | Fix moisture; dry within 24–48 hours 4 |
| Allergens | Pollen, pets, dust mites | Filtration, humidity control |
Example: a one-hour home IAQ check
Example only. These are owner-level checks drawn from EPA and CPSC guidance, in order of impact.
| Check | How | Target |
|---|---|---|
| CO alarms | Press test on each alarm | One on every level and outside sleeping areas 6 |
| Radon | Buy a short-term test kit | Below 4 pCi/L; EPA says fix the home at 4 or above 5 |
| Humidity | Use a cheap hygrometer in several rooms | Below 60%, ideally 30–50% 4 |
| Filter | Pull it and read the MERV and date | MERV 13 if your system allows 3 |
| Kitchen and bath fans | Hold a tissue near the grille | Fans that exhaust outdoors 2 |
| AC drain | Look at the pan and outlet | Dry pan, flowing drain 4 |
EPA's radon table shows the stakes: at 4 pCi/L, about 62 of every 1,000 never-smokers would get lung cancer over a lifetime of exposure, falling to about 32 at 2 pCi/L 5. No HVAC upgrade comes close to a radon fix for a home that needs one.
Where you will see the term
- On HVAC sales sheets for air purifiers, UV lights and "IAQ packages."
- In home inspection add-ons such as radon and mold testing.
- In ventilation codes based on ASHRAE 62.2 10.
- In wildfire smoke advisories from public health agencies.
What HVAC upgrades can and cannot do
Can help
Limits of air cleaning
EPA is plain about this: for most indoor air problems at home, "source control is the most effective solution" 2. A filter cannot keep up with a gas stove run without a hood, a damp basement, or a smoker indoors.
Ventilation has trade-offs too. EPA notes that bringing in more outdoor air can raise energy costs 2, and on bad outdoor air days, such as wildfire smoke, you may want less outdoor air, not more.
Cost drivers
HyreHVAC does not list IAQ product prices without enough dated quotes. The big cost drivers are the type of fix (filter upgrade, dehumidifier, ERV, radon mitigation), duct changes, electrical work, and ongoing filter or maintenance costs.
Cheap steps often help most: a radon test kit, CO alarms, a hygrometer, a better filter, and running the range hood. Start there before buying equipment. Our research hub lists what the evidence supports.
How to work on your home's IAQ
- Fix safety first: test CO alarms and radon 6 5.
- Remove or reduce sources: vent appliances outdoors, run exhaust fans, avoid smoking indoors 2.
- Control moisture: fix leaks, keep humidity in range, keep AC drains clear 4.
- Upgrade filtration to MERV 13 if the system allows 3.
- Add ventilation if the home is tight or stuffy.
- Only then consider add-on air cleaners.
Ongoing upkeep
Warning signs and when to get help
Rules and standards
There is no federal limit for most pollutants in private homes. The main standard for home ventilation is ANSI/ASHRAE 62.2. ASHRAE lists 62.2-2025 as the current edition 9; ASHRAE's fact sheet notes the standard is referenced in 19 state codes 10. Codes adopt editions at different times.
EPA recommends fixing a home with radon at 4 pCi/L or more 5. Many states require CO alarms in homes with fuel-burning appliances or attached garages; rules vary by state.
IAQ vs related terms
| Term | How it relates |
|---|---|
| Indoor air quality | The overall condition of indoor air |
| Relative humidity | One part of IAQ that HVAC controls directly |
| Ventilation rate | How much outdoor air comes in |
| Air filter | Removes particles from the air that passes through |
Common misconceptions
- Myth An air purifier fixes indoor air.
- Reality EPA ranks air cleaning third, after source control and ventilation 2.
- Myth Ducts should be cleaned every few years.
- Reality EPA recommends cleaning only as needed, such as visible mold or vermin 7.
- Myth Tight homes have the best air.
- Reality Tight homes need planned ventilation to dilute indoor pollutants 1.
- Myth If it smells fine, it is fine.
- Reality Radon and CO have no smell.
Regional IAQ issues
Humid Southeast states face mold and humidity issues; Western states face wildfire smoke; cold Northern homes are often tight and dry in winter. Radon can be high anywhere, which is why EPA says to test.
When IAQ matters to you
It matters for everyone, but most for homes with asthma, allergies, infants, older adults, smokers, gas appliances or recent renovations.
Next step: run the one-hour check above, then read our indoor air quality services page before buying anything. For ventilation, see ERV and HRV systems.
Questions about Indoor Air Quality
What is indoor air quality?
EPA defines it as the air quality within and around buildings as it relates to the health and comfort of the people inside. It covers particles, gases such as carbon monoxide and VOCs, radon, mold and humidity.
How can I improve indoor air quality at home?
Follow EPA's order. First, control sources: vent appliances, run kitchen and bath fans, fix leaks and avoid smoking indoors. Second, ventilate with clean outdoor air. Third, filter, using MERV 13 in your HVAC system if it allows and portable cleaners where needed.
Does my HVAC system clean the air?
Partly. The filter catches particles whenever the fan runs, and air conditioning removes moisture. Most systems do not bring in outdoor air or remove gases. Upgrading to MERV 13 and keeping humidity in range are the most useful HVAC steps.
Is air duct cleaning worth it?
EPA does not recommend routine duct cleaning and says it has never been shown to prevent health problems. EPA says to consider it if there is substantial visible mold inside hard ducts, vermin, or so much debris that dust blows out of the supply registers.
What humidity is best for indoor air?
EPA advises keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent. Higher humidity encourages mold and dust mites; very low humidity can cause dry skin and static. A hygrometer shows your level.
Should I test my home for radon?
Yes. EPA estimates radon causes about 21,000 lung cancer deaths a year in the US and says to fix homes at 4 pCi/L or above. Radon has no smell, and levels vary house to house. Test kits are inexpensive.
Do air purifiers help indoor air quality?
Portable air cleaners with a high CADR can reduce particles in a room, and EPA notes several studies found small health improvements with portable HEPA units. They do not remove most gases, and EPA advises avoiding units that make ozone.
How do I know if my indoor air is bad?
Clues include musty odors, visible mold, condensation on windows, lingering cooking smells, and headaches or irritation that ease when you leave home. Radon and carbon monoxide have no smell, so test for radon and keep CO alarms working.
Do UV lights in the ductwork improve indoor air quality?
Evidence for home duct UV lights is limited, and EPA notes that some UV lights without proper lamp coatings may emit ozone. UV is sometimes used on AC coils to limit biological growth. Fix moisture and drain problems first, and ask any seller for test data on the exact product.
Can a gas furnace hurt indoor air quality?
A properly working, properly vented furnace sends its exhaust outdoors. A blocked vent, cracked heat exchanger or poor combustion can release carbon monoxide into the home. EPA recommends having fuel-burning furnaces inspected, and CPSC advises CO alarms on every level and outside sleeping areas.
Sources
- U.S. EPA, Introduction to Indoor Air Quality, retrieved .
- U.S. EPA, Improving Indoor Air Quality, retrieved .
- U.S. EPA, Guide to Air Cleaners in the Home, retrieved .
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home, retrieved .
- U.S. EPA, Health Risk of Radon, retrieved .
- U.S. CPSC, Carbon Monoxide Information Center, retrieved .
- U.S. EPA, Should You Have the Air Ducts in Your Home Cleaned?, retrieved .
- U.S. EPA, Use and Care of Home Humidifiers, retrieved .
- ASHRAE, Standards 62.1 & 62.2 (bookstore listing, 2025 editions), retrieved .
- ASHRAE Government Affairs, Standard 62.2 fact sheet (December 2025), retrieved .
- U.S. DOE Energy Saver, Whole-House Ventilation (archived copy, 2025), retrieved .
Expert review
Written by the HyreHVAC Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Every IAQ position is EPA's or CPSC's own wording, read 10 October 2026. Radon risk figures are EPA's never-smoker lifetime table. No health outcome is claimed for any individual; the check table lists owner actions only.
Suggest a correction. We fix errors and say what changed.