Toxicity is the capacity of a substance to harm humans and other living things. That is, it describes how harmful a substance can be.
The toxicity of a substance depends on factors such as the dose, the duration of exposure, how the substance enters the organism and the individual susceptibility of the organism (such as age, genetics and overall health). As the physician Paracelsus* is often quoted as saying, "The dose makes the poison." Even substances essential for life, such as water and oxygen, can be harmful in excessive amounts.
Toxic effects are often described as acute or chronic. Acute toxicity refers to harmful effects that develop quickly, often after a single, high-level exposure, for example, breathing in carbon monoxide or swallowing a chemical cleaner. Chronic toxicity refers to harm that develops over time, usually after repeated or prolonged exposure, such as long-term exposure to lead or tobacco smoke.
Toxicity can affect different parts of an organism. For example, neurotoxicity is damage to the nervous system or brain, which may result from exposure to substances such as mercury or lead. Cytotoxicity is damage to cells; some chemotherapy drugs use this effect to kill cancer cells. Hepatotoxicity is damage to the liver, which can occur after a paracetamol (known as acetaminophen or Tylenol in North America) overdose.
Environmental toxicity examines how pollutants affect wildlife and ecosystems. As these pollutants interact with living systems, they can undergo bioaccumulation and biomagnification, becoming increasingly concentrated as they move up the food chain. This escalation explains why, for example, pesticide runoff can harm fish in rivers, and how heavy metals can contaminate the soil and the organisms that depend on it.
To compare how acutely toxic a substance is, toxicologists often look at the dose required to cause harm, sometimes called the "median lethal dose" or LD50. This measures the amount of a substance that causes the death of half a group of test animals under specific conditions, meaning a lower dose indicates higher toxicity. However, animal tests do not always directly reflect how a substance affects humans. For plants, researchers use similar benchmarks, such as the concentration that stunts growth or reduces seed germination in half of a test group.
Human-caused environmental toxicity, from sources such as industrial waste, agricultural runoff, mining and urban pollution, can be addressed through a combination of prevention, containment, and remediation.
Remediation is the removal of toxins or neutralizing or treating them so they become less harmful. This can be done through physical, chemical or biological methods. Bioremediation uses living organisms, such as plants, bacteria or fungi, to break down or remove pollutants.
One type of bioremediation is phytoremediation, which employs plants to absorb or accumulate contaminants, especially certain metals, from soil or water. Sunflowers and hemp are widely used for this purpose.
Among the chemical treatments for acidic water resulting from mining is to add lime, usually calcium hydroxide or calcium oxide. This raises the pH level and can cause dissolved metals to form solid particles that can be separated from the water and thus be disposed of more easily. Calcium carbonate may also be used in some treatments, but it reacts differently and may work more slowly.
Another approach is mechanical, such as digging up contaminated soil or scraping up polluted river sediment. The toxic material is then hauled away to specialized, lined hazardous waste landfills to prevent it from leaching into the surrounding environment. Likewise, contaminated groundwater can be pumped to the surface, passed through advanced filtration systems (such as activated carbon blocks) to remove the contaminants and then injected back into the ground.
Containment and mitigation strategies aim to stop the spread of toxic substances. If a site cannot be cleaned up immediately, strategies are deployed to lock the toxins in place and protect surrounding ecosystems and human populations. Capping involves placing a durable, impermeable barrier, such as thick layers of clay, synthetic liners and concrete, over a contaminated site, such as an old landfill. This prevents rainwater from soaking through the waste and carrying toxins down into the groundwater.
Subsurface barriers use deep trenches filled with slurry (a thick, semi-liquid mixture of water and clay) or sheet metal walls to physically block underground plumes (flows) of toxic chemicals from migrating toward drinking water wells or local rivers. Additionally, the root matrices of specific plants can be used to physically anchor contaminated soil, preventing wind erosion from turning toxic dirt into airborne dust and keeping rain from washing it away.
The most effective long-term strategy is preventing toxicity before it is ever created. This involves changing industrial and agricultural processes along with everyday human habits. It includes designing products and processes that reduce or eliminate the use and creation of hazardous substances from the start, for example biodegradable plastics or non-toxic industrial solvents.
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*Paracelsus (1493-1541), considered the father of toxicology, famously established the foundational principle of toxicology: "All things are poison, and nothing is without poison; only the dose permits something not to be poisonous." He was among the first to systematically evaluate toxicity based on dosage rather than treating substances as inherently good or bad.