"유해 (Harmful)" refers to anything that can cause damage or negative effects to living beings or the environment. This can include substances like toxic chemicals, which can lead to health problems, or behaviors such as smoking, which is known to harm both the smoker and those around them. In a broader context, harmful actions can also affect ecosystems. For example, pollution can degrade air and water quality, impacting wildlife and human health. Understanding what is harmful helps individuals and communities make better choices to protect themselves and their surroundings.