Radon Testing: The Complete Homeowner's Guide for 2026

One in 15 U.S. homes has radon testing results at or above the EPA action level of 4 pCi/L, according to the American Lung Association. In Wisconsin, the odds are far worse. The Wisconsin Department of Health Services reports a state mean indoor radon level of 5.7 pCi/L, nearly 4.5 times the national average of 1.3 pCi/L. Radon is the second leading cause of lung cancer, responsible for roughly 21,000 deaths each year.

The problem: radon is invisible, odorless, and you cannot predict your home's level based on what your neighbor measured. The only way to know is to test. The good news is that testing starts at $15 and takes 48 hours.

This guide covers which test type fits your situation, how to avoid the mistakes that produce false results, what your numbers actually mean, and special considerations for real estate transactions. We include Wisconsin-specific radon data and resources throughout, because nearly two-thirds of our state's 72 counties are classified EPA Zone 1, the highest radon potential category. National guides routinely understate the risk Wisconsin homeowners face. Start with why testing matters, then we walk through exactly how to do it right.

Why Every Home Needs a Radon Test - Regardless of Age, Location, or Foundation

"My neighbor tested fine." "I don't have a basement." "My home was built last year." These are the three most common reasons homeowners skip a radon test. All three are wrong.

Every home on every parcel of land can have a radon problem. The ANSI/AARST MAH-2019 national standard puts it plainly: radon concentrations cannot be predicted based on state, local, or neighborhood measurements. The only variable that determines your home's radon level is the interaction between your specific foundation, your soil, and your home's air pressure dynamics.

The Foundation Myth

Slab-on-grade homes are not immune. Documented cases from Branch Investigations show slab homes testing at 7.2 pCi/L and 9.4 pCi/L, both well above the EPA action level of 4 pCi/L. Concrete is porous. It cracks over time.

Water heaters, dryers, and furnaces exhaust air from the home, creating negative indoor pressure. That pressure differential actively pulls soil gas upward through slab openings and foundation cracks, turning your home into a low-grade vacuum drawing radon from the ground beneath it.

Basement homes face the most direct exposure because the lowest level sits underground and surrounded by radon-bearing soil. Crawl space homes are vulnerable too, especially when vents are sealed or the space connects to the living area through ductwork or gaps in the floor system. No foundation type offers protection.

Wausau 2006 Geological Survey

The Neighbor Fallacy

Adjacent homes can have dramatically different radon levels. Differences in foundation cracks,

soil composition beneath the specific footprint, HVAC pressure, and local geology all create unique radon conditions for each structure. ANSI/AARST confirms this explicitly in the national measurement standard. A home reading 1.5 pCi/L can sit 30 feet from a home reading 9 pCi/L.

The Health Stakes

"Radon causes approximately 21,000 lung cancer deaths annually in the United States. About 2,900 of those deaths occur among people who have never smoked." -- American Lung Association

According to the American Lung Association, radon is the leading cause of lung cancer among non-smokers. For smokers, the combination of radon exposure and tobacco use creates a tenfold increase in lung cancer risk.

No foundation type, home age, or neighborhood result substitutes for testing your own home. A $15 test kit resolves the question in 48 hours.

When to Test for Radon: Best Season, Timing Triggers, and the Re-Testing Schedule

Winter is the best season to test for radon, and a single test years ago does not tell you what your home reads today. We've seen considerable variation in both mitigated and non-mitigated homes from summer to winter months. Summer-only radon testing can produce a dangerous false negative.

Seasonal Variation, Quantified

A 13-year NIH study analyzing 2.4 million test records across 46 states found a clear seasonal pattern. The national mean peaked in January at 203.8 Bq/m3 and dropped to a low of 129.5 Bq/m3 in July. That represents a 57% increase from summer trough to winter peak. Among those 46 states, 33 showed their lowest radon concentrations in July.

Winter levels typically run 2 to 3 times higher than summer. In individual homes, the variation can reach 5 times or more. The mechanism is straightforward: during heating season, your furnace runs continuously, windows stay sealed, and the stack effect draws soil gas upward through the foundation. Sealed windows eliminate dilution while the HVAC system creates the negative pressure that pulls radon inside. ANSI/AARST notes that tests conducted when the heating system operates both day and night provide clearer characterization of potential radon hazards.

The best testing window runs from October through February. In Wisconsin, this aligns directly with peak heating season, when homes are sealed tightest and radon accumulation is greatest.

When to Re-Test

The EPA recommends re-testing every 5 years without a mitigation system and every two years WITH a mitigation system, regardless of previous results. Foundations shift and crack. Soil conditions change. A safe result today does not guarantee a safe result later.

Beyond the standard 2-5 year cycle, specific triggers warrant immediate re-testing:

• Before and after finishing a basement

• After HVAC replacement (changes pressure dynamics)

• After energy efficiency improvements like new windows or added insulation (a tighter envelope traps more radon)

• When you begin occupying a lower level

• After any major structural renovation or foundation work

  
  

Each of these changes alters the pressure relationship between your home and the soil beneath it, which is the primary driver of indoor radon levels.

For First-Time Testers

Test immediately, ideally during heating season. If you cannot wait for winter, test now and plan a winter follow-up. A summer test that reads below 4 pCi/L does not clear your home. It tells you what radon looks like under the most favorable conditions, when windows are open, ventilation is high, and the stack effect is weakest.

If your last test was more than 2 years ago or you have renovated since, it is time to re-test.

Types of Radon Tests: Short-Term, Long-Term, and Continuous Monitors Compared

A 48-hour test and a year-long test in the same home can produce wildly different results. According to a two-year accuracy study, the 2-day test might read anywhere from 1 to 13 pCi/L. The year-long test narrows that range to 4.5 to 5.0 pCi/L. Which number would you base a health decision on?

Understanding the accuracy trade-offs between radon testing methods is the first step toward choosing the right one for your situation.

Short-Term Passive (Charcoal Canister)

Charcoal canisters are the most common DIY radon test. They cost $10 to $30 at retail (or about $15 through a Wisconsin Radon Information Center with lab analysis included). You place the canister in your home for a minimum of 48 hours, then mail it to a lab for results.

The trade-off is accuracy. A two-year study found that 48-hour charcoal tests produce a 40% false negative rate, meaning 2 in 5 homes that actually exceed the EPA action level receive a falsely safe result. Charcoal continuously adsorbs and releases radon rather than accumulating it, and high humidity interferes with the reading.

Best for: Routine screening, first-time homeowner baseline. Skip if: You are in a real estate transaction (no tamper detection) or your short-term result falls in the 2 to 6 pCi/L borderline range.

For a detailed comparison of specific kits, see our radon testing kit guide.

Long-Term Passive (Alpha Track Detector)

Alpha track detectors run for 90 days to 12 months (6 or more months spanning the heating season is ideal). They cost $25 to $50 and capture seasonal variation that short-term tests miss entirely. The same study found a 25% false negative rate at 90 days, a significant improvement over the 48-hour charcoal result.

"A 48-hour test can read anywhere from 1 to 13 pCi/L in a home where the year-long average is 4.5 to 5.0 pCi/L. A 7-day test narrows to 1 to 11 pCi/L. A 90-day test narrows further to 3.0 to 7.6 pCi/L." -- Radon test variability data, two-year accuracy study

Closed-building conditions are NOT required for long-term tests per ANSI/AARST protocol. You live normally while the detector collects data.

Best for: Confirming borderline short-term results, establishing true annual exposure. Skip if: You need results in under 3 months.

Continuous Radon Monitor (CRM)

Continuous radon monitors record hourly measurements along with temperature, barometric pressure, humidity, and motion data. They cost $150 to $300 when deployed by a professional. Results are available immediately with no lab mailing required.

The key advantage is tamper detection and hourly readings. CRMs log environmental conditions every hour, so if someone opens a window or door during the test, the pressure and temperature shift appears in the data. This makes CRMs the required or preferred device for real estate transactions under the ANSI/AARST time-sensitive protocol.

Best for: Real estate transactions, post-mitigation verification. Skip if: You just want a general screening (a $15 charcoal kit is more cost-effective for that purpose).

CRM Test Report

Electret Ion Chambers and Smart Monitors

Electret ion chambers (EICs) are professional-grade devices that provide direct readings without lab mailing but carry higher cost and sensitivity to gamma radiation interference.

Smart monitors like Airthings devices are useful for ongoing tracking and trend monitoring. They are NOT listed by NRPP or NRSB, which means results are not accepted for real estate transactions or mitigation decisions under ANSI/AARST protocol. Treat them as supplemental tools, not primary measurement devices.

DIY Kits vs. Professional Radon Testing: How to Choose

If you are buying or selling a home, hire a professional with a continuous monitor. For everything else, a $15 DIY kit from your local Wisconsin Radon Information Center is a perfectly valid starting point.

That is the decision shortcut. Here is the full picture.

When DIY Is the Right Call

A do-it-yourself charcoal canister works well for routine first-time screening, periodic 2-year re-testing, and post-renovation baseline checks. According to a January 2026 Wisconsin DHS press release, Wisconsin RICs offer kits at approximately $15 including lab analysis, compared to $30 or more at hardware stores. For a straightforward "do I have a radon problem?" question, a properly placed DIY kit delivers a useful answer.

Visit our radon testing kit guide for a side-by-side comparison of available kits.

When You Need a Professional

Professional radon testing is essential (not optional) in four situations:

• Real estate transactions. ANSI/AARST requires a qualified measurement professional for the time-sensitive protocol. DIY kits lack tamper detection and are not accepted under this standard.

• Borderline DIY results (2 to 6 pCi/L). The error margin on consumer kits means a 3 pCi/L reading could actually be anywhere from 2.25 to 3.75 pCi/L. A professional CRM provides the precision needed to make a confident decision near the action level.

• Post-mitigation verification. After a mitigation system is installed, a professional confirms the system is reducing radon to target levels and operating as designed.

• Multi-zone homes. Homes over 2,000 square feet, split-level layouts, and structures with multiple foundation types require additional test locations per ANSI/AARST. A professional knows where and how to place devices for each zone.

Cost Comparison

• DIY kit (Wisconsin RIC): ~$15 including lab

• DIY kit (retail): $10 to $30

• Professional standalone test: $150 to $300

• Professional add-on to home inspection: $125 to $200

• Large multi-story home: Up to $500

Finding a Qualified Professional in Wisconsin

Wisconsin does not require state licensing for radon contractors, which means the burden of verification falls on you. Visit lowradon.org or call 1-888-LOW-RADON (1-888-569-7236) to find certified testers in your area. Always verify NRPP or NRSB certification independently. According to the Wisconsin DHS, Wisconsin has roughly 150 certified radon professionals statewide.

How to Run a DIY Radon Test Correctly: Placement, Closed-House Rules, and Common Mistakes

Most DIY radon tests fail not because the device is faulty, but because the homeowner places it in the wrong spot, skips the 12-hour pre-test window, or tests during a season that masks the true level.

Placement, closed-building conditions, and lab turnaround are the three accuracy pillars. Get all three right, and a $15 kit produces a reliable screening result.

Placement Specifications (ANSI/AARST MAH-2019)

Place the test device in the lowest level that IS regularly occupied (for general screening) or the lowest level that COULD be occupied (if you are making a mitigation decision). Position the device on a table or shelf with these minimum clearances:

• At least 20 inches above the floor

• At least 3 feet from exterior doors and windows

• At least 1 foot from any exterior wall

• At least 1 foot below the ceiling

• At least 4 inches from other objects

Do NOT place test devices in any of the following locations:

• Closets, cabinets, or enclosed spaces

• Sump pits or crawl spaces

• Near heat sources or HVAC vents

• Bathrooms, laundry rooms, or kitchens (humidity interference for charcoal)

• High-traffic areas near exterior doors

For homes over 2,000 square feet, each unique foundation area and each separate HVAC zone should receive its own test device.

The 12-Hour Rule: Closed-Building Conditions

This is the step most homeowners skip, and it is the single biggest source of inaccurate results.

Close ALL windows on ALL levels at least 12 hours BEFORE the test starts. Maintain those conditions throughout the entire test period (48 to 96 hours). This applies to every floor, not just the floor being tested. An open bedroom window on the second floor affects radon readings in the basement.

During the test period:

• Keep HVAC running at normal occupied temperature (65 to 80 degrees F)

• Do NOT operate whole-house fans

• Do NOT use fireplaces (unless the fireplace is your primary heat source)

• Minimize clothes dryer, range hood, and bathroom fan use

• Normal entry and exit through exterior doors is fine

Common Mistakes That Produce Inaccurate Results

These are the errors we see most often. Each one can skew your result enough to flip a reading from "safe" to "elevated" or the reverse:

• Placing the device near windows, vents, in bathrooms, kitchens, or laundry rooms

• Setting the device directly on the floor instead of an elevated surface

• Opening windows during the test period, even on upper floors

• Testing only in summer without planning a winter follow-up

• Using expired test kits

• Delaying mailing the kit to the lab after the test completes

• Testing a crawl space instead of the lowest living level

• Not testing each unique foundation area in split-level homes

According to Radonova Laboratories, most global radon labs lack ISO/IEC 17025 accreditation, meaning homeowners using non-accredited labs for DIY kit analysis may receive unreliable results with no quality assurance.

Follow the placement specs, maintain closed-building conditions for the full test period, and mail the kit the same day you retrieve it. These three steps eliminate the most common accuracy killers.

Radon Testing in Real Estate Transactions: Protocols, Costs, and Tampering Prevention

Real estate radon testing demands a stricter protocol, a tamper-evident device, and greater scrutiny than routine homeowner screening. A Wisconsin seller admitted to opening windows and sliding glass doors during a radon test. An earlier test had shown 8 pCi/L, double the EPA action level. The buyer's charcoal canister had no way to detect the interference.

The Time-Sensitive Protocol (ANSI/AARST MAH-2019 Section 8.2)

For real estate transactions, ANSI/AARST requires a qualified measurement professional (not a DIY approach). The protocol mandates:

• Two simultaneous short-term devices placed 4 to 8 inches apart, OR one continuous electronic monitor

• Testing the lowest level that COULD be occupied (including an unfinished basement)

• 12-hour closed-building conditions established before the test starts

• Base the mitigation decision on the average of both device results

When two results disagree: if both are above or both below the action level, use the average. If one is above and one is below, and the higher result is less than twice the lower, use the average. If the higher result exceeds twice the lower, a retest is required.

Who Pays for What

The buyer typically pays for the radon test, which runs $125 to $300 depending on whether it is a standalone service or an add-on to the home inspection ($125 to $200). If the result comes back at or above 4 pCi/L, the seller most commonly pays for mitigation.

Alternatives include a seller credit to the buyer or the seller completing mitigation before closing. In Wisconsin, mitigation systems cost $1,000 to $2,300.

Sellers have a strategic option: test and mitigate before listing. The EPA's Home Buyer's and Seller's Guide to Radon recommends this approach. Proactive mitigation removes buyer leverage, avoids transaction delays, and can shorten time on market. A radon mitigation system that brings levels below 2 pCi/L becomes a selling point rather than a negotiation liability.

How Tampering Works and How CRMs Prevent It

Charcoal canisters offer zero tamper detection. They can be moved, covered, or ventilated past with no record. Documented fraud cases include sellers opening windows and doors during tests, listing agents coaching sellers to ventilate basements, and contractors falsifying post-mitigation results.

CRMs log hourly radon concentrations alongside temperature, barometric pressure, humidity, and motion. Any window or door opening registers as a pressure and temperature event in the data. Professional testers also post "Radon Test in Progress" notices, obtain signed noninterference agreements, and conduct visual inspections at both placement and retrieval.

Wisconsin-Specific Context

Southeast Wisconsin has some of the highest radon levels in the state. In the Milwaukee area, radon has become a standard negotiation point in real estate transactions. With Wisconsin's mean indoor level already exceeding the EPA action level, skipping the radon test during a transaction means accepting a known risk in one of the highest-radon states in the country.

Find certified testing professionals at lowradon.org or call 1-888-LOW-RADON.

For real estate transactions, insist on a continuous electronic monitor deployed by an NRPP/NRSB certified professional. It is the only test method that produces tamper-evident results and hourly data. Budget $125 to $300 for the test. It protects a purchase worth hundreds of thousands.

View a your local county and zip code radon readings using our helpful zip code search for WI here.

Understanding Your Radon Test Results: What the Numbers Mean and What to Do Next

Your radon test result falls into one of four action zones. Here is exactly what to do with each one.

The Four Action Zones

Below 2 pCi/L: No action needed. Re-test in 2-5 years. This is below the World Health Organization reference level and represents low risk. Continue the standard re-testing schedule.

2 to 4 pCi/L (the gray zone): The EPA says "consider fixing." You have two reasonable paths. Run a long-term test (90 or more days) to establish a more accurate annual average, or mitigate now given the modest cost ($1,000 to $2,000 in Wisconsin). Many homeowners choose to mitigate at this range, especially in Wisconsin where seasonal peaks could push levels above 4 pCi/L in winter.

4 to 8 pCi/L: The EPA says "fix your home." Conduct a second short-term test to confirm, or proceed directly to mitigation. If you want a more complete picture, follow up with a long-term test, but do not delay the mitigation conversation.

Above 8 pCi/L: Fix immediately. At this level (twice the action level), ANSI/AARST recommends immediate action under the extended protocol. Conduct a second short-term test AND begin the mitigation process. Do not wait for a long-term test.

Confirmation Testing Protocol

If your initial short-term result warrants a second test:

• Both results above or both below the action level: use the average

• One above, one below, and the higher is less than twice the lower: use the average

• Higher result exceeds twice the lower: retest required

• For results at or above 8 pCi/L: run a second short-term test immediately (do not substitute a long-term test)

What Mitigation Looks Like

Sub-slab depressurization is the primary mitigation method. A fan and pipe system draws radon gas from beneath the foundation and vents it above the roofline.

Costs range from $800 to $2,500 nationally and $1,00 to $2,000 in Wisconsin in most cases. Post-mitigation radon testing is required: a minimum of 24 hours after system activation, under closed-building conditions, in the same location as the pre-mitigation test.

For a full overview of mitigation systems, costs, and what to expect, visit our radon mitigation system guide.

If your result is 4 pCi/L or above, contact an NRPP/NRSB certified mitigator. If you are in the 2 to 4 pCi/L range, a long-term test or proactive mitigation are both reasonable next steps.

Wisconsin Radon Testing: State Data, Free Resources, and Why WI Homeowners Face Higher Risk

Wisconsin's mean indoor radon level is 5.7 pCi/L. That is already above the EPA action level of 4 pCi/L, and it is nearly 4.5 times the national average of 1.3 pCi/L.

If you live in Wisconsin, radon testing is not a precaution. It is a necessity.

Wisconsin by the Numbers

• Nearly two-thirds of Wisconsin's 72 counties are classified as EPA Zone 1, the highest radon potential category (predicted average above 4 pCi/L)

• Wisconsin has NO Zone 3 counties (lowest risk). Every county in the state carries at least moderate radon potential.

• 46% of Wisconsin zip codes report that at least 50% of test results exceed 4 pCi/L

• Highest-risk counties include Waukesha, Washington, and Dodge, where up to 60% of homes test above guidelines

• The Wisconsin DHS estimates 1 in 10 Wisconsin homes has high radon levels

"Just because we can't see radon, doesn't mean it's not present in our homes and dangerous to our health." -- Paula Tran, Wisconsin State Health Officer, January 2026 DHS press release

Wisconsin Testing Resources

Wisconsin operates one of the most accessible radon testing programs in the country. According to the Wisconsin DHS, seventeen Radon Information Centers serve all 72 counties. The RICs have distributed over 100,000 free or reduced-cost test kits and provide more than 2,000 radon consultations each year.

• RIC test kits: approximately $15 including lab analysis (vs. $30 or more at retail)

• Consultation hotline: 1-888-LOW-RADON (1-888-569-7236)

• Certified professionals: lowradon.org

Wisconsin Regulatory Landscape

Wisconsin does NOT require state licensing for radon measurement or mitigation contractors, meaning anyone can advertise radon services regardless of training. Always verify NRPP or NRSB certification independently before hiring a tester or mitigator. Wisconsin has approximately 150 certified radon professionals statewide.

Wisconsin stands alone as the only state mandating radon testing for childcare facilities on a fixed schedule: every 2 years for family care centers and every 5 years for group care centers. New providers must test within 6 months of opening.

For county-level radon data and a full overview of Wisconsin's radon landscape, visit our Wisconsin radon guide.

Radon Testing FAQ

Does my slab-on-grade home need radon testing?

Yes. Documented cases from Branch Investigations show slab-on-grade homes testing at 7.2 and 9.4 pCi/L, both well above the EPA action level of 4 pCi/L. Concrete is porous and develops cracks over time. No foundation type protects against radon. Test every home regardless of foundation.

How often should I re-test for radon?

Radon levels are dynamic; a safe result today does not guarantee a safe environment forever. Foundations shift, settling creates new cracks, and changes to how you live in your home can all impact radon entry. To ensure your family remains protected, follow these standard testing schedules:

If You Do NOT Have a Mitigation System

• Every 5 Years: At a minimum, homes that previously tested below the Action Level ($4.0~pCi/L$) should be re-tested every five years.

• Real Estate Transactions: You should always test any residence whenever it is being bought or sold to ensure the health of the new residents.

  
  

If You HAVE an Active Mitigation System

• Every 2 Years: Homes with an active radon system (like a vent fan) should be tested at least every two years. This "stewardship" testing ensures the system is still operating effectively and that radon levels remain below the $4.0~pCi/L$ action level.

  
  

When to Test Sooner (The "Trigger Events")

Regardless of your last test date, you should re-test immediately if any of the following occur:

• Home Renovations: You build an addition, alter the building configuration, or perform extensive renovations.

• HVAC Changes: You install, significantly alter, or replace heating or cooling systems.

• Structural Shifts: Nearby construction blasting, earthquakes, or the formation of sinkholes can open new paths for radon gas.

• Lifestyle Changes: You begin occupying a lower level (like a finished basement) or a ground-contact area that wasn't previously used.

• Energy Repairs: Extensive weatherization or alterations to your home’s ventilation system can change the pressure balance and draw in more radon.

Can I test for radon in an apartment or rental?

Yes. Ground-contact apartments have radon levels similar to low-rise residential buildings in the same area, per ANSI/AARST MA-MFLB-2023. That standard requires testing all ground-contact units. Contact your landlord about testing, or purchase a DIY kit and test your unit independently. Wisconsin RICs offer kits for approximately $15 including lab analysis.

What's the difference between short-term and long-term radon tests?

Short-term tests run 2 to 90 days (typically 48 to 96 hours for DIY kits) and require closed-building conditions. Long-term tests run over 90 days, require no closed-building conditions, and capture seasonal variation. According to a two-year accuracy study, a 2-day test can read 1 to 13 pCi/L in a home where the year-long average is 4.5 to 5.0 pCi/L. Short-term tests work for initial screening and real estate transactions. Long-term tests reveal actual annual exposure.

What should I do if my result is between 2 and 4 pCi/L?

The EPA recommends "consider fixing" at this range. Run a long-term test (90 or more days) for a more accurate annual average, or mitigate now. Mitigation costs $900 to $2,000 in Wisconsin and typically brings levels below 2 pCi/L. No radon level is risk-free, and many Wisconsin homeowners mitigate in this range given the state's 5.7 pCi/L mean indoor level.