Colorado radon guidance for residential landlords

01 / Statutory obligations

What Colorado law requires before a residential lease

Before a tenant signs a lease for residential real property, the landlord must provide written disclosure stating whether the property has been tested for elevated radon. The disclosure covers known radon concentrations and testing history, and includes the required warning about radon dangers and the need for testing.

The landlord must also provide the most recent Radon and Real Estate and Rental Transactions brochure published by the Colorado Department of Public Health and Environment (CDPHE). These are statutory disclosure requirements, not optional property-management guidance.

• Written disclosure of whether the property has been tested for elevated radon.
• Known radon concentrations and testing history, when available.
• The required warning statement about radon dangers and the need for testing.
• The most recent CDPHE Radon and Real Estate and Rental Transactions brochure.

02 / The mitigation threshold

4 pCi/L and the 180-day reasonable-effort window

When radon is measured at or above 4 picocuries per liter (4 pCi/L), the landlord must make a reasonable effort to mitigate within 180 days after being notified of the elevated measurement. Property managers may want to retain testing, notice, and mitigation records, but the exact meaning of a reasonable effort depends on the applicable law and circumstances.

03 / Applicability and dates

Check the lease term, property type, and current source text

The disclosure and mitigation summary here is focused on Colorado residential lease transactions. It should not be treated as a summary of commercial-property rules or the separate disclosure rules for residential sales.

Beginning January 1, 2026, for lease agreements longer than one year, a tenant may void the lease if the required radon testing disclosures are not provided or if a reasonable effort is not made to mitigate within 180 days after notification of elevated levels. Review the linked statute and state guidance for the exact language, exceptions, and any later amendments.

04 / CDPHE guidance

State resources that support testing and mitigation

CDPHE provides testing and mitigation resources that support landlords, property managers, tenants, and homeowners. These resources are state guidance and assistance; they supplement the statutory disclosure and mitigation duties but do not replace them.

• Free radon test kits for households, generally one per household per year while supplies last, plus discounted kits available for purchase.
• Directories of certified radon testing and radon mitigation professionals.
• Information about radon testing for rental properties and water radon mitigation.
• The Low-Income Radon Mitigation Assistance (LIRMA) program for qualifying Colorado homeowners; eligibility is based on program requirements and county criteria.