Those who need to take care when it's hot
Heart disease patients
The hot environment can put extra stress on the heart. This can lead to a worsening of underlying heart disease, such as increasing angina for coronary artery disease patients and a shortness of breath in heart failure patients, says Dr Adam Leung, a specialist in cardiology and Health Post advisory panel member. Those with heart failure cannot pump blood effectively to lower body temperature. Heart drugs don't help, either: diuretics can cause worsening dehydration, and beta-blockers can prevent the heart from beating fast enough to allow body cooling.
Very fit or unfit people
Besides being poorer at thermoregulation, unfit people may also be less experienced at pacing, which is a recipe for disaster, says Dr Jason Lee, an adjunct assistant professor in physiology at the National University of Singapore. Conversely, the very fit might ignore internal cues to stop or slow down. 'Always listen to your body, and never push beyond what you're capable of,' says Lee.
The young or elderly
Dr Chua Siew Eng, a psychiatrist and Health Post advisory panel member, says: 'Kids have larger surface area to volume, so they tend to absorb more heat. They also have a higher metabolic rate, so they generate more heat. The elderly are also more prone to less efficient thermoregulation.'