Overview
As global temperatures inexorably rise, heat waves are becoming a serious threat to health, not only of humans, but all species. These periods of intense heat are already affecting food security, as well as longer term viability of certain areas of the world. Heat-related morbidity and mortality like heat exhaustion, heat stroke, along with dehydration and electrolyte disorders, diarrhoea and respiratory diseases have become more widespread around the world, particularly in tropical, subtropical, and temperate climate zones like Bangladesh. Heat-related illnesses (HRI) are the most severe health consequences of daily wage workers because of their long exposure to high temperatures. Bangladesh is one of the world’s most climate-sensitive health risk countries and future predictions indicate that both the frequency and intensity of risks (heatwaves) will rise (1). For example, recent studies in Australia (2), Bangladesh (3), China (4), India (5), Russia (6), the United Kingdom (7) and the United States (8) reported a significant number of fatalities due to illness caused by extreme heat waves.
Heat Impact on Health
Temperature plays a crucial role in human health (9). Internal organ temperature is referred to as body core temperature (Tc), and the body has multiple thermoregulatory ways to maintain Tc within a degree or two of 37°C, with a range from 36°C to 40°C. When Tc rises above 40.5°C hyperthermia results, with clinical manifestations of “sweating, flushing, tachycardia (rapid pulse), light-headedness, headache, muscle cramps, hypotension, confusion, delirium, seizures, and finally, coma” (Cheshire 2016). Heat waves commonly result in such symptoms of heat exhaustion, with progression to heat stroke, along with dehydration and electrolyte disorders.
Figure 1: Illustration of the Physiological pathways of human heat strain, illnesses, and death (Source: The Lancet (12)).
Dehydration can result in lower blood volume (and BP) which reduces cardiac filling, and in compensation increasing heart rate. The main cooling mechanism is sweating where salt and water can be lost at up to 2 L/hour. This cutaneous vasodilation dissipates the loss by radiant and convective pathways.
In summary, exposure to high temperatures for an extended period can result in direct damage to respiratory and renal systems, as well as indirect impacts through increased anxiety, violence, and substance [ab]use (10). There are reports of rising levels of kidney diseases, often with kidney stones, among farm workers in hot countries, like India and Egypt (11).
Heat Action Plans
One of the first heat action plans in the subcontinent was the Ahmedabad Heat Action Plan devised following a spike in deaths maximum temperature over 46°C on May 21, 2010. The HAP involved changing building construction to cooler designs (white painted roofs, etc.), improved hospital preparedness for heat stroke admissions, and establishing city-wide warning systems for coming heat waves.
Bangladesh currently has no established heat wave forecasting system, nor any system to convey such warnings to the general public. There is no widely accepted definition of heat wave for Bangladesh, but Nissan suggests “day and nighttime temperatures above the 95th percentile for 3 consecutive days (1). The establishment of these systems need to be a priority. It is well known that heat waves tend to occur in the pre-monsoon months (April-June) and are more likely if there has been a recent winter El Nino. There may be links also with the Indian Ocean Dipole.