causing or capable of causing harm
"too much sun is harmful to the skin"
"harmful effects of smoking"
of a kind likely to be damaging; injurious
"The harmful chemicals in the paint fumes caused several residents to develop respiratory issues."
In plain English: Harmful means something that causes damage, injury, or bad results to people or things.
"Smoking is harmful to your lungs and can lead to serious health problems over time."
Usage: Use harmful as an adjective before nouns like harmful chemicals or harmful effects, rather than using it after the noun. It describes anything that causes damage or injury, distinguishing itself from neutral terms by implying active detriment.
The word "harmful" comes from Middle English and Old English, combining the root for injury with a suffix meaning "full of." It has traveled into modern English with essentially the same meaning it held in its earliest forms.