Antonym: harmless. Similar words: charm, harmony, harmonize, harmonious, farm, arm, warm, farmer. Meaning: ['hɑrmfʊl /hɑːm-] 
adj. 1. injurious to physical or mental health 2. able or likely to do harm 3. causing or capable of causing harm 4. constituting a disadvantage 5. contrary to your interests or welfare 6. tending to cause great harm.
adj. 1. injurious to physical or mental health 2. able or likely to do harm 3. causing or capable of causing harm 4. constituting a disadvantage 5. contrary to your interests or welfare 6. tending to cause great harm. 
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(1) The drug can be harmful if taken in excess.
(2) We should stop factories from producing harmful gases.
(3) She actually considered fresh air harmful.
(4) Smoking is harmful to your health.
(5) This apparatus produces harmful radiations.
(6) It's important to protect your skin from the harmful effects of the sun.
(7) Rubbishy commercials and harmful programmes contribute to the rise in violence and crime in the street.
(8) The drug is not harmful per se, but is dangerous when taken with alcohol.
(9) Sunscreen provides a protective layer against the sun's harmful rays.
(10) It is generally believed that smoking is harmful to health.
(11) Biodegradable packaging helps to limit the amount of harmful chemicals released into the atmosphere.
(12) There is abundant evidence that cars have a harmful effect on the environment.
(13) Fruit juices can be harmful to children's teeth.
(14) To jerk off is harmful.
(15) Smoking is a harmful vice. Sentencedict.com
(16) These products are often positively harmful.
(17) Not all virus infections are necessarily harmful to vines.
(18) Smoking can be harmful to your health.
(19) It is distinct that smoking is harmful to health.
(20) A lot of these chemicals are potentially very harmful.
(21) It is evident that smoking is harmful to health.
(22) There is a general awareness that smoking is harmful.
(23) Sighing for sorrow can be harmful.
(24) It is conspicuous that smoking is harmful to health.
(25) Many household products are potentially harmful.
(26) Sun creams work by blocking harmful ultraviolet radiation.
(27) Water is not an effective shield against the sun's more harmful rays.
(28) Exposure of the body to strong sunlight can be harmful.
(29) Unfortunately, pesticides kill off beneficial insects as well as harmful ones.
(30) Depletion of the ozone layer leaves the earth's surface increasingly exposed to harmful radiation from the sun.