Your basement is not the bottom of the house.
In winter it is the top of the air supply.
The study
Liuliu Du, Stuart Batterman and colleagues published “Air exchange rates and migration of VOCs in basements and residences” in the journal Indoor Air in 2015. It was funded by the National Institute of Environmental Health Sciences and run as community-based participatory research with Detroit residents. Basements are, as the authors put it, “a building zone which is rarely studied.”
Households were recruited in Detroit between August 2011 and December 2012. Seventy-four of the homes had basements with paired measurements in both zones, and after exclusions for invalid or implausible measurements, 61 residences and 170 week-long measurement visits made the final analysis.
The method matters here. In each home they placed two perfluorocarbon tracer emitters in the living area and one in the basement, then measured both tracers in both zones over a week. Because each tracer has a known release rate and is measured where it did not start, you can solve for the flow in each direction separately. A single whole-house air-exchange figure cannot do that. Passive samplers ran alongside for roughly 100 target compounds, and a technician walked each accessible home with an inspection checklist.
What they found
The basement leaked far more than the living space above it. Air exchange averaged 0.51 per hour in the living area (median 0.35) and 1.52 per hour in the basement (median 1.15). That is roughly three times higher, and it runs against the intuition that a basement is a sealed box.
Then the seasons split, and they split in opposite directions. In summer, living-area air exchange was at its highest and basement air exchange at its lowest, and the median ratio between them was 1.3. In winter the living area was at its tightest and the basement at its leakiest, and that ratio jumped to 4.4.
The air moved with it.
The share of the living area’s air that arrived from the basement rather than from outdoors averaged 21% in summer and 58% in winter, with a winter median of 59%.
Measured as flow rather than share, basement-to-living-area movement averaged 37 cubic meters per hour in summer and 141 in winter.
That is the stack effect. Cold, dense outdoor air pushes into the leaky base of the house, warms, and rises through every gap it can find into the rooms above. The authors’ own summary is that the winter figures suggest “little isolation between basement and living areas.”
The paper gives several reasons for the high basement figures, including summer stratification and the make-up air a heating season demands. Two of them are worth a homeowner’s attention. Ninety-three percent of these basements had windows, and in 13% of the basements those windows did not appear to seal tightly. And most homes had a natural-gas furnace and water heater in the basement, most of those systems drawing their combustion air from that space.
Separately, the path upstairs was open. The paper records, flatly, that passage doors between basements and living spaces, present in all houses, “were not well sealed.”
What was stored down there
The walkthrough is the part worth reading twice.
| Potential source | Living area | Basement |
|---|---|---|
| Solvents | 13% | 75% |
| Household cleaners | 15% | 70% |
| Air fresheners | 92% | 67% |
| Gasoline-powered equipment | 0% | 31% |
| Glue, adhesives | 25% | 20% |
| Paint cans | 5% | 15% |
| Motor oil and lubricants | 2% | 10% |
| Gasoline | 0% | 8% |
| Paint thinner | 0% | 5% |
Solvents and gasoline-powered equipment are the two entries that matter most, because they can be strong sources and because they are overwhelmingly concentrated in basements. As the authors note, older equipment in particular “may leak and release fuel and vapors, e.g., benzene.”
The concentrations
Median levels in the living areas were 1.4 micrograms per cubic meter of benzene, 6.3 of toluene, 0.9 of naphthalene and 13.7 of limonene. Basement medians were close: 1.5, 5.9, 1.1 and 9.0. Outdoor medians were lower: 0.87 benzene, 1.86 toluene, 0.15 naphthalene and 0.22 limonene. One caution on that last point: the paper’s prose says every outdoor median fell below 1 microgram per cubic meter, which its own Table 4 contradicts for toluene. The table values are the ones used here.
The distributions are skewed hard to the right, which is where the story is. The 90th percentiles in living areas were 4.0, 19.1, 20.3 and 51.0. The maxima were 54 micrograms per cubic meter of benzene in a basement, 888 of toluene in a basement, and 1,117 of naphthalene in a living area, which the authors attribute to the “inappropriate use of repellents and deodorants.” That last home averaged 616 across four separate visits, so it was not a one-week fluke.
Across the sample, 65% of homes had benzene above 1 microgram per cubic meter, a level the authors tie to a lifetime leukemia risk of 6 in a million, and 24% had naphthalene above 3 micrograms per cubic meter, the chronic inhalation reference concentration they cite.
The result that cuts the other way
Here is where this study is honest and where most retellings of the idea are not.
When the authors apportioned the VOCs actually measured in the living areas back to their sources, the median basement contribution was small: 8% for benzene, rising to 19% for naphthalene. Most of the benzene traced to outdoor air, and most of the naphthalene and limonene to products used upstairs.
So how can the airflow be so large and the pollution share so small?
Because these basements were leaky enough to dilute themselves before the air went up. In the authors’ words, “an important factor limiting the influence of basements is the higher AER in this space.” In some individual homes the basement did account for 40 to 60% of an indoor concentration. But the median Detroit home in this sample was protected by a draft.
And then they say the uncomfortable part out loud:
“However, many new residences have very low AERs, and AERs in basements may be particularly low, especially in summer. In older residences, energy conservation measures will also tighten-up basements and reduce AERs, thus increasing VOC levels.”
Sealing a basement without first clearing it out removes the one thing that was helping.
The limits, stated plainly
Nobody was tested. No symptoms, no lung function, no blood, no diagnoses. This is a study of air and airflow. It does not show that anyone got sick, and this article does not claim that it does. The two health-referenced numbers above are regulatory benchmarks, not outcomes measured in these households.
Sixty-one homes in one city. The sample was mostly lower-income family homes, averaging 78 years old (based on the 64% of homes where a construction year was available) and smaller than the US average, so the results may not transfer to a newer or larger house.
Potential sources were identified by visual inspection, so some were missed. Only 75% of homes were fully accessible to the technician. The models assume steady-state, well-mixed air in each zone, which real rooms are not. Attached garages were present in 11% of homes and were not separated out in the apportionment. Outdoor concentrations came from four fixed Detroit monitoring sites rather than from immediately outside each home. Vapor intrusion of soil gas into basements was not accounted for. Building volumes were estimated.
One internal inconsistency is worth flagging: the methods section reports 266 week-long visits, while the results and every table report 170. The published paper does not reconcile the two, and the figures here all come from the 170-visit analysis.
What survives all of that is narrow and useful: a basement sits aerodynamically upstream of the bedrooms, most strongly in winter, and what is stored there does not stay there.
What gets lost in the marketing
Benzene, toluene and gasoline vapor are gases, not particles. A HEPA filter, which is the device most often sold as a home air cleaner, does not remove a gas at all. Activated carbon does adsorb VOCs, and it also loads up silently, with no indication that it is spent.
There is also a gap in the code that is worth knowing about. The authors note that ventilation codes recognize air migration from an attached garage into a house as a problem and “require measures to depressurize the garage or minimize air flow between these zones.” Then: “Such measures do not exist for basements and may not be practical.” Nobody is going to fix this for you at the permit stage.
What the data supports
Move the sources out. The gas can, the mower, the string trimmer, spare fuel, paint thinner and old solvent cans belong in a detached shed or an unattached garage, not next to the furnace. This is the authors’ own conclusion: “elimination of sources in basements is the most appropriate remedy.” It costs nothing.
Throw out the part-used containers. EPA’s guidance on volatile organic compounds is blunt about this: “Because gases can leak even from closed containers, this single step could help lower concentrations of organic chemicals in your home.” Use a household hazardous waste collection day rather than the garbage can.
Weatherstrip and sweep the basement door, but only after the sources are gone. Sealing first is the wrong order, because you tighten the path without removing what is traveling along it.
Check the draft before you tighten. If a furnace or water heater draws its combustion air from the basement, changing the pressure balance around it can cause backdrafting. Have that verified by someone who can test it.
Bottom line
In the coldest months of the year, most of the air in the typical living room had just come up the stairs.
Whatever is stored down there is stored in the bedroom.
The first step this weekend costs nothing: go down, look at the shelf next to the furnace, and move the fuel outside.
illumenair tests indoor air in Portland and Vancouver homes.
971-363-5626 · info@illumenair.com · www.illumenair.com
References
Du L, Batterman S, Godwin C, Rowe Z, Chin J-Y. Air exchange rates and migration of VOCs in basements and residences. Indoor Air 2015;25(6):598-609. doi:10.1111/ina.12178
US Environmental Protection Agency. Volatile Organic Compounds’ Impact on Indoor Air Quality. epa.gov, fetched 9 September 2026.