Heavy metals enter the environment from two broad categories of sources: natural geological processes and human activity. Earth’s crust naturally contains metals like lead, arsenic, mercury, and cadmium locked inside rocks and minerals. But the concentrations that pose real problems for human health overwhelmingly come from what we’ve built, burned, mined, and discarded over the past few centuries.
Natural Sources in Rock and Soil
Heavy metals exist naturally in the earth’s crust, embedded in rock formations that have been there for millions of years. Rock weathering, soil erosion, and the dissolution of water-soluble salts gradually release trace amounts of these metals into groundwater and surface water. The specific metals present in any area depend on local geology. Selenium, for example, occurs naturally in sedimentary rocks, shales, coal deposits, and phosphate-rich formations. Arsenic appears in certain volcanic and metamorphic rocks.
These natural background levels are typically low. In U.S. soils, median background concentrations of lead sit around 25 to 30 milligrams per kilogram, arsenic around 8 to 12, and cadmium around 0.5. At these levels, the metals are a normal part of the environment and rarely cause harm. The problems start when human activity concentrates them far beyond those baselines.
Mining and Smelting
Mining has caused the most widespread metal contamination of soil worldwide. When ore is extracted from the ground and processed through smelting, metals that were locked deep in rock get released into the surrounding environment as dust, wastewater, and airborne particles.
The scale can be staggering. In the Celje area of Slovenia, a century of zinc smelting released an estimated 1,700 tonnes of zinc and over 9 tonnes of cadmium into the surrounding landscape. Soil zinc concentrations there reached levels hundreds of times above natural background. Near Kosovska Mitrovica in Kosovo, topsoil in a former mining area spanning 302 square kilometers contained lead at 35,000 milligrams per kilogram and zinc at 12,000, orders of magnitude above what you’d find in uncontaminated ground. These aren’t isolated cases. Regions with 300-plus years of lead mining and smelting history still carry heavy contamination in their soil, dust, and indoor environments.
Burning Coal and Fossil Fuels
Coal contains trace amounts of heavy metals, and burning it sends those metals into the atmosphere. At temperatures above 1,000°C inside a power plant, roughly 90% of cadmium, 85% of mercury, and 80% of lead in the coal migrate from solid fuel into flue gas. From there, they either get captured by pollution controls or escape into the air.
Mercury, arsenic, lead, and chromium are the primary metals of concern from coal combustion. Studies of soil near coal-fired power plants have found cadmium and mercury concentrations that significantly exceed safety screening limits. Arsenic levels from some plants also surpass those thresholds. These emissions don’t stay local. Fine particles carrying metals can travel long distances on wind currents before settling onto soil and water far from the original smokestack. Car exhaust is another combustion source, historically contributing lead to roadside soils during the decades when leaded gasoline was standard.
How Metals Travel Through the Air
Once metals become airborne, whether from a factory, a coal plant, or a desert dust storm, they bind to tiny particles and can travel hundreds or thousands of miles. They eventually return to the ground through two main pathways. Wet deposition occurs when rain or snow scavenges metal-laden particles from the atmosphere and washes them down to the surface. Dry deposition happens when particles simply settle out of the air under gravity, influenced by wind patterns and terrain.
Wet deposition is the dominant pathway in most climates. An estimated 68% to 74% of atmospheric cadmium and zinc reach the ground this way, with 25% to 33% arriving through dry deposition. In arid and semi-arid regions, dry deposition takes over. Dust storms from places like the Gobi Desert and the Taklimakan Desert transport both natural and human-generated metals across East Asia, contaminating water bodies and soil along the way. This long-range transport means heavy metals from industrial regions can affect sensitive ecosystems in remote areas that have no local pollution sources at all.
Farming and Fertilizers
Agriculture is a quieter but persistent source of heavy metal accumulation. Phosphate fertilizers naturally contain cadmium, with concentrations ranging from 0.1 to 170 milligrams per kilogram depending on the source rock. Every application adds a small amount to the soil, and over years of repeated use, cadmium builds up. Copper and zinc also accumulate from long-term fertilizer use. Eventually, this buildup can reduce soil fertility and decrease crop productivity.
Pesticides contribute too. Certain insecticides release arsenic into agricultural soil. Livestock manure, often spread as fertilizer, can contain metals from animal feed supplements. Irrigating fields with treated wastewater introduces yet another pathway. The result is a slow, steady increase in metal concentrations in the very soil we grow food in, which plants then absorb through their roots.
Metals in Your Drinking Water
One of the most direct exposure routes for heavy metals is your home’s plumbing. Lead is the primary concern. Lead service lines, the pipes connecting water mains to homes, can be a major source of lead in tap water. Solder used on copper pipes installed before 1986 contained high levels of lead. Galvanized pipes accumulate lead particles on their interior surfaces over time, which then flake off into the water. Even fixtures like faucets and connectors can contain lead.
The EPA requires water systems to take action when lead levels exceed regulatory limits, including replacing lead service lines and optimizing corrosion control treatment. Corrosion control works by coating the inside of pipes with a mineral layer that prevents lead from dissolving into the water. When that protective chemistry gets disrupted, as happened in Flint, Michigan, lead levels can spike.
Electronics, Batteries, and E-Waste
Modern electronics contain a cocktail of heavy metals. When these devices are improperly recycled or dumped, those metals leach into the surrounding environment. The World Health Organization reports that informal e-waste recycling can release up to 1,000 different chemical substances, including known neurotoxicants like lead and mercury. Common practices at unregulated e-waste sites include open burning, acid baths to extract metals, and dumping directly onto land or into water. Each of these methods contaminates air, soil, dust, and water at the recycling site and in neighboring communities.
Lead is the most commonly released metal from e-waste processing. It’s found in solder on circuit boards, in older cathode-ray tube monitors, and in various electronic components. Mercury appears in certain switches and fluorescent backlights. These materials are safe when sealed inside a functioning device but become hazardous when that device is broken apart without proper controls.
Household Items and Consumer Goods
Heavy metals show up in everyday products more often than most people realize. Lead-based paint, banned for residential use in the U.S. in 1978, still coats surfaces in millions of older homes. The CDC identifies several categories of consumer products that may contain lead: imported or antique toys, children’s jewelry (where lead is used to add weight and brighten colors), ceramic dishware, lead crystal glassware, vintage painted tin, and antique furniture. Lead is also used in some plastics to make them more flexible and heat-resistant, and this use has not been banned.
Cosmetics, particularly those imported from countries with less stringent regulations, can contain lead, mercury, or arsenic. The leather tanning industry uses chromium extensively, with nearly 80% of tanneries worldwide relying on chromium-based processes and releasing an estimated 3,200 tonnes of chromium into the environment annually.
Buildup in the Food Chain
Once metals enter waterways, they don’t just stay in the water. Fish absorb them through their gills, skin, and by eating contaminated food. Over time, metals accumulate in fish tissue, a process called bioaccumulation. The real concern is what happens next: when larger predatory fish eat smaller contaminated fish, the metal concentration increases at each step up the food chain.
Mercury is the clearest example. In water, bacteria convert elemental mercury into methylmercury, a form that is far more toxic and far more easily absorbed by living tissue than the metal itself. Methylmercury concentrates most heavily in large predatory species. Swordfish and sharks carry some of the highest mercury levels of any commonly consumed seafood. This is why dietary guidelines specifically recommend that pregnant women and young children limit consumption of these species while still eating lower-mercury fish for nutritional benefits.