Passage: "The heat index measures how hot it actually feels when humidity is combined with air temperature. On a humid day, sweat evaporates slowly, so the body cools less efficiently, and a temperature of 33°C can feel like 40°C or more. Health officials advise people to drink water regularly even before they feel thirsty, to avoid strenuous outdoor work at midday, and to watch for warning signs such as dizziness, headache and confusion. Older adults, young children and outdoor workers face the greatest risk." Based on the passage, which group should be most careful during periods of high heat index?