harmful

adjective

harm·​ful ˈhärm-fəl 

: of a kind likely to be damaging : injurious

harmfully

ˈhärm-fə-lē 

adverb

harmfulness noun

Synonyms

• detrimental
• damaging
• dangerous
• adverse
• bad

Examples of harmful in a Sentence

DDT has been proven to be extremely harmful to the environment.

Recent Examples on the Web

While some individuals and groups have called for a slower approach to gender-affirming care for minors, other pediatric gender care experts and advocates have said ending such care can have a harmful effect on patients' mental health and well-being. —Mary Kekatos, ABC News, 23 Aug. 2025

Exposure to wildfire smoke, which contains fine particulate matter and other harmful pollutants, is unhealthy even for short periods, according to the air district. —Jason Green, Mercury News, 22 Aug. 2025

Beyond boosting efficiency, quantum dots may even help stabilize panels by converting harmful UV light into usable photons. —Dianne Plummer, Forbes.com, 22 Aug. 2025

But over the past 50 years, ecologists have come to value wetlands not only as wildlife habitats but also as carbon sinks, defenses against rising seas, and filters for harmful pollutants. —Kyra Morris, The Atlantic, 22 Aug. 2025

Word History

First Known Use

14th century, in the meaning defined above

Rhymes for harmful

• armful

Kids Definition

harmful

adjective

harm·​ful ˈhärm-fəl 

: causing damage : injurious

harmfully

-fə-lē 

adverb

harmfulness noun

Legal Definition

harmful

adjective

harm·​ful

: of a kind likely to cause harm

harmfully adverb

harmfulness noun