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Los Angeles, CA vs San Diego, CA

Water, air, radon and what is registered nearby, from the public record for each county.

San Diego, CA is lower on two of the four

In the typical US county, 48% of what a utility tests for comes back above a health guideline. Los Angeles, CA is at 36%, San Diego, CA at 49%.

Lower is not the same as low, and which of the four matters most depends on your house.

  • Drinking waterLos Angeles, CA
  • Traffic emissionsSan Diego, CA
  • RadonSan Diego, CA
  • Registered sitesNo comparison

What the utilities serving each area reported, compared against health guidelines rather than legal limits alone. A share rather than a count: a system that measures more analytes will always flag more of them.

Above a health guideline
Los Angeles, CA37 of 103 · 36%
San Diego, CA30 of 61 · 49%the higher of the two
Over a legal limit
Los Angeles, CA2the higher of the two
San Diego, CA1
Furthest over a guideline
Los Angeles, CA3,488×
San Diego, CA18,454×the higher of the two
Against every US county
Los Angeles, CAHigher than 9%
San Diego, CAHigher than 54%the higher of the two
What that is
Los Angeles, CAPerfluorohexane sulfonate (PFHXS)
San Diego, CAPerfluorohexane sulfonate (PFHXS)

San Diego, CA carries the further exceedance — about 5× the other's worst reading.

This is the water a utility delivers. A house on a private well is in none of it: a well files no report, so a test is the only way to know what is in one.

Utility reports as pulled on 7 July 2026.

What road traffic releases across each county every year, from EPA’s emissions inventory, divided by land area. These are county-wide totals of what goes into the air — not a reading taken at an address.

Worst pollutant
Los Angeles, CAFine particles (PM2.5)
San Diego, CAFine particles (PM2.5)
All eight, per square mile
Los Angeles, CA0.81 tthe higher of the two
San Diego, CA0.31 t
Against every US county
Los Angeles, CAHigher than 93%the higher of the two
San Diego, CAHigher than 85%

Los Angeles, CA releases about 2.6× as much per square mile.

EPA National Emissions Inventory, 2020.

EPA’s predicted zone for each county, from geology and the indoor measurements that already exist. Zone 1 is the highest. Radon comes in through the foundation, so two houses on one street can differ by more than these zones differ from each other.

EPA radon zone
Los Angeles, CAZone 2 — predicted 2 to 4 pCi/Lthe higher of the two
San Diego, CAZone 3 — predicted below 2 pCi/L

Los Angeles, CA sits in the higher-risk zone, but the building decides the answer in either.

Superfund sites and Toxics Release Inventory facilities on the federal registers in each county. Being on a register means a site is recorded, not that anything from it has reached a home.

Sites on file
Los Angeles, CA30+
San Diego, CA30+

At least one of these counties is at the limit of what we store, so the true counts may be further apart than they look.

If you test one thing

Taken from the record above, in that order: anything above a legal limit first, then a county EPA puts in radon zone 1, then the furthest reading over a health guideline. A starting point drawn from the county — not a finding about a house.

  • Los Angeles, CAYour tap water2 contaminants came back above a legal limit in this county's record, Perfluorooctanoic acid (PFOA) among them. A legal limit is the clearest line on this page: above it, a utility is out of compliance.
  • San Diego, CAYour tap waterOne contaminant came back above a legal limit in this county's record, Perfluorohexane sulfonate (PFHXS) among them. A legal limit is the clearest line on this page: above it, a utility is out of compliance.

None of this was measured in a house

Everything above is the public record for a county — what the utilities reported, what traffic releases, what EPA predicts from the geology. The pipes between the plant and one kitchen are not in any of it.

Get the full report for an addressSee the water test kits

The report is free and takes an address. Results from a kit arrive in about 10 days.

Other comparisons

Nestwell screens your home environment. Results describe your water, air, or dust and are not a medical diagnosis. Everything above is a public record for a county — none of it was measured inside a home.

About this comparison

Los Angeles, CA is read from the public record for Los Angeles County, and San Diego, CA from San Diego County. Each covers the whole county, so it describes the area around a home rather than any one street or house.

Drinking water is compared as the share of what each area’s utilities measured that sits above a health guideline, not as a count. A utility that tests for more will always flag more, so a raw count tends to make the less-tested place look better.

Registered sites are counted up to 30 per county. Most large metros reach that, and when both places do, the page says the two cannot be told apart rather than calling it a tie.

There is no overall score: in a city it mostly measures how dense the place is. Each category is shown on its own so you can weigh the ones that matter to you.

What to do with what you found

Tap water running into a lab sample bottle held in one hand
WaterIs my Brita doing anything?
An illustration contrasting a repeating PFAS molecular chain on the left with scattered, irregular microplastic fragments on the right.
WaterPFAS vs. microplastics: what's the difference?
A hand holding an air sampling tube against a warm backdrop
AirIs my air filter doing anything?
Two hands holding a jar of soil over a weathered wooden table
Radon and nearby sitesWhat to look for in your home

Questions about this comparison

Where does this comparison come from?
The public record for each county: what the local water utilities reported, traffic emissions from the EPA National Emissions Inventory, the EPA’s radon prediction from the area’s geology, and the sites registered nearby.
Why is there no overall score for each place?
Because in a city it mostly measures how dense the place is. Traffic and registered sites run high in almost every metro, so a single number would rank nearly every city you would want to compare at the bottom. Each category is shown on its own instead.
Does this tell me what is in my home’s water or air?
No. It describes a county, not a house. The pipes between the treatment plant and one kitchen are not in any of it, and neither is the air inside a particular home. A home test measures what is actually there.
Why do two cities show the same results?
The record is kept by county, so two cities in the same county share it. The page says so when that happens.