# deleterious

/ˌdɛɫəˈtɪɹiəs/ (US) · /dɪlɪtˈiəɹɪəs/ (UK)

Source: https://englishreference.com/word/deleterious
Updated: 2026-08-27

## Senses

### 1. harmful or damaging

adjective · CEFR C1 · Formal · Academic

harmful to someone or something, often in a slow or hidden way.

For teachers: causing harm or damage, particularly in a subtle, cumulative, or unexpected manner.

- Smoking has a deleterious effect on your long-term health.
- The new policy had a deleterious impact on staff morale, leading to a significant decrease in productivity over the following quarter.

Usage: Frequently used in the phrase 'deleterious effect' or 'deleterious impact'.

Teaching tip: This is a high-level academic synonym for 'harmful'; students should use it in formal essays rather than casual conversation.

### 2. genetically harmful

adjective · CEFR C2 · Technical · Academic

describing a gene or trait that makes a living thing less healthy or less likely to survive.

For teachers: reducing the biological fitness of an organism; specifically used for mutations that are harmful to survival or reproduction.

- The animal carried a deleterious mutation that made it difficult to find food.
- Natural selection typically acts to remove deleterious alleles from the population, though some may persist in recessive forms.

Usage: Commonly modifies nouns like 'mutation', 'gene', or 'allele'.

Teaching tip: This sense is specific to biology and genetics; it is rarely encountered outside of scientific literature.

## Examples

- The smoke from the factory has a deleterious effect on the local birds.
- Prolonged exposure to high levels of noise can have a deleterious impact on a person's mental health and hearing.
- While the initial economic stimulus provided a temporary boost, the long-term reliance on debt proved deleterious to the country's fiscal stability and international credit rating.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.