In severe heat strain the heat stress index (HIS) is –
**Core Concept**
Heat stress index (HIS) is a measure used to assess the risk of heat-related illnesses. It takes into account the air temperature, humidity, and wind speed to estimate the level of heat stress. HIS is a critical parameter in occupational health and safety, particularly for individuals working outdoors in hot environments.
**Why the Correct Answer is Right**
The heat stress index is calculated using the following formula: HIS = (air temperature + 0.3 * (absolute humidity)) - (0.2 * (wind speed)). In severe heat strain, the HIS value is typically above 90. This indicates a high risk of heat-related illnesses, such as heat exhaustion or heat stroke. The mechanism behind HIS is based on the concept of wet bulb globe temperature (WBGT), which measures the temperature that would be recorded by a thermometer wrapped in a wet cloth and placed in a small globe exposed to the environment.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately represent the formula used to calculate HIS. While air temperature is an important factor, absolute humidity and wind speed are also crucial components.
**Option B:** This option is incorrect because it implies that HIS is solely dependent on air temperature, which is not the case. HIS takes into account multiple environmental factors, including humidity and wind speed.
**Option C:** This option is incorrect because it suggests that HIS is a fixed value, rather than a calculated index. HIS values can vary depending on the environmental conditions.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that HIS values above 90 indicate a high risk of heat-related illnesses. In such cases, individuals should be provided with adequate cooling measures, such as rest, hydration, and shade, to prevent heat exhaustion or heat stroke.
**Correct Answer: D. 90.**