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Radon in Commercial Due Diligence
Radon sits outside the standard Phase I ESA. When a buyer or lender wants it answered, the measurement gets added to due diligence on the acquisition clock and delivered as a report built for the deal file.
Radon and the Phase I ESA, plain English
A Phase I Environmental Site Assessment under ASTM E1527 evaluates a commercial property for recognized environmental conditions. Radon is a naturally occurring soil gas, not a CERCLA hazardous substance, so it generally falls outside that standard scope. A Phase I can be fully compliant without ever addressing radon.
That does not mean radon stops mattering in a deal. Buyers and lenders who want it answered add radon to the user-defined or non-scope portion of due diligence, or order a separate radon measurement alongside the Phase I. The sections below cover why radon stays on the table in due diligence, how it gets added without changing the Phase I, and how the measurement is scoped to fit an acquisition timeline.
Why radon comes up in a commercial deal
Four situations cover almost every commercial radon inquiry. None of them require panic. All of them have a defensible answer that ends with a documented measurement in the deal file.
A lender requires it
Your lender or credit committee added radon as a closing condition, separate from the Phase I ESA itself. The test gets scoped and executed inside the deal timeline so it does not become the item that holds up funding.
Your consultant flagged it
The environmental consultant on the deal noted radon as a non-scope or user-defined consideration in the Phase I, especially in a higher-radon county. A separate radon measurement closes that open item with documented data.
The property sits in a higher-radon area
The building is in a county the EPA maps as a higher-radon zone, or nearby properties have tested elevated. Buyers want to know what they are acquiring before the basis is set, not after a tenant or future buyer asks.
Liability and exit planning
Buyers want a clean baseline in the acquisition file so a future disposition, refinance, or tenant question has a documented answer already on hand. A test at acquisition is cheaper than discovery during a later transaction.
Scope
Where radon fits relative to ASTM E1527
The most common confusion in commercial deals is whether the Phase I already covered radon. Usually it did not, and that is by design. Here is how the pieces fit together.
Why radon is out of the standard scope
ASTM E1527 governs the Phase I Environmental Site Assessment, the standard used to evaluate a commercial property for recognized environmental conditions. Radon is a naturally occurring soil gas, not a CERCLA hazardous substance, so it falls outside the standard inquiry. A Phase I that does not address radon is still a complete Phase I under the standard.
Where radon actually gets added
When a buyer or lender wants radon addressed, it goes into the user-defined or non-scope portion of the assessment, or it is handled as a separate radon measurement ordered alongside the Phase I. The ASTM standard explicitly leaves room for these additional, user-requested services without changing the underlying Phase I.
A measurement, not a guess
Radon cannot be inferred from a desktop file review. It has to be measured in the building under controlled conditions. That is what gets added: a real measurement with devices placed in ground-contact spaces, run for a documented window, and reported with the underlying data.
The 4.0 pCi/L reference point
The EPA action level of 4.0 pCi/L is the third-party benchmark most consultants and lenders reference. A result at or above that level is the trigger most deals use to require mitigation or a price/credit conversation. A documented result below it is the clean baseline buyers want in the file.
The practical takeaway: do not assume the Phase I answered the radon question, and do not treat radon as a reason to reopen a completed Phase I. It is a separate, parallel measurement that the deal team can reference on its own.
How the measurement is scoped to the deal
Radon in due diligence is not just “a canister and a number.” The measurement is scoped to fit the diligence clock, placed where soil gas would actually enter the building, run under documented conditions, and reported in a form that holds up to outside review.
The plan is set against your closing date and building type on the intake call, so the radon work fits inside the deal rather than holding it up.
Scope it to the deal clock
Most acquisitions run on a defined diligence period. The radon measurement is scoped to fit inside that window so it does not become the long pole. Short-term measurement devices run a documented window, then lab turnaround. The timeline is set against your closing date up front.
Place devices where it matters
Devices go in ground-contact spaces and a defensible sample of the occupied building, prioritizing the areas where soil gas would actually enter. Building type drives the plan: a slab-on-grade office is sampled differently than a building with a below-grade level or a large warehouse footprint.
Run under documented conditions
The measurement runs with the building in its normal operating mode, conditions noted, device serial numbers logged. The point is a result that holds up if a lender, a future buyer, or counsel reviews it later, not a number with no record behind it.
Deliver a defensible report
You get a report built for a deal file: device locations, serial numbers, raw data, conditions during the test, a narrative interpretation against the 4.0 pCi/L reference, and a clear recommendation. It is built to hand to the consultant, lender, or counsel without follow-up questions.
Why a measurement at acquisition is the cheap moment
The acquisition is the point of maximum leverage for the buyer. A documented radon result before closing lets the deal team price a known condition, negotiate a credit, or require a seller-funded fix while the seller still has a reason to cooperate. The same result discovered after closing is the buyer’s problem alone.
Geography matters here. In counties the EPA maps as higher-radon zones, the odds of an elevated building are meaningfully higher, and a clean Phase I says nothing about it. A measurement is the only way to know what is actually in the building. A documented result below the 4.0 pCi/L reference is the clean baseline a buyer wants in the file, and a result at or above it is the number the deal team needs to act on while there is still room to negotiate.
Carrying an open radon question into ownership means carrying it into every future event: the next refinance, the next disposition, the next tenant who asks. Answering it once, at acquisition, with a defensible report, closes the loop on the cheapest possible terms.
Frequently asked questions
Does a standard Phase I ESA include radon?▾
If it is out of scope, why does radon still matter in due diligence?▾
How does radon get added to a Phase I without changing the Phase I?▾
Can the radon measurement fit inside our diligence period?▾
What result triggers a problem in a deal?▾
Who orders the radon work, the consultant or the buyer?▾
What if the building tests elevated during diligence?▾
Do you provide the testing that goes into commercial due diligence?▾
Information on this page is general guidance about radon in commercial real estate due diligence and how it relates to the ASTM E1527 Phase I Environmental Site Assessment standard. It is not legal, financial, or engineering advice. ASTM E1527 and the EPA 4.0 pCi/L action level are referenced as third-party standards. Specific decisions about a property, transaction, scope of work, or lender requirement should be made with qualified counsel, your environmental consultant, and a site-specific evaluation.
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Due diligence radon inquiry