Radon Levels Around Los Angeles

Radon Levels Around Los Angeles

Los Angeles County is a low-radon area: county public health estimates approximately 1% of homes test above the EPA action level of 4.0 pCi/L. That countywide figure conceals a real hot spot along the southern San Fernando Valley hillsides, and it sits next to Ventura County, which is considered a high radon area outright.

We publish this because most radon marketing in Southern California overstates the risk, and a few pages understate it in the other direction by treating the county average as the whole story. Both are wrong in ways that cost building owners money.

The Regional Picture

Area	Radon potential	Source
Los Angeles County overall	Low, roughly 1% of homes above 4.0 pCi/L	LA County Dept of Public Health
Riverside and San Bernardino counties	Comparable to Los Angeles	LA County DPH
Orange County	Thought to be lower than LA	LA County DPH
Ventura County	High; over 14% elevated in parts	LA County DPH
Southern San Fernando Valley hillside zips	Moderate to high pockets	CDPH radon database

The Studio City Data Point

The clearest local evidence comes from the state's own radon database. In the 91604 zip code, 20 of 134 tests came back elevated, and the state's annual report described readings in the area averaging about 20 pCi/L, five times California's action level and ten times the EPA's recommended consideration threshold of 2.0. A state health department spokesperson at the time described the area as moderate radon potential and explained the cause: the hills along the southern edge of the valley contain marine sedimentary rocks with above-background naturally occurring uranium, and soil at their base is eroded from that material.

The control case in the same data is what makes it useful. Valley Village, zip 91607, a couple of miles north on flat ground, recorded 95 tests with essentially nothing. Same city, same climate, same era of construction, entirely different result, because one sits at the base of the hills and one does not.

Where This Matters for Commercial Buildings

• Test in the hillside pockets regardless of whether anyone is asking. Sherman Oaks, Studio City, Encino, Tarzana, Woodland Hills, West Hills, Chatsworth, and the equivalent hillside margins in Glendale and northern Pasadena.
• Test when a lender, tenant, district, or consultant asks, anywhere in the county. That is most of our work and the results are usually clean.
• Do not mitigate without a test. Anywhere. There is no building in Los Angeles County where the base rate justifies installing a system on assumption.
• Treat Ventura County property differently. Thousand Oaks, Simi Valley, and the rest of the county sit in a genuinely elevated area, and a portfolio spanning the county line should not be scoped with a single assumption.

What the Action Level Means

4.0 pCi/L is the level at which EPA and CDPH recommend action, and it is what every lender policy and agency program uses as the compliance threshold. It is not a line between safe and unsafe. EPA states there is no known safe level of radon and suggests considering action between 2.0 and 4.0 as well, and estimates that lifetime exposure at 4.0 pCi/L would cause between 13 and 50 lung cancer deaths per 1,000 people exposed. Radon is the second leading cause of lung cancer after smoking.

For commercial decision-making, 4.0 is the operative number. For deciding whether a marginal 3.6 result is worth addressing in a building where children or residents spend all day, the risk literature is worth reading rather than the compliance table.

Primary Sources

Everything above is checkable. The Los Angeles County Department of Public Health radon page is the source for the county and regional comparisons, including the Ventura County figure. The California Indoor Radon Program at CDPH maintains the state radon database, the zip-code testing data, and the list of certified radon professionals. We would rather you check us than take our word for it.

Getting Your Own Number

The countywide statistic tells you what to expect. It does not tell you about your building, which is the only number that matters to a lender, a tenant, or an occupant. Commercial testing runs $650 to $1,800 for a small building, and in this county it usually ends the conversation.

Questions About Radon in Los Angeles

How much radon is in Los Angeles?

Not much, on average. Los Angeles County Public Health estimates roughly 1% of homes here test above the EPA action level of 4.0 pCi/L, and the county is not considered a high radon area. Riverside and San Bernardino counties are comparable, Orange County is thought to be lower, and Ventura County is considered high, with over 14% of homes elevated in parts of it.

Which parts of Los Angeles have the highest radon?

The zip codes along the southern edge of the San Fernando Valley where it meets the Santa Monica Mountains. Studio City's 91604 is the best-documented case: the state's radon database recorded 20 elevated results out of 134 tests, with area readings averaging around 20 pCi/L in the state's annual report. Elevated results have also appeared in Encino, Sherman Oaks, Tarzana, Chatsworth, West Hills and Woodland Hills. Flat valley zips a short distance north, like Valley Village 91607, tested essentially at zero.

Why is radon higher there?

Geology. The California Department of Public Health has attributed it to marine sedimentary rock in those hills containing higher than background levels of naturally occurring uranium, which decays into radon, with soil at the base of the hills derived from that eroded material. Distance from the hills is the variable that matters, which is why the pattern tracks topography rather than city boundaries.

Should a commercial building in LA be tested at all?

Yes, if something is asking for it or if the building sits in one of the elevated pockets, and probably yes anyway because the cost is small relative to what it resolves. A current clean report answers a lender, a tenant EHS review, and the next buyer's consultant for years. What we would not do is install mitigation on a Los Angeles building without a test justifying it.

Does radon vary by season here?

Less than in most of the country, because our heating season is short and buildings are not sealed tight for months. It still varies: results run higher in winter when windows stay shut, and in tightly conditioned buildings running mechanical ventilation year-round. Short-term tests in October with the windows open produce numbers that mean nothing.

Is 4.0 pCi/L a safe level?

No, it is an action level, not a safety threshold. EPA is explicit that there is no known safe level of radon and recommends considering action between 2.0 and 4.0 pCi/L as well. For a commercial building, 4.0 is the number every agency policy and lender uses, so it is the operative threshold for compliance regardless of what the risk literature says about lower exposures.