Which hazardous condition requires monitoring for signs of heat illnesses?

High humidity is a significant condition that requires monitoring for signs of heat illnesses. When humidity levels are elevated, the body's ability to cool itself through evaporation of sweat is hindered. This can lead to an increased risk of heat-related illnesses such as heat exhaustion or heat stroke. In high-humidity environments, even moderate physical activity can raise body temperature and reduce the effectiveness of sweating, making it essential for those working in such conditions to be vigilant about hydration and recognizing early symptoms of heat stress.

In contrast, heavy rain can create challenging work conditions but typically does not inherently raise the risk of heat illnesses. Prolonged exposure to chemicals, while hazardous in its own right, does not specifically relate to the physiological effects of heat. Cold temperatures present a different set of risks, notably cold stress rather than heat-related illnesses. Thus, high humidity stands out as the primary concern for monitoring heat illnesses.