Radon is a soil gas. It moves out of the ground into the air, and where the ground is under a floor, some of it moves into the house instead. You cannot smell it or see it, which is why the only number that means anything is a measurement taken in your own house.
The EPA maps Washington County as Radon Zone 2, the middle of its three categories, defined as a predicted average indoor screening level between 2 and 4 picocuries per liter. That is a prediction built from geology, not a survey of houses. The action level is 4. A county in the middle band still contains houses well above it and houses well below it, and the map cannot tell you which one you own.
What a zone designation is, and what it is not
EPA's radon zones are predictions built from geology, not a survey of houses. A Zone 2 designation says the ground under this county is capable of producing indoor readings in the 2 to 4 range on average. It does not say your street does, and it does not say your neighbor's does.
An average would do less work than people expect, even a good one. Individual houses vary enormously with the soil under them, the foundation type, and how the house was built. A county figure is arithmetic performed on other people's houses. The only number that describes yours comes from a test in yours.
So the honest position is the boring one. County numbers do not predict your house. A test does, and a test is cheap. Utah's Division of Waste Management and Radiation Control publishes a page on testing your home, and a separate one on radon in real estate transactions, which is the moment most people first hear the word.
Why it is worth the trouble
The Utah Geological Survey calls radon Utah's leading geologic hazard in terms of fatalities. The number behind that phrase is in the Utah State Hazard Mitigation Plan: 5,630 Utah deaths attributed to radon induced lung cancer between 1973 and 2015, which is 92.7 percent of all Utah geologic hazard fatalities in that period. For comparison, the same plan counts 342 from landslides, 101 from flooding and 2 from earthquakes.
That is the entire fear, stated once. We are not going to keep repeating it. The practical point is that this is a hazard with a known mechanism, a cheap measurement and a mechanical fix, which puts it in a different category from most of what a state hazard plan covers. You cannot do anything about the Hurricane fault. You can do something about the pressure under your floor.
The survey has also published a document specific to this area, Radon Hazard Potential in the St. George Area, Washington County, Utah, issued as UGS Public Information 35. If you want the geology behind the zone designation rather than the summary, that is where it is.
The county hazard map will not help you here
Washington County publishes 24 free hazard map layers built on Utah Geological Survey Special Study 127, and they are genuinely useful: faults, surface rupture, liquefaction, rockfall, collapsible soil, expansive rock and soil, gypsiferous rock and soil, wind blown sand, piping and erosion, three separate flood layers, dam break inundation and wildfire history. Radon is not one of them.
That is a real gap and it is worth knowing before you go looking. Nine adverse construction conditions are mapped in that study and radon is not among them, so there is no parcel level layer to check. Nothing substitutes for the test kit.
Why the foundation type changes the job
Most of the national radon literature assumes a basement, because most of the country has one. Southern Utah largely does not, and the reasons are in the geology. Special Study 127 maps shallow bedrock as an adverse construction condition across the area. It maps caliche, which the survey says can be very difficult to excavate. The St. George Building Department publishes a frost line depth of 12 inches, so there is no frost driver pushing footings deep. And expansive clay and collapsible soil both make a deep excavation riskier and costlier. Everything in the ground argues for a slab.
For radon, that changes the whole picture. There is no basement to be the collection point and no crawlspace to ventilate. The floor of the living space is sitting directly on the soil the gas is in, separated by four inches of concrete and whatever sealing was done at the penetrations twenty years ago.
It also means the entry routes are the same openings a plumber already works at. The sewer stub through the slab. The water line penetration. The shrinkage crack that opened when the ground under the slab moved. Every one of those is a hole between the soil and the room.
What a mitigation system actually is
Sub-slab depressurization sounds like a technology. It is a pipe, a fan, and caulk.
The principle is pressure, not filtration. Nothing removes radon from the air in the house. Instead you make the space under the slab a lower pressure than the room above it. Once the pressure under the floor is below the pressure inside the house, soil gas takes the path of least resistance, which is now the pipe, rather than the path it was taking, which was the crack around the sewer stub. A suction point is cut through the slab, PVC pipe is run from it to a fan and out of the building, and every other opening through the slab is sealed so the fan is pulling from the soil instead of pulling conditioned air out of your living room.
That is a plumbing skill set doing plumbing work: core the slab, run rigid pipe, support it, seal the penetrations properly, and put the whole thing where it will not be in the way for twenty years. The parts of it that are not plumbing are radon measurement and system verification, which have their own protocols and their own certifications, and we will not pretend otherwise.
How you know it worked
A mitigation system is judged by a measurement after it runs, not by the installer's opinion of his own work. The reading before and the reading after are the whole argument. If the number did not move, something about the suction point, the sealing or the fan is wrong, and that is a diagnosable problem rather than a matter of taste.
Two things follow from that. First, do not accept a system without a post-installation test. Second, be suspicious of anyone who quotes mitigation before you have tested at all, because the entire point of the first number is that it decides whether you need the second.
There is also a maintenance dimension people forget. A fan is a mechanical device with a service life and it can fail quietly. A system that has been running for years without anyone looking at it is a system with an unknown status.
The slab penetrations you already have open
If you are having slab work done for another reason, that is the cheapest moment this will ever be. A slab leak repair opens the floor. A re-route opens the floor. A bathroom remodel with a moved drain opens the floor. Closing those penetrations properly instead of adequately costs almost nothing when the concrete is already broken out and a great deal when it is not.
The same sealing does more than one job. Utah State University Extension's termite guidance for southern Utah, where subterranean termites are most common in the state, lists sealing foundation cracks and repairing water leaks in the same short prevention list. Utah State University's scorpion guidance recommends sealing cracks around the foundation and at utility penetrations, and notes that bark scorpions do not burrow but shelter opportunistically, including inside houses. Radon, insects and moisture all use the same holes.
None of that is a reason to mitigate a house that has not been tested. It is a reason to do the concrete work correctly the first time, which is a defensible position regardless of what your radon number turns out to be.
What we’d do at your house
We will tell you to test before we tell you anything else, and we will point you at the state's own testing page rather than sell you a kit.
If your reading comes back low, we will say so and leave it alone.
Radon work is credentialed in Utah. A company providing radon testing or mitigation must hold a national certification through the National Radon Proficiency Program or the National Radon Safety Board, and the matching state contractor licence. Ask whoever you call to show you both, and ask us the same question. Where the work sits outside what we hold, we will say so and point you at someone who holds it.
We seal every slab penetration we open on any job, whether or not radon is the reason we are there.
We will tell you plainly which parts of a radon job are measurement and verification work rather than ours.
Whatever the system reads after it runs is the number that settles it, not our opinion of the install.
Sources
What this page says comes from the documents below, read in full. It describes them. It is not legal advice. Confirm with your association, your city, or an attorney before you act.
- EPA radon zones for Utah; Washington County as Zone 1 and the greater than 4 pCi/L definition 05-environment-water-geology-corpus.md §4.1
- Utah IBIS-PH long term county radon averages: Washington County 2.8 pCi/L, Utah 3.2 pCi/L 05-environment-water-geology-corpus.md §4.1
- Utah Geological Survey, radon as Utah's leading geologic hazard in terms of fatalities 05-environment-water-geology-corpus.md §4.1
- Utah State Hazard Mitigation Plan Chapter 8: 5,630 radon deaths 1973 to 2015, 92.7 percent of geologic hazard fatalities, versus 342 landslide, 101 flooding, 2 earthquake 05-environment-water-geology-corpus.md §4.1; playbook.txt §4
- UGS Public Information 35, Radon-hazard potential in the St. George area, Washington County, Utah 05-environment-water-geology-corpus.md §4.1
- Utah DEQ radon testing and radon in real estate transaction resources 05-environment-water-geology-corpus.md §4.1
- Utah Geological Survey Special Study 127: nine adverse construction conditions, shallow bedrock, caliche difficult to excavate 05-environment-water-geology-corpus.md §2.1, §2.3
- Washington County Hazards MapServer, 24 layers, radon not among them 05-environment-water-geology-corpus.md §2.9
- Slab-on-grade reasoning: shallow bedrock, caliche, no frost driver, expansive and collapsible soil; consequence for radon mitigation 05-environment-water-geology-corpus.md §5.1
- St. George Building Department published design criteria, 12 inch frost line depth 01-plumbing-corpus.md §1.4
- Sub-slab depressurization described as PVC pipe, a fan and slab penetration sealing 05-environment-water-geology-corpus.md §4.1; playbook.txt §4
- USU Extension subterranean termite guidance: most common in southern counties; seal foundation cracks, repair water leaks 05-environment-water-geology-corpus.md §6.3
- USU scorpion guidance: non-burrowing, opportunistic shelter indoors, seal cracks and utility penetrations 05-environment-water-geology-corpus.md §6.1