Researchers from Lawrence Berkeley National Laboratory visited 70 detached California houses built between 2011 and 2017, all required by state energy code to have mechanical ventilation. The systems were there, and on average the whole-house fan moved half again more air than the code minimum demanded.
On first arrival, the fan was running in 26% of them. In many houses the switch carried no useful label, though building standards required one. Brett Singer and colleagues published the findings in Indoor Air in 2020.
That is the honest state of home ventilation in the United States. The equipment is better than it has ever been. Nobody knows which switch it is.
The Environmental Protection Agency estimates people spend roughly 90% of their time indoors, and houses have gotten far tighter. Good for the heating bill, bad for everything you generate inside.
One note on order of operations. EPA ranks the fixes, and the ranking is not a suggestion. Get rid of whatever is putting the pollutant into the air, dilute what remains with outdoor air, and only then put a filter in the path of what is left. This article covers the middle step. Filters and air cleaners are covered separately, as are range hoods and cooking.
What a sealed house holds on to
Outdoor air gets into a house three ways, and EPA names all three. It leaks in through cracks, joints and gaps around windows and doors, which is infiltration. It comes through windows and doors you open on purpose, which is natural ventilation. Or a fan brings it in, which is mechanical ventilation. The first two run on wind and on the temperature difference between inside and out, so they vary with the weather. The number that matters is the air exchange rate, meaning how fast outdoor air replaces the air already inside.
EPA puts the consequence plainly: a home built to minimize air leakage can carry higher pollutant levels than a leaky one unless mechanical ventilation compensates. The agency makes a related point that catches people off guard. Weatherization, the storm windows and weather stripping and caulking and blown-in insulation everyone is encouraged to do, reduces infiltration. Indoor concentrations can therefore rise after a weatherization project, even though the work itself added almost nothing to the air.
So what accumulates? Start with the occupants, who exhale carbon dioxide and water vapor continuously, with showers, cooking and laundry adding moisture. Then the building: pressed-wood cabinetry, new flooring, upholstery, carpet, adhesives and finishes all release volatile organic compounds, a family of chemicals that evaporate from solids and liquids at room temperature. Then the products, including cleansers, disinfectants, aerosol sprays, paints and solvents. EPA reports many of these compounds run higher indoors than out, sometimes by a factor of ten, and that its TEAM studies found about a dozen common organic pollutants at two to five times higher levels inside homes.
The clearest demonstration of what ventilation does comes from a test house. The National Institute of Standards and Technology built a net-zero energy research home in Gaithersburg, Maryland, ventilated by a balanced system. Researchers reported that with that system off, indoor contaminants including formaldehyde, d-limonene and hexanal ran two to ten times higher. Same house, same furniture, same products. The only variable was whether air was moving.
Scented products add to the pile
Air fresheners sit on EPA’s list of household sources of volatile organic compounds, alongside paints, solvents, aerosol sprays, cleansers and disinfectants. EPA notes they emit more or less continuously, like building materials and unlike a hobby project that starts and stops. A scent does not remove anything. It adds a chemical mixture to air that already holds whatever was in it.
Be precise about the evidence tier. That these products emit volatile organic compounds is established. What those emissions do to health at the concentrations typically found in homes is not well characterized, and EPA says as much about indoor organic compounds generally. The case against air fresheners is not that they are proven dangerous. It is that they are the opposite of a solution and are sold as one, and that a stale-smelling room is free information about air exchange. Masking the smell deletes the signal and leaves the cause.
Why “just open a window” sometimes fails
Opening a window works. EPA lists it first among the practical ways to raise ventilation, along with window or attic fans when weather permits and a window air conditioner run with its vent open. Natural ventilation also moderates indoor temperature, which matters in a home without air conditioning or during a power outage.
The problem is that it is a method, not a system. It requires you to be home, awake and in agreeable weather. In a heat wave, opening windows through the afternoon imports heat into a house already struggling, and for older adults and people with heart or lung conditions, indoor heat is itself the hazard. Night flushing works in dry climates, not everywhere. In a cold snap the energy and comfort costs are both real, which is exactly why people shut everything and leave it shut. EPA notes that weather can sharply reduce the outdoor air entering a home, so pollutants build up even in a house most people would call drafty.
Then there is what is outside. EPA advises evaluating ventilation carefully when there are nearby outdoor pollution sources, naming smoke and refuse. Heavy traffic belongs in that category. A Boston study published by the Health Effects Institute found indoor nitrogen dioxide tracked traffic density within 50 meters of the home. A 2020 study of naturally ventilated urban homes found indoor particle levels closely matched outdoor ones, because open windows and doors carry outdoor particles straight in. If your window faces an arterial road at rush hour, it is not automatically the cleaner side. None of this argues against opening windows, only against that being the whole plan.
The exhaust fans you already own
Most American homes already have mechanical ventilation and almost nobody thinks of it that way. It is in the bathroom and over the stove. EPA recommends installing and using exhaust fans vented to the outdoors in kitchens and bathrooms, and venting the clothes dryer outdoors. Those fans do two jobs. Most people only know about the first. A local exhaust fan pulls contaminants and moisture out of the room where they are produced. It also raises the whole-home ventilation rate, because every cubic foot it throws out has to be replaced from outside.
Three things go wrong, all checkable this week.
The fan does not vent outdoors. Some terminate in the attic, moving moisture from one part of the building into another and growing a problem where you will not see it for years. Walk the outside of the house and find the termination.
The fan runs for two minutes. Steam clears fast; the moisture in the walls does not. Running a bathroom fan for fifteen to twenty minutes after a shower does more real work than most equipment purchases.
Nobody knows which switch it is. If the ventilation control is unlabeled, label it. Tape and a pen is the cheapest ventilation improvement available.
And ventilate hard during short bursts of heavy emission. EPA names painting, paint stripping, welding and sanding, and says to do those jobs outdoors when weather allows.
Balanced ventilation, and what heat recovery really saves
Whole-house mechanical ventilation comes in three broad shapes. An exhaust-only system pushes stale air out, and replacement air leaks back in wherever the building is weakest, which might be a crawlspace or an attached garage. A supply-only system pushes filtered outdoor air in, and air leaves wherever it can. A balanced system moves roughly equal amounts in and out through ducts, which means you decide where the incoming air comes from.
That is the real argument for balanced systems, and EPA’s Indoor airPLUS construction specification treats it as a certification requirement. The intake must draw directly from outdoors, never from an attic, crawlspace, garage or adjacent dwelling unit, and must sit at least two feet above grade or the roof deck and three feet from dryer exhausts.
Balanced systems are usually paired with heat recovery. The outgoing and incoming airstreams pass through a core that lets heat cross without letting the air mix. A heat recovery ventilator, or HRV, moves heat only. An energy recovery ventilator, or ERV, moves heat and moisture. That is a climate decision, not a quality ranking.
Now the honest part, because heat recovery gets sold on energy savings smaller than people expect. NIST ran its Gaithersburg test house on a continuously operating HRV for a full year. Compared with supplying the same outdoor air without heat recovery, the HRV cut heat pump energy by about 7% over the year. That figure hides a wide swing: 36% savings in heating months, and a 5% increase in cooling months, when outdoor air was sometimes cooler than indoor air and the exchanger worked against the house. The HRV’s own fan used 513 kWh, about 4% of everything the house consumed, and roughly 294 kWh more than a plain supply fan moving the same air. Lisa Ng and Vance Payne concluded the fan energy essentially paid for itself, which is another way of saying it broke about even.
So buy heat recovery for comfort, for control over where air enters, and for moisture management in the right climate. The energy case is strongest in cold climates, and equipment and ductwork are a renovation-scale expense, far cheaper during construction than as a retrofit.
The standard behind the fan in your bathroom
Residential ventilation has an actual standard, ASHRAE Standard 62.2, from the American Society of Heating, Refrigerating and Air-Conditioning Engineers. NIST researchers describe its structure clearly: it sets minimum ventilation rates for acceptable indoor air quality based on a home’s floor area and number of bedrooms, and it deliberately does not dictate how the air is delivered. Exhaust-only, supply-only, a system integrated with central heating and cooling, or an HRV or ERV are all acceptable paths to the same number. The full text sits behind a paywall, so treat any airflow figure quoted online with care unless the source names its edition.
What gives the standard teeth is adoption by other rules. EPA’s Indoor airPLUS specification requires a certified home to have whole-dwelling mechanical ventilation meeting ASHRAE 62.2-2010 or later, requires airflows to be tested against it rather than assumed, and requires local exhaust to the outdoors in every bathroom and kitchen. State energy codes do similar work, which is why the California study could measure compliance at all. The homeowner takeaway is modest: a bathroom fan’s airflow rating is tied to the size of the space it serves, and a contractor who picks one on price alone is skipping a calculation someone already did.
Humidity is a ventilation problem in both directions
EPA recommends keeping indoor relative humidity between about 30% and 50%, with attics and crawlspaces below 50% and basements in that same band, using a dehumidifier if needed.
Too damp is the better-known failure. Standing water, wet surfaces and water-damaged materials support mold, mildew, bacteria and insects, and dust mites thrive in warm, damp conditions. EPA’s advice on wet materials is time-sensitive: clean and dry water-damaged carpets and building materials thoroughly, ideally within 24 hours, or remove and replace them, because decontaminating them afterward is very difficult.
The agency also lists the signs of inadequate ventilation, free to check tonight. Condensation on windows or walls. Stuffy air when you walk in. Mold on books, shoes or stored items. Any of those is a reason to look at air movement before buying anything.
Too dry is the direction people forget. Cold outdoor air holds very little moisture, so heating it drops indoor relative humidity sharply, and dry air irritates airways, eyes and nasal passages. Here is the genuine conflict: in a cold snap, the same extra ventilation that lowers your carbon dioxide and your volatile organic compounds pulls your humidity down further.
This is where the HRV and ERV distinction earns its keep. An ERV transfers moisture along with heat, moderating both extremes. An HRV does not, which is sometimes exactly what you want: NIST’s test house used one on purpose, to avoid carrying bathroom moisture back into the living space. Mold beyond a small patch, and dampness you cannot trace to a source, belong to a professional rather than a spray bottle.
Carbon dioxide as a rough gauge of fresh air
People exhale carbon dioxide, and in an occupied room receiving little outdoor air it builds up. That makes a reading a usable stand-in for how much outdoor air a room is getting per person, and cheap sensors have made it something a homeowner can measure.
The limits matter as much as the tool, and the clearest statement of them comes from Andrew Persily of NIST, writing in Indoor Air in 2022. His argument is that a single number such as 1,000 parts per million gets used as a metric of ventilation and air quality by people who have not worked out what it signifies. Carbon dioxide is of limited value as an air quality metric in itself, and no one concentration can serve as a ventilation indicator across spaces with different occupancies and requirements. Persily’s alternative is a space-specific expected level calculated from the intended ventilation rate, the number of people, how fast they generate carbon dioxide, and how long the space has been occupied.
Translated for a house: a reading tells you about outdoor air per person, in that room, at that moment. It says nothing about formaldehyde off a new cabinet, particles from the stove, or radon. Low carbon dioxide in a room full of paint fumes is not good news.
As a relative instrument rather than a pass-fail line, it works well. A 2012 study in Environmental Research of 21 Montreal day-care centers found carbon dioxide was a better proxy for ventilation than knowing a mechanical system was installed, because having one did not mean it ran. More than 85% of those centers averaged above 1,000 parts per million.
So watch the trend in one room. Compare a closed bedroom overnight against the same bedroom with the door open, or with the fan running. If the number climbs steeply when the room fills and drops when you intervene, the intervention works. If it barely moves when you switch the fan on, you have learned something about the fan.
The days when ventilating is the wrong move
During a wildfire smoke episode or a high-pollution day, the rule inverts. Outdoor air is the contaminant, and the job is keeping it out.
EPA’s guidance when you are told to stay indoors: keep windows and doors shut, switch a central HVAC system to recirculate or close its outdoor air intake damper, close the outdoor air damper on a window air conditioner and do not run it if you cannot, and use an evaporative cooler sparingly because it pulls outdoor air inside by design.
Then stop adding particles of your own, the part people skip. EPA’s list of indoor sources to avoid during a smoke event includes smoking and vaping, gas, propane and wood-burning stoves, aerosol sprays, frying or broiling food, candles and incense, vacuuming without a HEPA filter, and strenuous activity.
Heat does not stop being dangerous because the air is smoky. EPA says to use fans and air conditioning, and if you cannot stay cool indoors, to seek shelter somewhere you can. Smoke guidance is not a reason to sit in a sealed, overheating house.
Reopening is a judgment call, and EPA gives no numeric threshold, which is worth saying rather than inventing one. Its advice is that when outdoor air quality improves, even temporarily, open windows or the fresh air intake and air the house out, running the kitchen and bathroom exhaust fans while you do. EPA points readers to AirNow.gov for current conditions, plus state air quality sites and local news when a fire is burning nearby.
What to do first
Most houses do not need a new machine. They need the machines already in them to be vented outdoors, labeled, and switched on.
In rough order of value for effort: find where your bathroom and kitchen fans terminate, label the switches, and run the bathroom fan for fifteen to twenty minutes after a shower rather than two. Open windows when outdoor air is clean and weather cooperates. Treat condensation, a stale smell on walking in, and mold on books or shoes as the free diagnostics they are. If you are building or remodeling, ask what the ventilation system is, whether its airflow was tested rather than assumed, and where the intake draws from.
Some situations need somebody with training. If a carbon monoxide alarm sounds and anyone has symptoms, get everyone out of the house, call 911, and tell the clinician you suspect carbon monoxide poisoning. EPA notes that carbon monoxide symptoms mimic the flu or food poisoning, which is why people talk themselves out of suspecting it. If the alarm sounds and nobody has symptoms, ventilate with fresh air, shut off the possible sources including the furnace, water heater, range, dryer and space heaters, and have a qualified technician inspect the appliances and chimneys.
EPA also advises having fuel-burning appliances inspected by a trained professional at the start of each heating season, and keeping flues and chimneys connected and unblocked. Never heat a home with a gas oven, never run a charcoal grill indoors, and never idle a car in an attached garage even with the door open.
And if someone in the house has breathing symptoms that ease when they leave and return when they come back, describe that pattern to a clinician in exactly those terms. When and where symptoms happen is information a doctor can use, and most patients never think to mention it.
Sources
- U.S. Environmental Protection Agency. “Improving Indoor Air Quality.” epa.gov
- U.S. Environmental Protection Agency. “Introduction to Indoor Air Quality.” epa.gov
- U.S. Environmental Protection Agency. “The Inside Story: A Guide to Indoor Air Quality.” epa.gov
- U.S. Environmental Protection Agency. “Volatile Organic Compounds' Impact on Indoor Air Quality.” epa.gov
- U.S. Environmental Protection Agency. “Wildfires and Indoor Air Quality (IAQ).” epa.gov
- U.S. Environmental Protection Agency. “Air Cleaners and Air Filters in the Home.” epa.gov
- U.S. Environmental Protection Agency. “Protect Your Family and Yourself from Carbon Monoxide Poisoning.” epa.gov
- U.S. Environmental Protection Agency. “Indoor airPLUS Construction Specifications, Version 1 (Rev. 05), December 2025.” epa.gov
- Applied Thermal Engineering (via PubMed). “Ng LC, Payne WV. “Energy Use Consequences of Ventilating a Net-Zero Energy House.” 2016;96:151-160. PubMed. DOI: 10.1016/j.applthermaleng.2015.10.100.” doi.org
- Indoor Air (via PubMed). “Singer BC, Chan WR, Kim YS, Offermann FJ, Walker IS. “Indoor air quality in California homes with code-required mechanical ventilation.” 2020;30(5):885-899. PubMed. DOI: 10.1111/ina.12676.” doi.org
- Indoor Air (via PubMed). “Persily A. “Development and application of an indoor carbon dioxide metric.” 2022;32(7):e13059. PubMed. DOI: 10.1111/ina.13059.” doi.org
- Environmental Research (via PubMed). “St-Jean M, St-Amand A, Gilbert NL, et al. “Indoor air quality in Montréal area day-care centres, Canada.” 2012;118:1-7. PubMed. DOI: 10.1016/j.envres.2012.07.001.” doi.org
- Journal of the Air & Waste Management Association (via PubMed). “Abdel-Salam MMM. “Outdoor and indoor factors influencing particulate matter and carbon dioxide levels in naturally ventilated urban homes.” 2020;71(1):60-69. PubMed. DOI: 10.1080/10962247.2020.1834009.” doi.org
- Research Report (Health Effects Institute), via PubMed. “Levy JI, Clougherty JE, Baxter LK, Houseman EA, Paciorek CJ. “Evaluating heterogeneity in indoor and outdoor air pollution using land-use regression and constrained factor analysis.” 2010;(152):5-80. PubMed. PMID: 21409949 (no DOI assigned).” pubmed.ncbi.nlm.nih.gov