**Core Concept**
Heat stress index (HSI) is a measure of the combined effects of temperature and humidity on the human body. It is a crucial factor in determining the risk of heat-related illnesses and the need for protective measures in occupational environments.

**Why the Correct Answer is Right**
The heat stress index is used to categorize the level of heat stress, with higher values indicating greater risk. The heat stress index ranges from 0 to 150, with higher values indicating more severe heat stress. At a heat stress index of 90, the risk of heat-related illnesses increases significantly, and it is not possible for individuals to work comfortably, posing a threat to their health. The body's ability to cool itself through sweating is impaired at this level, and prolonged exposure can lead to heat exhaustion and heat stroke.

**Why Each Wrong Option is Incorrect**
**Option A:** This is not a valid heat stress index value, and it is not a recognized level of heat stress.
**Option B:** No information is provided about this value, and it is not relevant to the question.
**Option C:** This is not the correct answer, as the question asks for the level at which it is not possible to work comfortably, causing a threat to health.

**Clinical Pearl / High-Yield Fact**
Remember that a heat stress index of 90 or higher indicates a high risk of heat-related illnesses, and individuals should take regular breaks, stay hydrated, and wear protective clothing to minimize the risk of heat stress.

**Correct Answer:** C.