# Environmental Toxicology and Human Health

## Overview

Environmental toxicology studies the adverse health effects of environmental contaminants on human populations. Exposures occur through air, water, soil, food, and consumer products. The field is built on key principles including dose-response relationships, exposure assessment, biomonitoring, and the identification of vulnerable populations. Major contaminants of public health concern include lead, mercury, arsenic, PFAS, pesticides, and air pollutants. Environmental health disparities disproportionately affect low-income and minority communities, making environmental justice a central concern.

## Fundamental Principles

### Dose-Response Relationships

Two primary dose-response models guide toxicological assessment. The threshold model assumes a dose below which no adverse effect occurs, applicable to most non-cancer endpoints. Key reference points include the No Observed Adverse Effect Level (NOAEL), the highest tested dose showing no adverse effect, and the Lowest Observed Adverse Effect Level (LOAEL), the lowest dose at which adverse effects are observed. The Reference Dose (RfD) is calculated by dividing the NOAEL by uncertainty factors, representing the daily dose unlikely to cause harm over a lifetime. The non-threshold (linear) model assumes any dose carries some risk and is used for genotoxic carcinogens, with risk extrapolated linearly from high-dose animal data to low-dose human exposure and expressed as excess lifetime cancer risk. Hormesis — a U-shaped or J-shaped dose-response showing low-dose stimulation and high-dose inhibition — remains controversial.

### Routes of Exposure

Inhalation exposes humans to particulate matter, volatile organic compounds, and gases. Ingestion routes include contaminated water, food, and soil, with children particularly vulnerable through hand-to-mouth behavior. Dermal absorption is relevant for solvents, pesticides, and some metals. Transplacental transfer exposes the developing fetus during pregnancy.

### Toxicokinetics (ADME)

Absorption depends on the route of exposure, chemical properties, and physiologic factors. Distribution occurs through blood to tissues and fat stores; lipophilic chemicals tend to bioaccumulate. Metabolism is primarily hepatic, involving Phase I reactions (CYP450 system) and Phase II conjugation reactions. Excretion occurs through renal, biliary, respiratory, and breast milk pathways. The half-life of a chemical determines both the duration of its effects and the window for biomonitoring detection.

### Vulnerable Populations

Children face higher exposure per unit body weight, have immature metabolic pathways, possess developing organ systems, and engage in hand-to-mouth behavior that increases ingestion exposure. Pregnant women transmit contaminants transplacentally, affecting fetal development during critical windows. The elderly have reduced detoxification capacity and cumulative lifetime exposures. Workers in certain industries face occupational exposures that exceed general population levels. Low-income communities experience greater proximity to pollution sources, older housing stock, and limited access to clean water.

## Major Environmental Contaminants

### Lead

There is no safe blood lead level for children. The CDC reference value is 3.5 mcg/dL. Sources include deteriorating lead paint in pre-1978 housing, contaminated water from lead service lines, contaminated soil near highways and industrial sites, and imported products. In children, lead causes neurodevelopmental impairment including decreased IQ, learning disabilities, and behavioral problems, along with anemia. In adults, it causes hypertension, renal disease, reproductive toxicity, and peripheral neuropathy. Blood lead level is the standard biomonitoring measure. Primary prevention through lead paint abatement, lead service line replacement, and soil remediation is the only effective strategy, as chelation therapy for severe poisoning (blood lead above 45 mcg/dL in children) does not reverse neurodevelopmental damage.

### Mercury

Mercury exists in elemental (metallic), inorganic (salts), and organic (methylmercury) forms, with methylmercury being the most toxic. The primary exposure pathway is methylmercury through fish consumption, as it bioaccumulates up the aquatic food chain. Health effects include neurotoxicity (especially to the fetus, as demonstrated by Minamata disease), renal toxicity, tremor, and cognitive impairment. EPA and FDA issue fish consumption advisories for pregnant women and children. Biomonitoring uses blood and hair mercury levels.

### Arsenic

Inorganic arsenic is found in groundwater, rice, pressure-treated wood, and some pesticides. Chronic exposure causes skin lesions (keratoses, hyperpigmentation), peripheral neuropathy, cardiovascular disease, diabetes, and cancers of the bladder, lung, and skin. The EPA drinking water standard is 10 parts per billion, though some advocate for a lower limit. Millions of people worldwide are exposed through contaminated groundwater, particularly in Bangladesh, West Bengal, and parts of the western United States.

### PFAS (Per- and Polyfluoroalkyl Substances)

PFAS are nicknamed "forever chemicals" because they are extremely persistent in both the environment and the human body. Sources include nonstick cookware, food packaging, firefighting foam (AFFF), and waterproof textiles. Health effects include immunotoxicity with reduced vaccine responses, thyroid disease, elevated cholesterol, kidney and testicular cancer, and reproductive effects. Long-chain PFAS such as PFOA and PFOS have half-lives of 2-8 years in humans. The EPA set drinking water standards at 4 parts per trillion for PFOA and PFOS in its 2024 final rule. NHANES data shows detectable PFAS in more than 95% of the U.S. population.

### Pesticides

Organophosphates and carbamates are cholinesterase inhibitors that cause acute toxicity (the SLUDGE syndrome) and chronic neurobehavioral effects. Organochlorines such as DDT and dieldrin are banned in many countries but persist in the environment and cause endocrine disruption. Glyphosate, the most widely used herbicide globally, was classified by IARC as "probably carcinogenic" (Group 2A), though this classification is contested by the EPA. Neonicotinoids are systemic insecticides linked to pollinator decline with emerging human health concerns. Farmworkers face particular exposure through pesticide drift, inadequate PPE, and heat stress that limits PPE use. Biomonitoring uses cholinesterase levels for organophosphate exposure and urinary metabolites for other classes.

| Contaminant | Primary Sources | Key Health Effects | Biomonitoring Measure | Regulatory Standard |
|---|---|---|---|---|
| Lead | Paint (pre-1978), water pipes, soil | Neurodevelopmental impairment (children), hypertension, renal disease | Blood lead level | CDC reference: 3.5 mcg/dL (children) |
| Mercury (methylmercury) | Fish consumption (bioaccumulation) | Neurotoxicity, fetal brain damage | Blood/hair mercury | FDA/EPA fish advisories |
| Arsenic (inorganic) | Groundwater, rice, pressure-treated wood | Skin lesions, cancers (bladder, lung, skin), CVD | Urine arsenic | EPA drinking water: 10 ppb |
| PFAS (PFOA/PFOS) | Nonstick cookware, food packaging, AFFF | Immunotoxicity, thyroid disease, cancer, elevated cholesterol | Serum PFAS levels | EPA drinking water: 4 ppt (2024) |
| Organophosphate pesticides | Agriculture, pest control | Cholinergic crisis (SLUDGE), neurobehavioral effects | Cholinesterase levels, urinary metabolites | EPA tolerance levels |

## Exposure Assessment Methods

Environmental monitoring measures contaminant levels in air, water, and soil at the point of exposure. Personal monitoring provides individual-level measurement using dosimeters and personal air samplers. Biomonitoring measures chemicals or their metabolites in biological specimens including blood, urine, hair, and breast milk; the CDC/ATSDR National Report on Human Exposure to Environmental Chemicals uses NHANES biomonitoring data and reflects actual internal dose integrating all routes and sources. Exposure modeling uses mathematical approaches including physiologically-based pharmacokinetic (PBPK) models to estimate dose from environmental concentrations. Questionnaires capture residential history, occupational history, and dietary patterns.

## Environmental Health Regulatory Framework

The EPA sets and enforces environmental standards including NAAQS, drinking water standards, and Superfund site cleanup. ATSDR (the Agency for Toxic Substances and Disease Registry) conducts health assessments at hazardous waste sites and publishes toxicological profiles. TSCA (the Toxic Substances Control Act, amended by the Lautenberg Act in 2016) governs chemical safety evaluation and regulation. CERCLA (Superfund) manages cleanup of contaminated sites listed on the National Priorities List. RCRA governs hazardous waste management from cradle to grave.

## Environmental Justice

Low-income and minority communities bear disproportionate environmental burdens due to historical zoning practices, redlining, and facility siting decisions that placed polluting industries in minority neighborhoods. EPA's EJScreen is an environmental justice screening and mapping tool. Executive Order 12898 (1994) requires federal agencies to identify and address environmental justice concerns. The Justice40 initiative directs 40% of benefits from federal environmental investments to disadvantaged communities. The Emergency Planning and Community Right-to-Know Act (EPCRA) and the Toxics Release Inventory (TRI) support community awareness of local environmental hazards.

<image>A diagram showing the dose-response curve with two models side by side: (1) a threshold model showing no adverse effect below the NOAEL, with NOAEL and LOAEL marked, and the RfD derived by dividing NOAEL by uncertainty factors; (2) a linear non-threshold model for carcinogens showing risk increasing linearly from the origin with no safe dose. Both curves are clearly labeled with dose on the x-axis and response/risk on the y-axis. Environmental toxicology education illustration.</image>

<image>An infographic showing the major environmental contaminants and their health effects organized by contaminant: Lead (brain/neurodevelopment icon), Mercury (nervous system/fish icon), Arsenic (skin/cancer icon), PFAS (immune system/kidney icon), and Pesticides (nervous system/agriculture icon). For each contaminant, primary sources and key health effects are listed. Biomonitoring specimen types (blood, urine, hair) are shown. Environmental health education illustration with chemical hazard symbols.</image>

<image>A conceptual diagram of bioaccumulation and biomagnification in an aquatic food chain, showing mercury concentration increasing from water to phytoplankton to small fish to large predatory fish to humans. Concentration values increase at each trophic level. An arrow shows the pathway from industrial mercury release to atmospheric deposition to waterway contamination. Annotations explain why predatory fish (tuna, swordfish) have the highest mercury levels and why pregnant women receive consumption advisories. Environmental toxicology education illustration.</image>

## Clinical Pearls

There is no safe blood lead level in children — even levels below 3.5 mcg/dL are associated with measurable cognitive deficits, making primary prevention through source removal the only truly effective strategy. PFAS represent the emerging environmental health crisis of this era, detectable in nearly all Americans, extremely persistent, and associated with immune, metabolic, and carcinogenic effects. Always ask about residential history and water source when evaluating potential environmental exposures — zip code is itself a risk factor. Biomonitoring through NHANES has been instrumental in tracking population-level exposure trends and documenting the success of interventions such as the dramatic decline in blood lead levels following the leaded gasoline phase-out. For board preparation, know the dose-response models (threshold versus linear non-threshold), the major contaminants and their health effects, and the concept of environmental justice.

## References
- ATSDR. Toxicological Profiles. atsdr.cdc.gov; 2024.
- CDC. National Report on Human Exposure to Environmental Chemicals. CDC; 2022.
- Landrigan PJ, et al. The Lancet Commission on pollution and health. Lancet. 2018;391(10119):462-512.
- EPA. PFAS Strategic Roadmap. epa.gov; 2024.
- Bellinger DC. Very low lead exposures and children's neurodevelopment. Curr Opin Pediatr. 2008;20(2):172-177.
- Grandjean P, Landrigan PJ. Neurobehavioural effects of developmental toxicity. Lancet Neurol. 2014;13(3):330-338.