Radon testing, levels & next step in St. Louis County, MO
Local service search answer: If you searched for radon gas testing, mitigation services, commercial radon, or a local contractor in St. Louis County, use this page to separate the reading from the hiring decision. Confirm the result first, then open the local cost or quote path. St. Louis County searchers are often looking for testing, mitigation services, or a commercial radon answer. The useful sequence is still the same: confirm the reading, then decide cost, credit, or quote.
Do not hire from a blind reading
A service search is only useful after the number is real. Use this page to verify the result band, open the cost path, and capture the quote signal if you talk to a contractor.
Service intent detected: testing, mitigation, or commercial radon searches need a result-to-cost path.
No reading yet means test first. 4.0+ means pricing and quote comparison become relevant.
Test result -> county cost path -> quote ledger if a contractor gives a number.
Measured Radon Data
St. Louis County evidence before the next step
St. Louis County, MO has more than the EPA map: Missouri DHSS exposes 7,814 reported tests, 3.7 pCi/L county average, 28.0% of reported tests at or above 4.0 pCi/L, and 133.1 pCi/L high-end signal for 2005-2017.
Source window
2005-2017
County evidence type
Elevated measured burden
County context only; your home test controls the decision.
Primary result
3.7 pCi/L
57th percentile in-state
4.0+ signal
28.0%
56th percentile in-state
High-end signal
133.1 pCi/L
97th percentile in-state
Official evidence dossier
Source record for St. Louis County, MO
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.
Primary public source
Official county measurements
Measurement window
2005-2017
Retrieved / checked
2026-05-06
County FIPS
29189
Evidence stack
County source
Official county measurements
Primary source used for the county evidence row.
Federal map
EPA Map of Radon Zones
Zone 2 is context, not a property result.
Housing context
US Census ACS housing context
30.1% built before 1980; 444,860 housing units.
State follow-up
State radon program
Use the state source before hiring or interpreting local rules.
Primary field
3.7 pCi/L
Median field
Not available
4.0+ field
28.0%
Sample / volume
7,814 reported tests
Metric shape
This is the most useful setup: county average, 4.0+ share, high-end readings, and 7,814 reported tests/properties can be read together instead of relying on one number.
Source limitation
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.
Property-level limit
Not a property-level diagnosis. The county record explains local evidence; your home's own test result controls the next decision.
County-specific interpretation
St. Louis County is elevated enough that map-reading should turn into a home test.
St. Louis County is a priority-test case because 3.7 pCi/L average, and 28.0% of reported tests at or above 4.0. In-state rank: 57th percentile for average, and 56th percentile for 4.0+ share. The county signal is strong enough to justify testing or retesting before cost decisions.
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.
St. Louis County has enough measured elevation that buyers and owners should not stop at the county signal; confirm the home and price mitigation if the result crosses 4.0.
Use This EvidenceOfficial state path
Open the state radon program
Use state rules and local program contacts before choosing a contractor.
EPA hiring guidance
Find a qualified radon provider
EPA points homeowners to state programs and national proficiency listings.
Certified professional search
Search NRPP-certified pros
Use this after you have a valid result or need professional testing.
Elevated-intent answer
Are radon levels elevated in St. Louis County?
St. Louis County has enough measured elevation that the answer should not stop at the EPA zone. 28.0% of reported tests at or above 4.0 pCi/L, 3.7 pCi/L primary measured result, and 133.1 pCi/L high-end signal makes a first test or confirmatory retest the right next step before cost decisions.
Enter the result. Pick the deal side. Get the route.
Service intent detected: testing, mitigation, or commercial radon searches need a result-to-cost path.
County signal
3.7 pCi/L
At or above 4.0
28.0%
Decision side
Foundation clue
Choose your next step
Start from the exact job, not another generic radon article.
Pick the situation that matches St. Louis County, MO. Each route keeps the reading, deal side, or foundation clue attached so the next page answers the search instead of resetting the user.
Failed inspection
I need a repair or seller credit number
Open the local credit path with a 4.0+ buyer scenario already selected.
Open negotiation route4.0+ pCi/L
I have a high result and need the cost path
Go straight to the county estimate, quote context, and contractor checklist.
Open cost route2.0-3.9 pCi/L
I need to know if this number is bad
Use the level explanation first, then decide whether to retest, monitor, or price mitigation.
Open level routeNo test yet
I need the first valid result
Start with kit placement, timing, closed-house conditions, and result interpretation.
Open testing routePick 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.
Use this as the pre-contractor triage for St. Louis County: test first, price after 4.0+, and add any real quote to the ledger.
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 confirm the reading first. If it keeps showing up, use the county evidence and official guidance before deciding 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 need the cleanest follow-up path?
Review retesting stepsLocal service next step
Looking for a service or cost answer?
St. Louis searches need the official Missouri test signal and the local contractor path in one place, because generic state guidance is too slow for a user with a real reading. RadonVerdict is not a contractor directory. It turns the search into a decision: get or confirm the reading, open the local cost path after 4.0+, then record any real quote so the benchmark gets less generic.
Local provider search pack
Before contacting a radon provider, write down the test type, pCi/L result, lowest tested level, foundation type, deadline, and whether you need testing, mitigation, seller-credit support, or post-mitigation retest documentation.
Provider lookup
Use the official state radon program and EPA provider guidance to verify licensing or certification before treating any local service listing as enough.
Local call script
I have a radon result of ___ pCi/L in St. Louis County, MO. Before I compare quotes, can you confirm what foundation and suction method your price assumes, whether sump or crawl-space work is included, and how the post-mitigation retest will be documented?
St. Louis disambiguation
St. Louis searchers may mean County, City, or the metro. Route them before the quote.
The safest answer is not a generic St. Louis article. Choose the right geography, get a valid result, then use Missouri DHSS testing, buyer/seller, and mitigation guidance before you compare contractor bids.
St. Louis County
Use this page for county municipalities and county-level evidence context.
St. Louis City
Open the separate city page if the property is inside the independent city boundary.
Missouri testing
Start with DHSS testing instructions and test-kit resources when there is no result.
Missouri mitigation
Use DHSS contractor-selection guidance before treating a bid as fair.
County Evidence Snapshot
St. Louis County testing context
St. Louis County has enough official radon signal that the answer should not stop at the EPA map. Official county data shows 3.7 pCi/L and 28.0% at or above 4.0. Confirm the home before pricing mitigation, but do not dismiss the county as low urgency.
EPA map signal
Zone 2
County-level predicted indoor screening range, not a home-level test result.
Housing base
444,860
100th percentile among 115 MO counties with data.
Older housing share
30.1%
10th percentile in-state; older homes often need clearer test placement decisions.
Median home value
$249,400
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
3.7 pCi/L
At or above 4.0
28.0%
Maximum reported
133.1 pCi/L
Reported tests
7,814
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.
County evidence interpretation
Elevated measured burden
Primary result rank
57th percentile
3.7 pCi/L
4.0+ rank
56th percentile
28.0% at or above 4.0
High-end rank
97th percentile
133.1 pCi/L
Test volume rank
100th percentile
7,814 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 7,814 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: Johnson County (27.8%) is just lower, and Franklin County (28.2%) is just higher.
How this helps
Use this to decide whether the county signal is strong enough to justify testing or retesting before cost decisions.
What the data says
St. Louis County, MO is measurement-backed for 2005-2017. The measured average is 3.7 pCi/L, and 28.0% of reported results are at or above 4.0 pCi/L. The high-end signal reaches 133.1 pCi/L.
St. Louis County, MO sits at the 57th percentile for measured average, 56th percentile for 4.0+ share, 97th percentile for high-end readings, and 100th percentile for test volume among 115 measured counties in the state. Closest counties by county average: Randolph County (3.7 pCi/L) is just lower, and Boone County (3.8 pCi/L) is just higher.
What to do with it
St. Louis County has enough measured elevation that buyers and owners should not stop at the county signal; confirm the home and price mitigation if the result crosses 4.0.
Retest trigger: a 2.0-3.9 pCi/L home result should be confirmed here because 3.7 pCi/L average, and 28.0% of reported tests at or above 4.0 keeps the county from being a dismiss-it signal.
Source-backed context from Missouri DHSS based on about 7,814 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
Are radon levels elevated in St. Louis County?
St. Louis County has enough measured elevation that the answer should not stop at the EPA zone. 28.0% of reported tests at or above 4.0 pCi/L, 3.7 pCi/L primary measured result, and 133.1 pCi/L high-end signal makes a first test or confirmatory retest the right next step before cost decisions.
| Evidence | Value |
|---|---|
| Area | St. Louis County, MO |
| EPA Zone | Zone 2 |
| 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.
Elevated - Consider Action
Your reading is below the US EPA action level (4.0 pCi/L), but this range still warrants follow-up testing. The World Health Organization uses 2.7 pCi/L as a tighter reference point.
If this was just a one-time snapshot, confirm it with another test or with longer tracking. If this level persists, planning mitigation is reasonable, especially for homes with frequent basement use, children, or pending real-estate transactions. Scroll down to see your estimated cost.
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 Testing Priority in St. Louis County
The EPA map is only the starting layer for St. Louis County. Official county data shows 3.7 pCi/L and 28.0% at or above 4.0. That source-backed signal is why the practical move is testing or retesting before treating the county as low urgency.
This still is not a diagnosis for any single home. Foundation type, lowest-level use, pressure differences, and small geological changes can move one house above or below the county pattern.
If you do not have a reading yet, start with a short-term test in the lowest lived-in level. If the result is 2.0-3.9 pCi/L, confirm it; if it is 4.0+ pCi/L, move into mitigation pricing or seller-credit planning.
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 and county data tell you the regional signal, but your home could be significantly higher or lower than the countywide pattern. Start with a valid test setup before using any cost path.
State radon programs and EPA provider guidance are the right reference before hiring or confirming local requirements.
Open the state radon programAlready Know Your Level?
If your test shows 4.0 pCi/L or higher, get an itemized cost estimate specific to St. Louis 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 St. Louis 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 St. Louis 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 St. Louis 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)