Contaminants are substances that are present in the environment or in a product at levels that can cause harm or adverse effects. They can be chemical, biological, or physical agents, and may include pollutants like heavy metals, pesticides, bacteria, and particulate matter. Contaminants can affect air, water, soil, and food quality, posing risks to human health, ecosystems, and biodiversity. Their presence often results from industrial activities, agricultural practices, or improper waste disposal.