Harmful substances are materials that can cause damage to living organisms or the environment. These can include chemicals like pesticides, heavy metals such as lead and mercury, and even certain types of plastic. Exposure to these substances can lead to health issues, including respiratory problems, skin irritations, and long-term diseases. In addition to health risks, harmful substances can also contaminate soil, water, and air, affecting ecosystems and wildlife. For example, oil spills release toxic compounds into oceans, harming marine life. Understanding and managing these substances is crucial for protecting both human health and the environment.