Radon Testing in Saint Augustine, FL

Radon testing measures how much radon gas has accumulated inside your home, reported in picocuries per liter (pCi/L). What that number means for you depends on where you live, and St. Johns County is a more interesting case than most companies will admit.

Why county radon maps cannot answer this for you

Almost every county-level radon claim you will encounter traces back to one source: EPA's Map of Radon Zones, built in 1993 from geology, aerial radioactivity, soil parameters and housing stock. It sorts counties into three tiers of predicted potential. It is a planning tool for agencies deciding where to direct outreach — and EPA says so.

Three things follow from that, and they pull in different directions:

• EPA's own instruction on that map is that it "should not be used to determine if individual homes need to be tested." Anyone citing a county tier to tell you whether to test — in either direction — is using it for the one job its author says it cannot do.
• Zone assignments are modelled predictions across a whole county, not measurements of your address. EPA's standing advice is the same everywhere: no matter where you live, test.
• Florida DOH states plainly that a lack of data does not mean low radon potential. About 99% of Florida's radon measurements come from 25 counties, so for much of the state the local picture is genuinely thin — which is a reason for uncertainty, not a clean bill of health.

The practical upshot: a test is cheap and definitive, and it is the correct first purchase. A mitigation system bought without one is a guess.

How the test works

A calibrated continuous radon monitor goes on the lowest level you actually live in. It samples hourly and logs every reading, so you get a curve rather than a single averaged number — which matters, because it shows whether a high result came from a genuine soil source or from one stormy night with the house shut tight.

Placement is not casual. The monitor sits at least twenty inches off the floor, away from exterior walls, drafts, and direct sun, and it stays put. The house is kept under closed-house conditions: windows and exterior doors shut except for normal entry and exit, HVAC running as usual, no whole-house or window fans.

Short-term or long-term

Radon is not a steady number. It moves with barometric pressure, rainfall, soil moisture and how tightly the house is closed up. That variability is why two test lengths exist.

• Short-term, two to seven days: the right call when you need an answer now — a real estate contingency, or a first look at a home never tested.
• Long-term, ninety days or more: averages out weather swings and seasonal shifts, and is the better estimate of what you actually breathe year-round.

The long-term test earns its keep whenever a result lands close to the action level. A single short-term reading taken in one February week carries enough weather and closed-house noise to fall either side of 4.0, and a borderline number is exactly the situation where people get sold a system they may not need. If a short-term test comes back at 8.0 or higher, nobody waits ninety days — confirm and fix.

Reading your result

• Below 2.0 pCi/L: low, and the most likely outcome in this county. Retest every couple of years or after work that cuts into the slab.
• 2.0 to 3.9 pCi/L: the EPA suggests considering mitigation. A judgment call driven by how many hours a day you spend in the house and whether anyone in it smokes.
• 4.0 pCi/L and above: the EPA action level. Fix it.
• 8.0 pCi/L and above: fix it promptly, and confirm with a second measurement first.

The slab myth, which is the one thing the sceptics get wrong

There is a durable belief that radon is a northern basement problem and a slab-on-grade Florida house is exempt. That part is genuinely false. Radon is produced by uranium decaying in soil and rock beneath the house, and a slab is not a seal — gas enters through shrinkage cracks, expansion joints, plumbing and conduit penetrations, and the perimeter where slab meets foundation. Air conditioning slightly depressurises the house and helps draw it in.

So the slab myth cuts the other way from how it is usually deployed. Slab-on-grade construction does not protect you, which is why testing is worth an afternoon in Florida. But that is an argument for a $20 measurement, not for a $2,000 system — and the order matters. Test first. Buy a system only if the number says to.

How this works

We are not a contractor and we do not do the work. What we do is put Saint Augustine homeowners in front of local radon mitigation contractors, and give you enough of the picture beforehand that you can tell a fair quote from a padded one. Call (904) 395-5498 or send a few details and we will connect you.