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Not health advice. Contact your local water utility for concerns.
Data from EPA Water Quality Reports. For official information, contact your water utility or health department.
Real EPA measurements of total trihalomethanes (tthm) for California water systems — Six-Year Review compliance samples 2012–2019 (historical). Methodology
These cities have total trihalomethanes (tthm) levels above 50% of the EPA limit but within compliance
| City | Level | % of Limit | Population |
|---|---|---|---|
| Hawthorne | 58.12 ug/L | 72.6% | 44,718 |
| El Centro | 56.02 ug/L | 70.0% | 43,772 |
| San Luis Obispo | 52.64 ug/L | 65.8% | 47,788 |
| Napa | 52.45 ug/L | 65.6% | 84,804 |
| Inglewood | 48.22 ug/L | 60.3% | 80,824 |
| Santa Cruz | 48.07 ug/L | 60.1% | 94,626 |
| West Sacramento | 47.97 ug/L | 60.0% | 53,355 |
| Mountain View | 47.73 ug/L | 59.7% | 81,501 |
| La Mirada | 47.54 ug/L | 59.4% | 56,793 |
| Yuba City | 47.26 ug/L | 59.1% | 70,256 |
Showing 10 of 26 cities with elevated levels
Every California system with a total trihalomethanes (tthm) result in EPA's data, sorted by level. Averages of 2012–2019 finished-water samples, non-detects counted as zero — historical, not today's tap.
| City | Level | Period | EPA Limit | % of Limit | Status |
|---|---|---|---|---|---|
| HawthorneGrade A | 58.12 ug/LAverage of 371 samples | 2012–2019 | 80.00 ug/L | 72.6% | Warning |
| El CentroGrade A | 56.02 ug/LAverage of 92 samples | 2012–2019 | 80.00 ug/L | 70.0% | Warning |
| San Luis ObispoGrade A | 52.64 ug/LAverage of 117 samples | 2012–2019 | 80.00 ug/L | 65.8% | Warning |
| NapaGrade A | 52.45 ug/LAverage of 378 samples | 2012–2019 | 80.00 ug/L | 65.6% | Warning |
| InglewoodGrade B | 48.22 ug/LAverage of 227 samples | 2012–2019 | 80.00 ug/L | 60.3% | Warning |
| Santa CruzGrade A | 48.07 ug/LAverage of 472 samples | 2012–2019 | 80.00 ug/L | 60.1% | Warning |
| West SacramentoGrade D | 47.97 ug/LAverage of 29 samples | 2012–2019 | 80.00 ug/L | 60.0% | Warning |
| Mountain ViewGrade A | 47.73 ug/LAverage of 252 samples | 2012–2019 | 80.00 ug/L | 59.7% | Warning |
| La MiradaGrade C | 47.54 ug/LAverage of 124 samples | 2012–2019 | 80.00 ug/L | 59.4% | Warning |
| Yuba CityGrade A | 47.26 ug/LAverage of 99 samples | 2012–2019 | 80.00 ug/L | 59.1% | Warning |
| HanfordGrade A | 47.01 ug/LAverage of 64 samples | 2012–2019 | 80.00 ug/L | 58.8% | Warning |
| PowayGrade A | 46.42 ug/LAverage of 216 samples | 2012–2019 | 80.00 ug/L | 58.0% | Warning |
| VallejoGrade A | 46.10 ug/LAverage of 32 samples | 2012–2019 | 80.00 ug/L | 57.6% | Warning |
| CovinaGrade A | 44.60 ug/LAverage of 140 samples | 2012–2019 | 80.00 ug/L | 55.7% | Warning |
| YucaipaGrade F | 44.42 ug/LAverage of 105 samples | 2012–2019 | 80.00 ug/L | 55.5% | Warning |
| EscondidoGrade A | 44.17 ug/LAverage of 276 samples | 2012–2019 | 80.00 ug/L | 55.2% | Warning |
| SunnyvaleGrade A | 43.85 ug/LAverage of 248 samples | 2012–2019 | 80.00 ug/L | 54.8% | Warning |
| Redwood CityGrade A | 43.02 ug/LAverage of 249 samples | 2012–2019 | 80.00 ug/L | 53.8% | Warning |
| Santa BarbaraGrade A | 42.99 ug/LAverage of 278 samples | 2012–2019 | 80.00 ug/L | 53.7% | Warning |
| AnaheimGrade C | 42.88 ug/LAverage of 398 samples | 2012–2019 | 80.00 ug/L | 53.6% | Warning |
| ChinoGrade A | 42.68 ug/LAverage of 232 samples | 2012–2019 | 80.00 ug/L | 53.3% | Warning |
| EncinitasGrade A | 42.45 ug/LAverage of 330 samples | 2012–2019 | 80.00 ug/L | 53.1% | Warning |
| LancasterGrade A | 41.55 ug/LAverage of 175 samples | 2012–2019 | 80.00 ug/L | 51.9% | Warning |
| StocktonGrade F | 41.13 ug/LAverage of 614 samples | 2012–2019 | 80.00 ug/L | 51.4% | Warning |
| TracyGrade A | 40.87 ug/LAverage of 259 samples | 2012–2019 | 80.00 ug/L | 51.1% | Warning |
Showing 25 of 153 cities. View individual city pages for detailed reports.
Trihalomethanes are disinfection byproducts formed when chlorine reacts with organic matter in water. Long-term exposure to high levels of TTHMs has been associated with increased cancer risk, particularly bladder cancer. Studies have also linked TTHMs to reproductive problems including miscarriage and birth defects. Chloroform, the most common THM, can cause liver and kidney damage.
Formed when chlorine disinfectants react with natural organic matter in water; higher in surface water systems with more organic material
Based on our analysis of EPA Safe Drinking Water Information System (SDWIS) data and local water utility reports,total trihalomethanes (tthm) has been detected in 153 California water systems. The average level across all tested cities is 29% of the EPA Maximum Contaminant Level (MCL), with the highest detection at 72.6% of the limit.
The EPA MCL for total trihalomethanes is 80 ug/L (80 ppb). The EPA MCL for total trihalomethanes (tthm) is 80 ug/L. This limit is set to protect public health based on available scientific evidence about the contaminant's health effects.
153 California cities have total trihalomethanes (tthm) detected in their water supply. All tested cities are within EPA safety limits.
Increased cancer risk (bladder), liver and kidney damage, potential reproductive effects. The most vulnerable groups include pregnant women and people with liver disease.
The most effective methods for removing total trihalomethanes (tthm) include Activated Carbon, Reverse Osmosis. Look for filters certified by NSF/ANSI for total trihalomethanes (tthm) reduction.
The EPA Maximum Contaminant Level (MCL) for total trihalomethanes (tthm) is 80 ug/L.The EPA MCL for total trihalomethanes is 80 ug/L (80 ppb).
This page shows what California utilities reported for total trihalomethanes (tthm). It cannot tell you what is coming out of your own tap: utility samples are taken across a whole system and averaged, levels vary by address and season, and anything picked up after the water leaves the main never appears in their report.
Home water test kitscheck your own tap before buying any filterRecommended filtration methods for total trihalomethanes (tthm) removal:
Very effective for TTHMs
Removes 90%+ of TTHMs
THMs are volatile and can be removed by aeration
Paid links. We earn a commission if you buy, at no extra cost to you. Filters are matched to the removal methods listed above for total trihalomethanes (tthm).