Environmental health is the science of understanding how your surroundings affect your body. It covers the physical, chemical, and biological hazards in air, water, soil, and food, along with social stressors that can harm human well-being. The goal is straightforward: prevent illness and injury caused by the environment people live in. In 2021, environmental and occupational risk factors were responsible for nearly 19% of all global deaths, roughly 12.8 million people, making this one of the largest and most overlooked drivers of disease worldwide.
What Counts as an Environmental Health Hazard
The field spans a surprisingly wide range of exposures. Physical hazards include extreme heat, ultraviolet radiation, and noise. Chemical hazards cover everything from lead in old paint to industrial compounds in drinking water. Biological hazards include bacteria, viruses, and parasites that spread through contaminated water or insect bites. Social stressors, like living near highways or waste facilities, also fall under the umbrella because chronic stress and pollution exposure compound each other.
What ties all of these together is the idea that the places where you live, work, and play shape your health in ways that are often invisible. You may not taste the chemicals in your tap water or feel the particulate matter entering your lungs, but the cumulative effect of these exposures drives rates of cancer, heart disease, respiratory illness, and developmental problems in children.
Air Pollution: The Leading Environmental Killer
Fine particulate matter, the tiny particles known as PM2.5, is the single largest environmental risk factor for death. Outdoor air pollution from PM2.5 caused an estimated 4.7 million deaths globally in 2021. Household air pollution from burning wood, charcoal, or other solid fuels for cooking added another 3.1 million deaths, mostly in low-income countries. Together, these two sources of air pollution account for the majority of all environment-related deaths.
The World Health Organization cut its recommended annual PM2.5 exposure limit in half in 2021, dropping it from 10 to 5 micrograms per cubic meter after evidence showed health damage at even low concentrations. Most regions of the world still exceed even the older, more lenient guideline. The particles are small enough to pass through lung tissue into the bloodstream, where they contribute to heart attacks, strokes, lung cancer, and chronic respiratory disease.
Water Safety and Chemical Contamination
About 2.1 billion people globally still lack access to safely managed drinking water. That includes 106 million who drink directly from untreated surface sources like rivers and ponds. Another 3.4 billion lack safely managed sanitation, and 354 million still practice open defecation. Contaminated water spreads cholera, typhoid, and parasitic infections that kill hundreds of thousands of children each year.
In wealthier countries, the threats are different but still serious. A class of synthetic chemicals called PFAS, sometimes nicknamed “forever chemicals” because they don’t break down naturally, have contaminated drinking water systems across the United States. The EPA finalized enforceable limits in public water systems, setting the maximum allowable level for the two most common PFAS compounds (PFOA and PFOS) at 4 parts per trillion. That’s an extraordinarily small concentration, reflecting how toxic these substances are even in tiny amounts. PFAS exposure has been linked to certain cancers, thyroid disease, immune suppression, and reproductive problems.
Lead Exposure in Children
Lead is one of the most well-studied environmental toxins, and there is no safe level of exposure for children. Even small amounts can cause irreversible damage to brain development, lowering IQ and increasing behavioral problems. The CDC’s current blood lead reference value for children is 3.5 micrograms per deciliter, updated in 2021. About 2.5% of U.S. children aged one to five have levels at or above that threshold.
The most common sources are lead-based paint in homes built before 1978, contaminated soil near old buildings or industrial sites, and aging water pipes. The damage is cumulative and largely silent. Children rarely show obvious symptoms until exposure has been significant, which is why screening and prevention matter more than treatment.
Microplastics: A Newer Concern
Researchers have now detected microplastics in human lungs, blood, heart tissue, placental tissue, and semen. These tiny plastic fragments, shed from packaging, synthetic clothing, and degrading waste, are small enough to be absorbed by individual cells. Early evidence suggests they may trigger tissue inflammation, allergic responses, cellular damage, and problems in the liver, lungs, and gastrointestinal system. The full health consequences are still being mapped, but the sheer ubiquity of microplastics in the human body has moved this from a fringe concern to a central question in environmental health.
Climate Change as a Health Threat
Climate change amplifies nearly every other environmental health risk. Rising temperatures increase heat-related deaths, even in cities that are already adapted to hot weather. Shifting rainfall patterns alter where mosquitoes and other disease-carrying insects can survive, expanding the geographic range of dengue, chikungunya, and malaria. Warmer conditions also increase the likelihood of food-borne and water-borne illness.
The downstream effects compound quickly. More intense heat waves and wildfires raise the risk of injury and death. Diminished crop yields in already vulnerable regions increase malnutrition. Coastal shipping routes can inadvertently introduce disease-carrying mosquitoes to new areas through freight containers. These connections illustrate why environmental health professionals increasingly view climate change not just as an ecological issue but as a public health emergency.
Environmental Justice
Environmental hazards do not affect everyone equally. Low-income communities and communities of color are far more likely to live near hazardous waste sites, highways, industrial facilities, and other pollution sources. The EPA defines fair treatment as the principle that no group of people should bear a disproportionate burden of environmental harms, whether from industrial operations, government programs, or commercial activity.
These disparities are closely linked to health outcomes. Communities near pollution sources have higher rates of asthma, cancer, and cardiovascular disease. Children in older, poorly maintained housing face greater lead exposure. The overlap between poverty, race, and environmental risk is one of the most consistent findings in public health research, and addressing it is a core priority of the field.
The One Health Connection
Environmental health doesn’t exist in isolation from animal health or ecosystem health. The One Health framework, adopted by major global health organizations, recognizes that human well-being is inseparable from the well-being of animals and the environment. Its five pillars are food safety, the human-animal bond, antimicrobial resistance, water contamination, and zoonotic diseases (infections that jump from animals to humans).
About 75% of emerging infectious diseases are zoonotic in origin. Deforestation, wildlife trade, and industrial farming bring humans and animals into closer contact, creating more opportunities for viruses and bacteria to cross species. COVID-19 reinforced what environmental health scientists had been warning about for decades: that degrading ecosystems and ignoring animal health creates direct risks for human populations.
Who Works in Environmental Health
Environmental health professionals include scientists, inspectors, and policy specialists who monitor hazards and enforce safety standards. In the U.S., credentials typically require a bachelor’s or master’s degree in environmental health from an accredited program, often paired with certification as a Registered Environmental Health Specialist through the National Environmental Health Association. These professionals inspect food facilities, test water systems, investigate disease outbreaks linked to environmental exposures, respond to chemical spills, and shape the regulations that set limits on pollutants in air, water, and soil.