**Core Concept**
Polysulphide impression materials are a type of thermoplastic elastomer used in dentistry for making accurate impressions of the oral cavity. They are known for their dimensional stability and ability to capture fine details. However, they also have certain disadvantages that limit their use.

**Why the Correct Answer is Right**
Polysulphide impression materials have a distinct disadvantage due to their high toxicity, particularly due to the release of hydrogen sulfide gas, which can be hazardous to the patient and the dentist. This toxicity is a significant drawback, making them less preferred for use in certain patients, such as those with respiratory issues. The release of hydrogen sulfide gas is a result of the chemical reaction between the polysulphide material and the oral environment.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a recognized disadvantage of polysulphide impression materials. While they do have some limitations, such as requiring a longer setting time, this is not their primary disadvantage.
**Option B:** While polysulphide impression materials do have a higher cost compared to other materials, this is not their primary disadvantage. Their toxicity is a more significant concern.
**Option C:** This option is not accurate. Polysulphide impression materials are actually known for their dimensional stability, which is a significant advantage.

**Clinical Pearl / High-Yield Fact**
It's essential to note that polysulphide impression materials should be used with caution, especially in patients with respiratory issues, and adequate ventilation should be provided during their use.

**Correct Answer:** D.