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Prepositions after adjectives

Harmful: which preposition to use

Harmful usually takes to when describing negative effects. Other prepositions may appear in different sentence structures but are not standard complements after harmful.

Quick answer

The usual pattern is harmful to

Use harmful to to say that something damages or negatively affects someone or something. Harmful for is sometimes used in informal speech, but harmful to is preferred in standard English. In, by, and of usually belong to separate phrases rather than fixed combinations with harmful.

Smoking is harmful to your lungs.
The chemicals are harmful to marine life.
Too much stress can be harmful to children.

Choose the pattern by meaning

These forms are not interchangeable. Choose the construction by meaning, not by which preposition appears most often.

1
harmful to + person/thing

Describing negative effects

Use harmful to for damage, danger, or bad effects.

The smoke is harmful to animals.
This substance may be harmful to the skin.
2
harmful to + activity/process

Describing damage to progress or success

Harmful to can also describe negative effects on plans, relationships, or development.

The rumours were harmful to the company's reputation.
Lack of sleep is harmful to recovery.
3
harmful in + situation

Adding a context phrase

In may introduce the situation where harm occurs, but it is not the main complement after harmful.

These gases are harmful in enclosed spaces.
The product becomes harmful in large quantities.

See the difference in context

The same subject can take different constructions because each sentence answers a different question.

Construction
Example
Meaning
harmful to
This chemical is harmful to fish.
standard pattern for negative effects
harmful for
Too much sugar is harmful for children.
heard in some varieties, but harmful to is usually preferred
harmful in
The fumes are harmful in small rooms.
in introduces a situation or place
harmful by
The area was harmed by pollution.
by is normally used with the verb harm, not after harmful
harmful of
That phrase is not standard English.
harmful of is generally not idiomatic

Common mistakes

Some combinations appear in real text but do not mean what learners often assume.

Using harmful for in formal writing

Standard English strongly prefers harmful to.

✕ This additive is harmful for health. ✓ This additive is harmful to health.

Using harmful by as an adjective pattern

By usually follows the verb harmed, not the adjective harmful.

✕ The river is harmful by waste chemicals. ✓ The river is harmful to wildlife because of waste chemicals.

Using harmful of

Harmful of is not a standard adjective-preposition combination.

✕ The smoke is harmful of babies. ✓ The smoke is harmful to babies.

Confusing harmful in with harmful to

In gives context, while to names what is affected.

✕ This medicine is harmful in children. ✓ This medicine is harmful to children.

Quick practice

Choose the expression that matches the intended meaning.

1. This gas is harmful ___ the environment.
2. The fumes are harmful ___ closed rooms.
3. Too much noise can be harmful ___ babies.

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Frequently asked questions

Is harmful for incorrect?

It appears in some English varieties, but harmful to is the standard and safest choice.

Can I use harmful with people and things?

Yes. You can say harmful to children, harmful to health, or harmful to the environment.

Why do I sometimes see harmful in?

In usually introduces a context such as a place or condition, not the object affected.

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