Radon Testing For Multi-Family Buildings

Date Published: October 26, 2023

With the CO Senate Bill 23-206 now in full effect, landlords and home sellers are required to disclose information about radon to the tenant or buyer. This newly-implemented standard of practice will encourage prospective buyers/tenants to have radon testing at the property. Thus, Colorado will likely see an increase in radon testing altogether.

As a home inspection company, one of our longest-standing flagship services is radon testing. So we often have clients asking us the specifics about it. We have addressed the process for single-family homes in past articles, but never multi-family properties. While there are quite a few similarities between the two testing methods, there are also differences. That’s why we are going to take a closer look at radon testing for multi-family buildings. But first, a quick recap.

CO Senate Bill 23-206: A Quick Recap

Let’s take a brief step back to get everyone on the same page about the recent, radon-related Colorado bill. The presence of radon can be a serious heath threat to Colorado residents statewide. That is why the Colorado General Assembly saw it fit to implement a law that increases radon awareness and safety. As a result, CO SB 23-206 took effect on August 7, 2023. And it established a new law requiring more transparency about radon during housing transactions.

Sellers/ landlords now need to do three main tasks. First, they must include an official disclosure about radon issued by the Colorado Department of Public Health and Environment in their contract of sale or lease agreement. Next, they must disclose any known information about radon on the property, including past radon tests, radon mitigation systems, etc. Finally, they should provide a brochure from the Colorado DPHE that discusses radon and its effects.

Radon can be present in any type of structure, which is why this law applies for both single-family and multi-family properties. Tenants/buyers will need to read and acknowledge all of the above mentioned info. It is at this time during the transaction that any tenant/buyer is encouraged to test for radon (if the property has not already been recently tested.)

For more information about the CO Senate Bill 23-206, check out this article.

Radon Testing In Multi-Family Homes

There is a series of important guidelines to follow when it comes to testing for radon in multi-family buildings. Depending on how many units there are in the building, it can be a much more extensive process than testing in single-family homes. The American Association of Radon Scientists and Technologists (AARST) has established the correct way to test an entire building. Since you must have an NRPP license to test for radon in the state of Colorado now, all radon testing companies must adhere to these rules. Let’s break it down!


It’s worth noting that not just anyone can conduct these radon tests. According to the the ANSI/AARST guidebook, Protocol for Conducting Measurements of Radon and Radon Decay Products in Multifamily, School, Commercial and Mixed-Use Buildingsan individual must be certified by an AARST-accredited national program and meet any requirements established in a given local and/or statewide jurisdiction.

In Colorado, this means that you must be certified by the National Radon Proficiency Program (NRPP) in order to conduct any type radon testing procedures. This has been a requirement since the bill, HB21-1195, took effect in July of 2022. 

Official AARST guide for radon testing in multifamily buildings

Preparation is an important part of the process. The radon specialist must discuss the process with the client(s) and let them know the test conditions that are required to yield valid test results. The conditions are as follows:

  • All exterior doors and windows must be kept closed 12 hours before the test and during the test.
  • Heating and cooling systems should be set to normal (between 65 and 80 degrees F).
  • Systems that ventilate with outdoor air should set be to the lowest seasonal ventilation.
  • Excessive use of exhaust fans should be avoided.
  • Fireplaces should not be operated.

If the client adheres to all the above conditions properly, the test should yield accurate results.


Just like the testing individual, the testing device must also be approved by a qualified local program (if one such program exists in your area), or by a nationally accredited program, such as the NRPP or the National Radon Safety Board (NRSB).

At Scott Home Inspection, we utilize the Airthings Corentium Pro Digital Radon Monitor – a fully AARST/NRPP-certified device (shown below). 

An Airthings Corentium Pro radon testing device

One of the devices used by Scott Home Inspection to test radon levels.


Now it’s time to actually set the devices. Where do we need to place the monitors in the multi-unit building?

According to section 3.1 of the ANSI/AARST guidebook, when dealing with multi-family housing, devices need to be set in all units that have floors and/or walls in contact with the ground; and in units that are closest to ground over untested areas (such as crawlspaces or parking garages).

When testing in ground-contact dwellings, the testing specialist must set the device in the lowest lived-in space. This could be a bedroom, office, man-cave, etc. And note that if any of the lowest ground-contact units are inaccessible for some reason, the unit above it must be tested.

Test locations for radon testing in multi-family buildings

A selection of upper floors will also need to be tested in addition to the ground-level units. The guidebook requires that “at least one and not less than 10%” of dwellings on each floor must be tested (see section 3.2 in the diagram above).

The next question is – where in the room can the device be placed? Table 3.8 in the guide gives you an extensive breakdown about the dos and don’ts of radon device placement. But we can give you a brief summary here. The devices should be set…

  • At least 3 feet away from doors/windows/other openings.
  • At least 20 inches above the floor.
  • At least 1 foot from exterior walls.
  • At least 1 foot below the ceiling.
  • In a position where it will not be disturbed or moved.

Additionally, the device should not be set in enclosed spaces like closets, cabinets, drawers and so on. It should also be placed away from heat sources (fireplaces, radiators). Radon pros also know to keep the test apparatus away from HVAC and high humidity areas like bathrooms, kitchens, and laundry rooms.

Believe it or not, some household objects (such as granite countertops and slate pool tables) have been known to produce trace amounts of radiation. It is best to avoid these types of objects so as not to affect the device’s radon readings.

How Long?

The short term test is the most common, EPA-approved testing duration. This means that the test device(s) should be running continuously and taking incremental measurements in closed-building conditions for a minimum of 48 hours. At Team Scott, this is the kind of test we conduct, as they are most ideal for the quick turn-around of housing transactions.

However, there are also other options to evaluate radon over longer periods of time.

Yielding Radon Test Results

After the 48-hour testing period is over, the radon specialist will retrieve the devices and extract the results. According to the EPA, if the results are greater than or equal to 4.0 pico curies per liter (pCi/L), radon mitigation is needed.

In the case that levels read between 2 and 4 pCi/L (like the results shown below), it is still a good idea to mitigate. Believe or not, the EPA states that levels between 2 and 4 pCi/L can still pose health risks.

Keep in mind that even if the test yields very low levels, radon concentrations can still fluctuate over time. So we always recommend retesting periodically, just to be safe!

Radon Test Results

Results from a Scott Home Inspection radon test. Measured by an Airthings Corentium Pro Digital Radon Monitor

Radon Testing with Scott Home Inspection

As a company with over 23,000 radon tests under our belt, one thing is for sure – we know how to do it right! We have a full staff of NRPP-certified inspectors, equipped to handle radon testing in single or multi-family dwellings. We understand the dangers of radon. And that’s why we’re here to help you evaluate the risk in your home.

To learn more about our radon testing visit us online, or contact us to book your services today.

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About the Author: Chris Kimmel

Chris Kimmel worked as an Associate Home Inspector for two years, handling numerous services including sewer scope inspections, pest inspections, mold air sample testing, radon testing, and water quality testing. Chris now works with Scott Home Inspection as a Content Writing Specialist.

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