Even low PFAS in drinking water raise blood levels, California study shows

By Priyanjana Pramanik, MSc.Reviewed by Susha Cheriyedath, M.Sc.Nov 2 2025

A new California study reveals that “safe” levels of PFAS in tap water may still lead to elevated toxic chemicals in the bloodstream, raising new concerns about everyday exposure and regulatory limits.

Study: Associations between PFAS in public water system drinking water and serum among Southern California adults. Image Credit: Megane Ad / Shutterstock

In a recent article in the Journal of Exposure Science & Environmental Epidemiology, researchers examined blood chemical levels in adults exposed to per- and polyfluoroalkyl substances (PFAS) through public drinking water systems.

Their findings suggest that even in areas without industrial PFAS manufacturing, people can be significantly exposed to these “forever chemicals” through contaminated drinking water, requiring ongoing monitoring.

PFAS Persistence and Environmental Impact

PFAS are man-made chemicals used for resistance to heat, stains, and water. They persist in the environment and human body, accumulating in water and living organisms due to their long half-lives.

Major Human Exposure Pathways

Humans are mainly exposed through contaminated water and food. There is also exposure via skin contact, dust, or consumer products. Certain PFAS have been linked to health problems such as cancer, hormone disruption, reduced birth weight, and immune system effects.

Nationwide Contamination and EPA Findings

High PFAS contamination is often found near chemical manufacturing plants or sites using firefighting foams, but low-level contamination is now widespread. The U.S. Environmental Protection Agency (EPA) estimates that thousands of water systems serving millions of Americans exceed newly established limits for at least one PFAS.

California’s PFAS Monitoring Efforts

Although California lacks PFAS manufacturing facilities, it has contamination sources such as airports, military bases, and landfills. To address this, California’s State Water Resources Control Board began monitoring PFAS in public water systems in 2019.

Gaps in Research on General Populations

Most previous studies focused on areas with severe contamination, leaving uncertainty about PFAS exposure in general populations.

Study Linking Water Quality and Blood PFAS

In this study, researchers linked biomonitoring data from Southern California adults to public water system data to assess whether PFAS in drinking water corresponded with higher PFAS levels in blood. They used data from the California Regional Exposure (CARE) study, a biomonitoring program assessing environmental chemical exposure across Eastern and Southern California.

Participant Selection and Laboratory Analysis

Adults aged 18 and older who had lived in their region for at least a year provided blood and urine samples and completed questionnaires during 2018–2020. Blood samples were analyzed for 12 PFAS using advanced chromatography and mass-spectrometry methods with rigorous quality controls.

Drinking Water Data and Geographic Mapping

Drinking-water data were obtained from the California State Water Resources Control Board’s monitoring program (2019–2022), which tested public water systems, primarily untreated source wells with some post-treatment samples, for 18 PFAS compounds. Participant addresses were geocoded and matched to specific public water system boundaries to determine whether their water supply had detectable PFAS levels at State Consumer Confidence Report Detection Limits of 2–4 ng/L (for the modeled analytes, 4 ng/L).

Sample Size and Statistical Adjustments

After excluding participants with private wells or missing data, 563 individuals were included in the final sample. Serum PFAS concentrations were compared across participants with and without PFAS detections in their local water systems, adjusting for race/ethnicity, sex, education, age, income, parity, and nativity. Additional sensitivity analyses considered the type of drinking water and water purchasing practices between systems.

Demographic Profile of Participants

Among 563 adults from Southern California, the majority of participants were middle-aged women, with nearly 40% identifying as Hispanic and 54% primarily drinking tap water.

Prevalence of PFAS in Blood Samples

PFAS were detected in almost all blood samples, with PFOS, PFOA, and PFHxS being the most prevalent. Serum PFAS levels were generally lower than national averages but still indicated a potential for health effects in 86% of participants, according to National Academies of Sciences, Engineering, and Medicine (NASEM) guidance.

PFAS Detections Across Public Water Systems

Seventy public water systems serving the study area had PFAS testing data, and 56% of participants were connected to systems in which at least one PFAS was detected. PFBS, PFHxS, PFOA, PFOS, and PFHxA were the most frequently detected chemicals, particularly in urban Southern California.

Blood PFHxS Levels and Water Exposure Correlation

Participants whose water systems had PFHxS detections exhibited 31.9% higher serum PFHxS concentrations, with even stronger associations when detections were frequent (≥50% of sampling locations, 64% higher).

Post-Treatment Water Findings and Drinking Habits

When analysis was limited to post-treatment water samples, participants exposed to detected PFAS had significantly higher serum levels for PFHxS (~80% higher), PFOA (~30% higher), PFOS (~31% higher), and total PFAS (~42% higher). The association between drinking water and blood PFHxS was strongest among tap-water drinkers, while overall serum PFAS were lower among bottled-water drinkers, although some effect estimates for PFOS and ∑5 PFAS were approximately 20% higher but not statistically significant.

Low-Level PFAS Still Influence Blood Levels

These results indicate that even relatively low PFAS levels in drinking water contribute to measurable increases in blood PFAS concentrations.

Health Implications of PFAS Exposure

This study demonstrated that even at low environmental levels, PFAS in drinking water contribute to elevated blood concentrations, particularly for PFHxS, a compound with a long biological half-life.

Regional Context and Firefighting Foam Legacy

Compared with national and highly contaminated regions, PFAS levels in Southern California were lower but still of health concern. The strong link between PFHxS in drinking water and serum supports previous findings and highlights the legacy of aqueous film-forming foam (AFFF) contamination.

Study Strengths and Key Limitations

Strengths include robust linkage of biomonitoring data with statewide water-quality records and adjustment for key demographic variables. However, notable limitations included potential exposure misclassification from raw (pre-treatment) water data, the selection of larger or higher-risk systems that may bias associations upward, a temporal mismatch between some water sampling and serum collection, and a lack of information on other exposure routes (such as dust or diet).

Policy Implications and Need for Regulation

Despite these limitations, the findings emphasize that PFAS-contaminated drinking water remains a significant exposure pathway, even in non-industrial areas, reinforcing the need for stricter PFAS regulations, expanded monitoring, and improved water treatment across California.