Chemical toxicants are harmful chemical substances that can cause adverse effects on living organisms, including humans, animals, plants, and microorganisms. These substances may occur naturally or be synthesized artificially through industrial, agricultural, or domestic activities. When chemical toxicants enter the environment, they can disrupt biological processes, damage cellular structures, and lead to acute or chronic health problems. Environmental toxicology studies the sources, movement, transformation, and effects of these toxic chemicals in ecosystems and evaluates their risks to living systems. Chemical toxicants include heavy metals, pesticides, industrial solvents, petroleum products, polycyclic aromatic hydrocarbons, radioactive substances, and persistent organic pollutants. Their toxicity depends on factors such as dose, duration of exposure, route of entry, chemical properties, and susceptibility of the exposed organism. Exposure to chemical toxicants occurs when living organisms come into contact with these harmful substances. Exposure classes describe the ways toxicants enter biological systems and the environmental media through which exposure occurs. The three major environmental exposure classes are air pollutants, water pollutants, and soil pollutants. Each class represents a major pathway through which chemical toxicants interact with living organisms and ecosystems. Air pollutants represent one of the most significant exposure classes because air is essential for respiration and is constantly in contact with humans and other organisms. Chemical toxicants in air may exist as gases, vapors, or particulate matter. Common air toxicants include carbon monoxide, sulfur dioxide, nitrogen oxides, ozone, volatile organic compounds, and suspended particulate matter. These pollutants are released mainly from industrial emissions, vehicle exhaust, power plants, burning of fossil fuels, agricultural burning, and household fuel combustion. When inhaled, air toxicants directly affect the respiratory system and may enter the bloodstream, affecting other organs such as the brain, heart, and liver. Short-term exposure can cause irritation of the eyes, nose, and throat, breathing difficulties, headaches, and dizziness. Long-term exposure can lead to chronic respiratory diseases, cardiovascular problems, neurological disorders, and cancer. Air toxicants can also settle on land and water surfaces, contributing to secondary contamination of soil and water. Certain air pollutants participate in atmospheric reactions forming acid rain, which damages crops, forests, aquatic ecosystems, and infrastructure. Water pollutants form another important exposure class because water is essential for drinking, agriculture, sanitation, and industrial activities. Chemical toxicants enter water bodies through industrial discharge, agricultural runoff, sewage disposal, mining operations, oil