Headaches, constipation and muscle cramps are the top three illnesses people suffer from during summer, according to the Department of Health.

The department said 21.9% of the 682 respondents in a survey on health and wellness during the summer frequently suffered from headaches, followed by constipation (13.6%) and cramps in their legs and belly (12.7%).

The survey, collected from March 1 to April 4, also showed that 52.8% of respondents had moderate concerns about health problems caused by heat, and 19.8% showed a high level of concern.

According to the Meteorological Department, 15 provinces, including Bangkok, will be affected by a red-level, or a highly dangerous heat index until April 27. As the high heat index directly affects health conditions, the department warned people about the risk of heatstroke.

Heatstroke leads to the body's core temperature rising to 40 degrees Celsius or more. Symptoms include skin redness, a faster pulse, headache, dizziness, vomiting, unconsciousness or even death in some cases.

Heatstroke can be treated by applying ice or cooling cloth over the body, especially the back of the neck, armpits, and the crotch, to lessen the body temperature before sending the patient to the hospital, the department said.