Biomonitoring: How Scientists Measure Pollution Inside Our Bodies

In 2001, the CDC published the first National Report on Human Exposure to Environmental Chemicals — a comprehensive biomonitoring survey measuring blood and urine levels of environmental chemicals in a representative sample of the US population. It tested for 27 chemicals. By 2021, the seventh edition of the report was measuring 352 chemicals and chemical metabolites.

The expansion reflects two things: the growth of analytical technology (we can now detect chemicals at parts per trillion in blood samples) and the growing recognition that the human body is a chemical archive — carrying within it a detailed record of environmental exposures accumulated over a lifetime.

Biomonitoring is the science of measuring this archive. It answers a question that environmental monitoring cannot: not what is in the air or water near you, but what has actually made it into your body. The NHANES programme — the National Health and Nutrition Examination Survey, the source of the CDC report — provides the population-level baseline against which any individual's biomonitoring results can be compared. It's the statistical foundation that transforms a blood PFAS level from a number into a meaningful piece of health information.

NHANES has been running since 1960 in various forms, but the modern biomonitoring component — measuring environmental chemical levels in blood and urine — has been continuous since 1999. Its design is deliberately representative: a complex stratified sample of the US population that allows results to be extrapolated to the national population.

What NHANES biomonitoring measures The programme measures over 350 chemicals across six broad categories: • Heavy metals (blood lead, mercury, cadmium, arsenic) • Persistent organic pollutants (PCBs, PBDEs, organochlorine pesticides, dioxins) • PFAS (now over 30 individual compounds) • Non-persistent chemicals (phthalates, parabens, phenols — measured in urine) • Organophosphate pesticide metabolites • Acrylamide, cotinine (tobacco smoke marker), volatile organic compound metabolites

Why NHANES matters for individual interpretation NHANES data provides the percentile distributions that allow an individual biomonitoring result to be contextualised. If your blood PFAS level comes back at 3.5 ng/mL, knowing whether that places you at the 50th percentile of the US population or the 95th percentile is essential for understanding its significance.

What NHANES has revealed Among the major findings that NHANES biomonitoring has produced: • Lead: blood lead levels have fallen dramatically since the 1970s following the phase-out of leaded gasoline — from a geometric mean of about 13 µg/dL in 1976 to under 1 µg/dL today — demonstrating the direct public health benefit of regulatory action • PFAS: detectable in the blood of virtually all Americans tested, including newborns, demonstrating the extent of environmental ubiquity • Phthalates: detectable in the urine of essentially all Americans, with higher levels in women (reflecting personal care product use) and children

The most important conceptual tool for interpreting biomonitoring data is the distinction between a reference range and a health threshold — two things that are frequently, and dangerously, conflated.

Reference range A reference range derived from NHANES data is a statistical description of where the general population falls. The 95th percentile for blood lead among US adults is currently around 2.0 µg/dL — meaning 95% of adults have blood lead below this level. If your blood lead is below the 95th percentile, you're "within the normal range" in the statistical sense.

Health threshold A health threshold — the level below which no adverse health effects are expected — is a fundamentally different concept. For lead, the health threshold is zero. For PFAS, the health thresholds implied by the 2024 EPA MCLs are in the parts-per-trillion range for water — levels that translate to blood PFAS concentrations well below the US population mean.

Why this distinction matters When NHANES reference ranges and health thresholds are confused, "normal" gets interpreted as "safe" — and for chemicals like lead and PFAS where the entire US population has been contaminated, the population average is not a safety benchmark. A blood PFAS level at the 50th percentile of the US population is not reassuring about health safety; it means you have about as much PFAS in your body as the average American, which is more than health researchers consider protective.

The regulatory catch-up problem For many environmental chemicals, the science of harm has outpaced regulatory action. The population is broadly exposed before health thresholds are established. In this context, comparison to the general population average is at best a measure of relative exposure, not of safety.

For most people, the question of whether to pursue personal biomonitoring testing — beyond what a physician might order for clinical reasons — doesn't have a clear yes or no answer. It depends on what you'd do with the results.

When personal biomonitoring may be worth pursuing:

Known high-exposure situations If you have a specific, documented high-exposure history — years working with benzene, living near a Superfund site with specific groundwater contamination, consuming high levels of a specific fish species — biomonitoring that tests for the relevant chemicals provides direct information about your body burden. Examples: • Blood lead and urinary lead after working in lead-exposed trades • Blood PFAS after drinking water from a contaminated system • Urinary arsenic after consuming high-arsenic well water over years

Monitoring a known exposure change If you've recently made significant exposure reduction changes — installed a PFAS-removing water filter, left a high-exposure job, moved away from a contaminated area — sequential biomonitoring can demonstrate whether body burden is declining, providing both confirmation of effective intervention and motivation to maintain the change.

When biomonitoring is less useful: For most people without specific high-exposure situations, general biomonitoring panels are likely to find the same mix of detectable PFAS, phthalate metabolites, and heavy metals that NHANES finds in almost everyone — and without specific health thresholds, the results don't yield clear action steps.

PollutionProfile's Historical Exposure Recorder helps you build the exposure history that determines whether targeted biomonitoring is indicated for your specific situation — and what to ask your doctor to test for if it is.