Radon mitigation and testing for San Diego homes

Radon is produced by the radioactive decay of uranium, an element present in trace quantities in nearly all soil and rock. Uranium decays into radium, and radium decays into radon. Radon is the only step in that chain that is a gas, and that single property is what turns a geological curiosity into an indoor air quality problem. Gas migrates. It moves through pore spaces between soil particles, through fractures in bedrock, and through any opening where the ground meets a building.

A house is not a sealed box. Warm indoor air rises and escapes through the upper envelope, exhaust fans push air outward, and clothes dryers and combustion appliances draw air from inside. The result is a slight negative pressure at the lowest level of the building relative to the soil beneath it. That pressure difference is small, but it is persistent, and it is enough to draw soil gas through slab cracks, plumbing penetrations, control joints, sump openings, and the perimeter gap where the slab meets the foundation wall.

San Diego County complicates the intuition most people carry about radon. The region's reputation for low averages is built on aggregated data across very large geographic units, and averages describe regions rather than individual buildings. Elevated results have been documented in homes across the county, including in neighborhoods where the surrounding average is unremarkable. Two houses on the same street, built in the same decade, can measure very differently because of a fracture pattern in the underlying rock, a difference in slab detailing, or a change in how the occupants ventilate.

The climate itself introduces a specific local wrinkle. Mild temperatures year round mean windows in San Diego homes stay open far more of the year than in most of the country. Open windows dilute indoor radon, sometimes dramatically. A casual test taken during an open-window week can read low while the same home reads considerably higher during a stretch of closed-house conditions in winter or during a heat wave when air conditioning runs continuously. This is not a flaw in the testing device. It is a reflection of the fact that radon concentration is a function of both entry rate and ventilation rate, and ventilation here swings widely across the year.

Health guidance from the U.S. Environmental Protection Agency identifies radon as the second leading cause of lung cancer in the United States after smoking, and the leading cause among people who have never smoked. The mechanism is not the radon gas itself so much as its short-lived decay products, which are electrically charged solids that attach to dust and airborne particles. When those particles are inhaled, the decay products lodge in lung tissue and emit alpha radiation at close range over the following minutes and hours. The dose is small on any given day. The concern is cumulative exposure across years of living in the same building.

Active soil depressurization works by making the space under the foundation slightly more negative in pressure than the space above it. When that pressure field extends across the whole footprint of the building, soil gas travels to the suction point and up the vent pipe instead of finding its way through slab openings. Achieving that field is straightforward in a house built over clean gravel. It is considerably harder over the compacted native soils and decomposed granite found across much of inland San Diego County.

Sub-slab communication testing is what separates a designed system from a guess. A small test hole is drilled, a vacuum is applied, and pressure is measured at points across the slab. The results show how far the pressure field reaches and whether one suction point will cover the building or whether three are required. Skipping this step is the single most common reason a system fails to bring levels down: the fan runs, the pipe is quiet, the homeowner assumes the problem is solved, and the post-mitigation test comes back barely changed.

Fan selection follows directly from the communication results. Tight soils need a high-suction, lower-flow fan. Loose or gravel-bedded soils need higher flow at lower static pressure. Choosing the wrong curve wastes electricity, produces unnecessary noise, and in some cases depressurizes the building enough to affect combustion appliance draft. Coastal installations add a corrosion dimension, since salt air is unkind to exterior fan housings, clamps, and fasteners that were selected for inland conditions.

Discharge placement is governed by the same reasoning. The vent terminates above the roofline and away from windows, doors, and mechanical air intakes so that exhausted gas disperses rather than re-entering. On flat-roofed modern homes and on multi-unit buildings with rooftop equipment, that clearance requires deliberate routing rather than the shortest path up the wall.