causing damage or injury to somebody/something, especially to a person’s health or to the environment the harmful effects of alcohol the sun’s harmful ultra-violet rays Many household products are potentially harmful. harmful to somebody/something Fruit juices can be harmful to children's teeth.Extra examples A lot of these chemicals are potentially very harmful. Not all virus infections are necessarily harmful to vines. She actually considered fresh air harmful. The spraying could prove harmful to humans. These pesticides are environmentally harmful. These products are often positively harmful. pesticides that are harmful to the environment Keep food chilled to prevent the spread of harmful bacteria. The harmful effects of alcohol are well established. The ozone layer filters out the sun’s harmful ultraviolet rays.