Radon Levels & Zone Map in Clay County, MO
Direct Answer for basement and lowest-level tests: Clay County is a strong county-level radon signal. You still need a home test, but this is not a county where skipping the first test makes sense.
Do not guess in a higher-risk county
Most users should either get a first test or move a 4.0+ result straight into mitigation pricing.
Higher countywide chance of 4.0+ readings.
2.0-3.9 pCi/L usually means retest or track. 4.0+ is where EPA action and quote planning start to matter.
No reading: test now. Reading at 4.0+: move into mitigation planning.
Measured Radon Data
Clay County evidence before the next step
Clay County, MO has more than the EPA map: Missouri DHSS exposes 2,128 reported tests, 4.3 pCi/L county average, 37.5% of reported tests at or above 4.0 pCi/L, and 65.0 pCi/L high-end signal for 2005-2017.
Source window
2005-2017
Processed verdict
High measured burden
High confidence - 92/100
Primary result
4.3 pCi/L
65th percentile in-state
4.0+ signal
37.5%
77th percentile in-state
High-end signal
65.0 pCi/L
86th percentile in-state
County-specific verdict
Clay County crosses the action threshold in the official county data.
Clay County is a test-now case because 4.3 pCi/L average, and 37.5% of reported tests at or above 4.0. In-state rank: 65th percentile for average, and 77th percentile for 4.0+ share. No reading means get the first number; a 4.0+ home result should move straight to mitigation quotes or seller-credit math.
Real-estate use
Buyer or seller use: ask for a fresh lowest-level test before inspection deadlines, tie any 4.0+ result to a contractor quote, and do not negotiate from the county signal alone.
Clay County should be treated as a county where a first test is urgent and a 4.0+ result should move directly into mitigation pricing or seller-credit math.
Choose Next StepHigh-risk intent answer
Is radon bad in Clay County?
Clay County should be treated as a high-priority testing market because 37.5% of reported tests at or above 4.0 pCi/L, 4.3 pCi/L primary measured result, and 65.0 pCi/L high-end signal in Missouri DHSS data. A missing home reading means test now; a 4.0+ result means mitigation pricing or seller-credit math should start.
Pick the situation that matches you
You should not need to read the whole guide before clicking one of these. Start with the lane that matches your current stage, then come back for the deeper reference only if you still need it.
Jump into a prefilled Clay County action plan based on the result you already have, instead of starting from a generic cost page.
I have not tested yet
Do not price mitigation blind. Get the first number, then decide whether you need monitoring, quotes, or nothing at all.
My result is 2.0-3.9
Usually retest or track first. If the reading keeps showing up, use the local action plan to decide whether pricing makes sense.
My result is 4.0+
This is the EPA action line. Use the local cost page before calling contractors so you know the likely scope, timing, and budget.
I am buying or selling
Turn the reading into a credit or repair number before negotiation starts. This is the faster path than arguing from a generic article.
Already tested once and just want to watch the number trend?
View Airthings Corentium HomeAffiliate disclosure: we may earn a commission if you buy through these links.
County Evidence Snapshot
Clay County testing context
Clay County is a higher-priority testing county because the EPA zone signal is high and the page now ties that signal to local housing context.
EPA map signal
Zone 1
County-level predicted indoor screening range, not a home-level test result.
Housing base
105,571
96th percentile among 115 MO counties with data.
Older housing share
56.3%
82th percentile in-state; older homes often need clearer test placement decisions.
Median home value
$237,200
Used as context for whether mitigation is a small maintenance item or a negotiation issue.
Measured Radon Data
Missouri DHSS Residential Radon Testing in Missouri
2005-2017
Average result
4.3 pCi/L
At or above 4.0
37.5%
Maximum reported
65.0 pCi/L
Reported tests
2,128
Missouri DHSS publishes residential radon testing through a public ArcGIS dashboard. County totals, average final result, and maximum final result come from the county summary layer. RadonVerdict computes the 4.0+ share from non-negative point-level Final_Result records because the point layer also contains -999 privacy/suppression sentinel values. These are reported test records, not a statistically designed survey of every home.
RadonVerdict Processed Verdict
High measured burden
Primary result rank
65th percentile
4.3 pCi/L
4.0+ rank
77th percentile
37.5% at or above 4.0
High-end rank
86th percentile
65.0 pCi/L
Test volume rank
97th percentile
2,128 reported tests
How to use this county data
Data source
Official county measurements
Missouri values come from the DHSS residential radon dashboard. County averages and maximums come from the summary layer, while the 4.0+ share is computed only from non-suppressed point records.
What the numbers show
Fuller county picture
This is the most useful setup: county average, 4.0+ share, high-end readings, and 2,128 reported tests/properties can be read together instead of relying on one number.
Nearby comparison
Nearby comparison: Closest counties by share of tests at or above 4.0 pCi/L: Marion County (37.4%) is just lower, and Ralls County (37.8%) is just higher.
How this helps
Use this to decide how urgently to test, retest, or plan mitigation after a 4.0+ result.
What the data says
Clay County, MO is measurement-backed for 2005-2017. The measured average is 4.3 pCi/L, and 37.5% of reported results are at or above 4.0 pCi/L. The high-end signal reaches 65.0 pCi/L.
Clay County, MO sits at the 65th percentile for measured average, 77th percentile for 4.0+ share, 86th percentile for high-end readings, and 97th percentile for test volume among 115 measured counties in the state. Closest counties by county average: Johnson County (4.2 pCi/L) is just lower, and Wayne County (4.3 pCi/L) is just higher.
What to do with it
Clay County should be treated as a county where a first test is urgent and a 4.0+ result should move directly into mitigation pricing or seller-credit math.
Retest trigger: a 2.0-3.9 pCi/L home result should be confirmed here because 4.3 pCi/L average, and 37.5% of reported tests at or above 4.0 keeps the county from being a dismiss-it signal.
High confidence (92/100) from Missouri DHSS based on about 2,128 reported tests/properties plus comparable county-level measurement fields.
No reading yet
No reading yet: run a short-term test now, then confirm or price mitigation quickly if the result is elevated.
2.0-3.9 result
2.0-3.9 pCi/L: retest or track longer-term rather than dismissing the result, because the county distribution has meaningful elevated readings.
4.0+ result
4.0+ pCi/L: use the result for mitigation quotes, repair scope, or seller-credit negotiation; the county signal is no longer the deciding input.
Source hierarchy: Missouri DHSS is used for this county, with EPA zone and Census housing data kept as supporting context. Missouri values come from the DHSS residential radon dashboard. County averages and maximums come from the summary layer, while the 4.0+ share is computed only from non-suppressed point records.
Direct Answer
What radon risk level should homeowners assume in Clay County?
Clay County is currently categorized as EPA Zone 1 (High Risk). Prioritize testing now and prepare for possible mitigation.
| Evidence | Value |
|---|---|
| Area | Clay County, MO |
| EPA Zone | Zone 1 |
| Primary Recommendation | Perform direct radon testing in the lowest livable level |
Your Radon Reading
Enter your home's measured level; the starting value is only a planning example until you have your own result.
Warning: Action Required - EPA Threshold Exceeded
At 5.5 pCi/L, this reading is above the EPA action level. Prompt mitigation planning is recommended after confirmatory testing.
Typical mitigation systems reduce radon by 80-99%. See your itemized cost estimate below.
Understanding Radon Levels: Complete Reference
Below 2.0 pCi/L - Lower Concern, Keep Testing
Below both the EPA (4.0) and WHO (2.7) action reference levels. This usually means mitigation is not the next immediate step after a confirmed result. The average outdoor radon level is approximately 0.4 pCi/L, and there is no known risk-free indoor level. Periodic testing is still recommended because levels can change over time due to seasonal variations, changes in home ventilation, or foundation settling.
-4.0
2.0 - 4.0 pCi/L - Elevated, Consider Action
Exceeds the World Health Organization's reference level of 2.7 pCi/L but falls below the US EPA action threshold. The EPA states that homeowners should "consider fixing" homes in this range, especially if the home has a basement used as living space, if children are present, or in connection with a real estate transaction. Practical next step: run a confirmatory long-term test, then compare mitigation quotes if levels remain elevated.
-8.0
4.0 - 8.0 pCi/L - Action Recommended
Exceeds the EPA action level of 4.0 pCi/L. The EPA and Surgeon General strongly recommend mitigation within a few months. At this level, prioritize confirmatory testing and contractor planning. Standard sub-slab depressurization systems typically reduce indoor levels by 80-99%.
Above 8.0 pCi/L - Urgent Action Required
At these levels, the EPA recommends expedited mitigation - ideally within weeks, not months. Occupants should minimize time in lower-level rooms until the system is installed. Use a certified mitigator and request priority scheduling to shorten high-exposure time. Many mitigators offer priority scheduling for homes above 8.0 pCi/L.
Why Radon is a Serious Concern in Clay County
Clay County sits in a geological region with elevated uranium concentrations in the underlying bedrock and soil. As uranium naturally decays, it produces radium, which further decays into radon gas. This gas migrates upward through soil and enters homes through foundation cracks, gaps around pipes, and sump pits.
In Zone 1 counties like Clay, the EPA predicts indoor screening levels are commonly above 4.0 pCi/L. Individual homes can still vary dramatically - even neighboring houses can differ by a factor of 10 or more. This is why every home needs its own test, regardless of what a neighbor's reading shows.
Factors that amplify radon entry include: basement foundations (more soil contact area), granitic or shale bedrock, tight energy-efficient construction (less natural ventilation), and negative indoor air pressure from HVAC systems, exhaust fans, and dryers.
Radon & Health: What the Science Says
lung cancer
from radon
4.0 pCi/L
Radon is a Class A carcinogen - the same classification as asbestos and tobacco smoke. The National Academy of Sciences estimates that radon causes approximately 21,000 lung cancer deaths annually in the United States, making it the leading environmental cause of cancer death.
Unlike smoking, radon exposure is involuntary and often invisible. There is no safe level of radon - risk increases linearly with exposure. The good news: radon mitigation systems are highly effective, typically reducing indoor levels by 80-99% within hours of activation.
Source: US Environmental Protection Agency, "A Citizen's Guide to Radon" (EPA 402/K-12/002). National Academy of Sciences, Biological Effects of Ionizing Radiation (BEIR VI) Report, 1999.
Step 1: Test Your Home
Testing is the only way to know your home's radon level. Zone data tells you the regional risk, but your home could be significantly higher or lower than the countywide pattern. For most homeowners, the right first purchase is a low-cost short-term test kit.
A digital monitor is a better fit after your first result, for seasonal re-checks, or to keep tracking levels after mitigation.
View Airthings Corentium HomeAs an Amazon Associate, RadonVerdict earns from qualifying purchases.
Already Know Your Level?
If your test shows 4.0 pCi/L or higher, get an itemized cost estimate specific to Clay County - including regional labor rates and permit requirements.
Get Mitigation Cost Estimate ->MO Radon Regulations
Missouri requires sellers to complete a Seller's Disclosure Statement, which includes known environmental hazards.
Missouri requires radon professionals to be licensed by the Missouri Department of Health and Senior Services.
How to Test for Radon in Clay County
Buy a Test Kit
Purchase a short-term charcoal test kit online or from a local hardware store. Cost: $15-$30. Place it in the lowest livable level of your home.
Wait 2-7 Days
Keep doors and windows closed (except normal entry/exit) during the test period. Avoid running whole-house fans. Mail the kit to the lab provided.
Read Your Results
If results are below 4.0 pCi/L, re-test every 2 years or use a monitor for ongoing tracking. If above 4.0, use our cost calculator to see mitigation options.
Related Radon Resources for Clay County
Explore Other MO Counties
Official State Resource
Missouri radon program and rules
Use the state program link to verify local radon guidance, disclosure language, and contractor credential expectations before you act on an estimate.
Disclosure rule tracked
Missouri requires sellers to complete a Seller's Disclosure Statement, which includes known environmental hazards.
State licensing required
Missouri requires radon professionals to be licensed by the Missouri Department of Health and Senior Services.
Sources & Methodology
Radon zone classifications for Clay County are sourced from the EPA's Map of Radon Zones, which uses geological surveys, indoor radon measurements, and soil permeability data to assign each county a risk tier.
Disclaimer: Zone data represents county-level screening ranges and cannot predict the radon level in any specific home. Testing is the only reliable method to determine your home's radon concentration. This content is for informational purposes and is not medical advice.
Content review: Source-level retrieval dates
Editorial and Data Transparency
- Author
- RadonVerdict Data Team (Public Data and Cost Modeling)
- Content Review
- Source-level dates shown below
- Data Retrieved At
- 2026-02-24
Primary Sources
- EPA Map of Radon Zones (retrieved 2026-02-21)
- EPA A Citizen's Guide to Radon (retrieved 2026-05-05)
- CDC Environmental Public Health Tracking - Radon Testing (retrieved 2026-05-05)
- Missouri DHSS Residential Radon Testing in Missouri (retrieved 2026-05-06)
- Official MO radon program
- US Census Bureau, 2018-2022 American Community Survey 5-Year Estimates (retrieved 2026-02-24)