Why radon testing is essential in Plymouth
Radon is an invisible, odorless, naturally-occurring radioactive gas that seeps up from soil into homes through micro-cracks in the foundation, crawl-space floors, and slab penetrations. It's the second-leading cause of lung cancer in the United States after smoking, according to the EPA.
The Minnesota Department of Health estimates that roughly 2 in 5 Minnesota homes test above the EPA action level of 4.0 picocuries per liter (pCi/L). Hennepin County, which contains Plymouth, sits in EPA Zone 1 — the highest-risk category. Geological surveys put Plymouth squarely in a high-uranium-soil corridor that drives elevated indoor radon.
The only way to know your home's radon level is to test. There is no "look" or "smell." There is only the measurement.
How our radon test works
We use a continuous radon monitor — a calibrated electronic device that takes hourly readings over a minimum 48-hour test period. Continuous monitors are the gold standard because they capture variation across the test period (which charcoal canister tests cannot) and produce tamper-evident data.
The test follows EPA protocol:
- Closed-house conditions for 12 hours before and during the test
- Device placed in the lowest livable level of the home
- Minimum 48-hour test period
- Hourly readings logged and averaged
- Results delivered in your digital report
Understanding your radon result
EPA action level: 4.0 pCi/L. Any reading at or above this level warrants radon mitigation.
- Below 2.0 pCi/L — Low. Re-test every 2–5 years.
- 2.0 to 3.9 pCi/L — Elevated. Consider mitigation; re-test annually.
- 4.0 pCi/L and above — High. Mitigation recommended. Mitigation systems typically run $1,200–$2,500 in Plymouth and reduce radon by 80–95%.
Radon testing during a home purchase
If you're buying a Plymouth home, get radon tested during your inspection contingency period. If the test comes back elevated, you have multiple options:
- Request the seller install a mitigation system before closing
- Negotiate a credit at closing to install one yourself
- Walk away (rare, but possible under your contingency)
Most Plymouth-area sellers are familiar with this conversation and resolve elevated readings as a standard part of closing.
Why Plymouth specifically has elevated radon risk
Three converging factors put Plymouth and the surrounding West Metro in EPA Zone 1 — the highest radon-risk classification:
Underlying geology
Minnesota's glacial till and bedrock contain elevated uranium concentrations. As that uranium decays over geologic time, it produces radium, which decays into radon gas. Radon then migrates upward through soil pore space toward the surface.
Foundation depth and construction
Twin Cities homes are built with full basements far more often than homes in warmer regions. A full basement means more foundation surface area in contact with soil — more pathways for radon to enter. Sump pits, slab cracks, block-foundation hollow cores, and slab-foundation expansion joints all serve as entry routes.
Heating-driven stack effect
Minnesota winters mean six months of substantial indoor-outdoor temperature differential. Warm indoor air rises and exits through upper-floor leaks, creating negative pressure in the basement that actively pulls soil gas — including radon — into the home. Stack effect concentrates radon levels in winter.
The combination is why Hennepin County data shows roughly 40% of homes testing above the EPA action level of 4.0 pCi/L, and 5–10% testing above 10 pCi/L — levels associated with substantial long-term lung cancer risk.
Test methods: which is right for which situation
Three test types are commonly used in residential settings. Each has appropriate use cases:
Short-term charcoal (2–7 days)
Inexpensive, simple, single-result. Good for: initial screening on long-tenure properties. Limitations: subject to short-term weather variability and tampering.
Continuous monitor (48+ hours, what we deploy)
Electronic device records hourly readings over the test period. Output: full curve showing how levels varied during the test, plus average. Good for: real estate transactions, where tampering resistance and granular data matter. Limitations: more expensive than charcoal.
Long-term alpha track (90+ days)
Passive device deployed for 3+ months. Output: time-averaged result smoothing out seasonal variation. Good for: post-mitigation verification and long-term occupational concern. Limitations: too slow for real estate use.
For Plymouth-area real estate transactions, the continuous monitor is the appropriate tool. The 48-hour result with full data curve is defensible, tamper-resistant, and arrives in time for inspection contingency negotiation.
Reading a radon test result: what the numbers actually mean
Test results come back in picocuries per liter (pCi/L). The thresholds that matter:
- Below 2.0 pCi/L — low. No action typically recommended.
- 2.0–3.9 pCi/L — moderate. EPA recommends consideration of mitigation. Below the action level but not negligible.
- 4.0 pCi/L or higher — EPA action level. Mitigation strongly recommended.
- 10+ pCi/L — high. Mitigation should be a closing-condition negotiation item.
- 20+ pCi/L — very high. Some carriers and lenders flag at this level.
Plymouth-area test results vary substantially: many homes in the 2–6 pCi/L range, some in the 8–15 range, occasional results above 20. Recent radon mitigation history in a home reduces but doesn't eliminate the need to retest — system condition matters.
Mitigation cost ranges in the Twin Cities
Active soil depressurization is the standard mitigation method. A PVC pipe is run from beneath the slab up through the home (often through a closet or garage) to a fan that exhausts above the roofline. Cost ranges:
- Standard active soil depressurization (basement, accessible installation): $1,200–$2,500
- Slab-on-grade or walkout-basement homes (more complex routing): $1,800–$3,500
- Crawl space encapsulation with depressurization: $3,500–$8,000
- Sump pit cover + sub-membrane depressurization: add $400–$800 to standard system
- Post-mitigation verification testing: $150–$300
For real estate transactions, sellers typically credit the cost of mitigation at closing rather than completing it before — which lets the buyer choose their preferred contractor and verify the result.