The EPA recommends that every apartment or condo unit on the ground floor through the third floor be tested for radon, because radon is a soil gas that enters multi-family buildings at the foundation and dilutes rapidly with each storey above grade — that guidance comes from the Citizen’s Guide to Radon (EPA-402-K-12-002). Above the third floor the risk is generally very low, but it is not categorically zero: underground parking, pressurised stairwells, and HVAC design can carry soil gas upward into specific units, which is why ANSI/AARST MS-PMD-2014 still requires a representative sample of measurements in any building with an identified soil-gas pathway.
Why floor level changes the answer
Radon (Rn-222) is the decay product of radium in soil and rock. It is a gas, it is chemically inert, and it migrates by diffusion and by pressure-driven advection through soil pores. In a building, the route in is almost always the foundation envelope: foundation cracks, slab penetrations around utility chases, sump pits, floor-wall joints, expansion gaps, and unsealed crawlspaces. EPA’s Consumer’s Guide to Radon Reduction (EPA-402-K-10-005) lists those entry pathways in the order they typically dominate.
Once radon is in the lowest level of a building, two things happen. First, it decays — Rn-222 has a half-life of 3.82 days, so absent a continuing source it is gone within a few weeks. Second, it mixes upward. In a single-family home that mixing is relatively efficient and a basement reading of 8 pCi/L might correspond to a first-floor reading of 4 pCi/L and a second-floor bedroom around 2 pCi/L. In a multi-storey apartment or condo building the dilution is steeper because each floor is a separate sealed enclosure connected to the next floor only through stairwells, elevator shafts, and small mechanical penetrations.
The practical consequence is that concentration tends to drop with each floor above grade, and by the time you are three floors above the slab the contribution from soil-gas radon is generally negligible. That is the empirical basis for the EPA’s ground-floor-through-third-floor recommendation.
The EPA’s exact apartment guidance
The Citizen’s Guide to Radon (EPA-402-K-12-002) is the document homeowners and tenants are most often pointed to. On apartments and condos it says, in essence: test every unit on the ground floor up to and including the third floor; for units above the third floor, the probability of an elevated reading is low enough that routine testing is not specifically recommended. The same 4 pCi/L action level applies to multi-family units as to single-family homes — the floor-level rule is about which units to test, not about a different action threshold.
| Unit location | Typical relative risk | Recommended action |
|---|---|---|
| Basement / partially below grade | Highest — direct soil-gas contact | Test every unit; closed-house short-term test, then long-term confirmation if ≥ 4 pCi/L |
| Ground floor (slab-on-grade) | High — floor membrane is the only barrier | Test every unit per EPA guidance |
| 2nd floor | Moderate — dilution begins, stack effect can still carry soil gas | Test every unit per EPA guidance |
| 3rd floor | Low — but EPA still recommends testing | Test every unit per EPA guidance |
| 4th floor and above | Generally very low | Routine testing not specifically recommended unless building has a known soil-gas pathway (parking garage, pressurised shaft) |
Both the threshold and the testing protocol live in the same EPA document, alongside the underlying Protocol for Radon Measurements (EPA-402-R-92-003) which specifies the placement and duration rules for a valid measurement. See our DIY radon testing guide for the step-by-step procedure.
Ground-floor units aren’t categorically high. Upper-floor units aren’t categorically safe.
The floor-level rule is a population-level prior, not a verdict on any particular unit. A ground-floor unit in a Zone 3 county with a well-sealed slab and balanced HVAC may test below 1 pCi/L. A fifth-floor unit directly above an underground parking garage with a pressurised lift shaft can, and occasionally does, test above 4 pCi/L. The only way to know what an individual unit is doing is to test it.
ANSI/AARST MS-PMD-2014 — the multi-family measurement standard
For any building larger than a single-family home, the operative measurement standard in the United States is ANSI/AARST MS-PMD-2014, the Protocol for Conducting Measurements of Radon and Radon Decay Products in Multi-family Buildings. This is the consensus standard adopted by HUD, by most state radon programmes, and by reputable commercial testing firms.
MS-PMD-2014 does not require every single unit in a building to be tested. It requires a representative sample, sized as a function of the building’s ground-contact footprint:
- Every unit on or below grade (basement units, ground-floor units, units over a crawlspace) is in scope.
- A defined sample — commonly described in practice as approximately 10% of ground-contact units, rounded up, with a minimum number specified by the standard — is tested.
- Any unit on any floor that has an identifiable soil-gas pathway (sitting directly above a parking garage, sharing a wall with a vertical chase, located adjacent to a pressurised shaft) is added to the sample regardless of floor.
- If any tested unit reads at or above 4 pCi/L, the standard requires expanding the sample, escalating to long-term measurement, and considering building-wide diagnostic testing.
For very large buildings — high-rises, schools, mixed-use towers — the companion standard is ANSI/AARST RMS-LB-2018, Protocol for Conducting Measurements of Radon in Schools and Large Buildings, which adds requirements for HVAC characterisation and zone-by-zone diagnostic testing.
Building features that change apartment radon risk
Floor level is the strongest single predictor, but it is not the only one. Several common multi-family building features can either pull soil gas upward into otherwise safe units or, conversely, mitigate radon in lower units that would normally be high-risk.
Underground parking garages
A subgrade parking deck is, in radon terms, an enormous occupied basement with vehicle exhaust ventilation. Soil gas enters through the slab joints and the cores around utility penetrations. Where the residential tower sits directly above the garage, the slab between the garage ceiling and the lowest residential floor is the only barrier, and elevators, stair pressurisation, and fire-stopping penetrations all provide vertical pathways into living units several floors up.
Pressurised stairwells and lift shafts
Fire-code-required stair pressurisation systems and the natural “stack effect” in any tall building act as chimneys. In winter, warm building air rises, the lower-level pressure goes negative relative to outside, and soil gas is actively drawn in at the foundation and then carried upward by the stack. This is the mechanism that occasionally puts a fifth- or sixth-floor unit above 4 pCi/L in a building with significant subgrade infiltration.
Building age and HVAC type
Older buildings with leakier envelopes and decentralised heating (window AC, baseboard hot water) tend to dilute radon by natural infiltration but also offer fewer engineered controls. Newer tightly-sealed buildings with central HVAC, balanced supply and return, and energy-recovery ventilators can hold soil gas in if the foundation is not properly sealed, but a well-commissioned positive-pressure HVAC system can also actively suppress radon entry. Building age alone does not determine risk — the foundation detailing and the HVAC pressure balance do.
Foundation type
Slab-on-grade, basement, and crawlspace foundations each present a different surface area in contact with soil. Crawlspaces in particular — especially unsealed ones — can supply a large reservoir of radon-laden air to the units directly above. See our explainer on sub-slab depressurization for how the standard mitigation techniques work on each foundation type.
Apartment vs. condo — who is legally responsible
Radon mitigation in a multi-family building is rarely a single-person decision. Whether the building is rental apartments or owner-occupied condominiums materially changes who has the authority and the obligation to test and mitigate. The general framing below is not a substitute for jurisdiction-specific legal advice — consult your jurisdiction’s landlord-tenant law or your HOA bylaws.
Rental apartments
In a rental building, the landlord generally controls the building envelope, the foundation, and the central HVAC — the three systems that determine whether radon enters in the first place. As a general matter, responsibility for testing and mitigation falls on the building owner. Several states have specific landlord disclosure or notification statutes that touch radon — Illinois, Florida, and New Jersey are among the most commonly cited examples — but the specifics vary, and many states have no radon-specific statute at all. A tenant can almost always test their own unit with a short-term kit, and in most jurisdictions habitability law gives the tenant standing to ask the landlord to address a known indoor-air contaminant. The exact procedure, notice requirements, and remedies are state-specific; consult your jurisdiction’s landlord-tenant law.
Owner-occupied condominiums
In a condominium building, the legal split is between the individual unit owner (who controls the air inside their unit) and the homeowners’ association (which controls the common elements: the building envelope, the foundation, the central HVAC, the parking garage). Mitigation of soil-gas entry almost always requires modifying common elements — sealing a slab, drilling a sub-slab depressurization point, modifying HVAC pressure balance — which means the HOA must approve and, depending on the bylaws, may need to assess all owners. An individual condo owner can test their own unit unilaterally, but they cannot install a building-wide mitigation system without HOA action. Consult your HOA bylaws and your jurisdiction’s condominium law before commissioning any building-side work.
Practical testing for a tenant or unit owner
The testing protocol for an apartment or condo unit is the same as for a single-family home, with the unit’s lowest livable level treated as “the lowest level” for measurement-placement purposes. EPA EPA-402-R-92-003 sets the rules:
- Place the kit in a regularly occupied room — a living room or bedroom — never in a kitchen, bathroom, laundry, or closet.
- Keep the kit at least 20 inches above the floor, at least 4 inches from any other object, and away from drafts, exterior walls, and direct sunlight.
- For a short-term test, maintain closed-house conditions (windows and exterior doors closed except for normal entry and exit) for 12 hours before and throughout the test.
- Short-term: 2 to 7 days. Long-term: 90+ days. See our guide to short-term vs. long-term radon testing for when to use which.
An alpha-track or electret long-term test is usually the right answer for a unit you actually live in, because it averages out the day-to-day variation that a 48-hour charcoal kit cannot. DIY kits cost under $30; a certified professional measurement runs $100 to $200 per unit. Our DIY radon testing guide walks through both options.
What to do with an elevated result
If you are a tenant
Notify the landlord in writing, attach the test result, and cite the EPA action level of 4 pCi/L from EPA-402-K-12-002. Ask the landlord to commission a confirming measurement following ANSI/AARST MS-PMD-2014 and, if the confirming result is also elevated, to engage a certified mitigation contractor. Many states’ warranty-of-habitability or implied-warranty doctrines cover known indoor-air contaminants; the specifics are jurisdictional, so consult your jurisdiction’s landlord-tenant law. Keep dated copies of every communication.
If you are a condo owner
Notify the HOA in writing, attach the test result, and cite both EPA-402-K-12-002 and ANSI/AARST MS-PMD-2014. Request that the association commission a building-wide measurement of all ground-contact units, because a single elevated reading in a multi-family building is usually a building-side problem, not a unit-side one. Ask the HOA to engage a contractor certified by NRPP or NRSB. Mitigation work on common elements typically requires a board vote and may require an assessment; review your bylaws before assuming the association will pay. If you are selling, see our guide on selling a home with high radon.
Mitigation in multi-family buildings
Multi-family mitigation is a different engineering problem from single-family mitigation, but it uses the same toolkit. The EPA Consumer’s Guide to Radon Reduction (EPA-402-K-10-005) and the AARST mitigation standards cover the design principles:
- Sub-slab depressurization (SSD). A fan-driven system pulls air from beneath the foundation slab and vents it above the roofline. A single SSD system can usually serve multiple ground-floor units in the same slab-on-grade block, with suction points spaced according to slab communication tests. This is the dominant mitigation strategy for slab-on-grade and basement multi-family buildings. See our sub-slab depressurization explainer.
- Sub-membrane depressurization (SMD). Where the building sits over a crawlspace, a sealed polyethylene membrane is laid across the soil and a fan pulls air from beneath it. SMD serves the units immediately above the crawlspace.
- Block-wall depressurization. For hollow concrete-block foundation walls in older multi-family buildings, suction can be applied to the wall cavities themselves.
- HVAC pressure modification. Where stack effect or pressurised stairwells are pulling soil gas upward into higher-floor units, the engineered response is to rebalance the central HVAC to make the lower-level pressure neutral or slightly positive relative to outside. This is the right answer in buildings where SSD alone leaves residual elevated units several floors above the slab.
A properly designed multi-family mitigation system typically costs more per building than a single-family SSD install but far less per unit. Post-mitigation testing per ANSI/AARST MS-PMD-2014 is required to confirm success, and the system fan’s pressure monitor should be checked at every annual inspection. For context on why the 4 pCi/L threshold matters in absolute health terms, see our radon and lung cancer risk numbers guide.
How this interacts with the EPA zone map
The 1993 EPA Map of Radon Zones (EPA-402-R-93-071) is a county-level planning tool that says nothing about individual buildings, let alone individual units within a multi-family building. A high-rise condo in a Zone 1 county is more likely to have elevated ground-floor units than a high-rise in Zone 3, but the floor-level rule and the building-feature checklist above apply regardless of zone. The EPA recommends testing every ground-through-third-floor unit in every zone. See our EPA radon zones explainer for what the county tier actually represents.
One sentence answer
If you only remember one thing: test every apartment or condo unit on the ground floor through the third floor; above the third floor, only worry about radon if the building has a parking garage, a pressurised shaft, or a known soil-gas pathway — and treat the 4 pCi/L EPA action level the same way you would in a single-family home.
Sources
- A Citizen’s Guide to Radon — EPA-402-K-12-002 — The federal recommendation to test every unit from the ground floor through the third floor in multi-family buildings, and the 4 pCi/L action level used throughout this guide.
- ANSI/AARST MS-PMD-2014 — Protocol for Conducting Measurements of Radon and Radon Decay Products in Multi-family Buildings — The consensus standard governing sample size, unit selection, and follow-up requirements for multi-family radon testing.
- ANSI/AARST RMS-LB-2018 — Protocol for Conducting Measurements of Radon and Radon Decay Products in Schools and Large Buildings — The companion standard for high-rises and other large buildings beyond the scope of MS-PMD-2014.
- EPA Protocol for Radon Measurements — EPA-402-R-92-003 — The federal placement and duration rules for a valid in-home radon measurement, applied per unit in multi-family buildings.
- Consumer’s Guide to Radon Reduction — EPA-402-K-10-005 — Source for the soil-gas entry-pathway list and the standard mitigation techniques used in multi-family buildings.
- American Lung Association — Radon — Background on radon as a Class A human carcinogen and the population-level risk that motivates the EPA action level.
Related guides
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