For materials:
* Non-breathable: This is the most direct antonym, meaning the material does not allow air to pass through it.
* Impermeable: This refers to a material that does not allow any substance, including air, to pass through it.
* Airtight: This describes a material that is completely sealed and does not allow air to enter or exit.
For environments:
* Suffocating: This describes an environment that lacks sufficient oxygen and makes it difficult to breathe.
* Claustrophobic: This describes an environment that feels enclosed and oppressive, potentially making breathing difficult.
For situations:
* Restrictive: This describes a situation that limits breathing, either physically or metaphorically.
* Oppressive: This describes a situation that feels heavy and suffocating, making it difficult to breathe freely.
The best antonym for "breathable" will depend on the specific context you are using it in.