Global warming has resulted in severe heat waves around the world. Recently, Pakistan, too, has been a victim of the phenomenon. Heat waves cause heat strokes which are often a serious health hazard and sometimes fatal. According to the World Meteorological Organization, “a heat wave occurs when the daily maximum temperature exceeds the average maximum temperature by 5 degrees centigrade for five consecutive days.” Heat strokes are caused when body temperature goes above 40.5 degrees centigrade. A heatstroke is more closely related to heat index than the air temperature. The National Oceanic and Atmospheric Administration calls the heat indexes from 41 to 51 degrees as the “danger zone”. Heat indexes above 51 degrees are referred to as the “extreme danger zone.” The heat index is a combination of ambient temperature and relative humidity. The heat index is also called humiture, which is the human-perceived equivalent temperature. It is also called ‘the feel temperature’ and nowadays appears on some smart phone applications. If air temperature is 40 degrees centigrade and relative humidity is 40 per cent, the feel temperature shall be 48 degrees centigrade. For the same air temperature if relative humidity is 5 per cent the feel temperature shall be 36 degree centigrade. This is one of the reasons why hot weather kills more people in Karachi than Larkana, Sukkur, Jacobabad and Nawab Shah etc. The 2015 heat wave from June 17 to June 24 killed around 1,200 people in Karachi according to the Ministry of Climate Change (in Technical Report on Karachi Heat-wave June 2015). The maximum air temperature recorded during this time was 44.8 degrees centigrade. However, during the peak hours the heat index was as high as 66 degrees centigrade. Human bodies respond to heat by sweating so that they can cool down by evaporation on the skin. If the air contains a lot of vapour, the evaporation slows down. The sweat thus stays on the skin. As water has a very high capacity of trapping heat, it accumulates more heat on the body resulting in a higher temperature feel. On the other hand, if air contains little or no vapours, the sweat evaporates quickly leaving a cooling effect on the body. The heat index, a combination of ambient temperature and relative humidity, is also called humiture or the feel temperature. Nowadays, it appears on some smart phone applications Dr Eric E Coris and his colleagues from the Department of Family Medicine, Division of Sports Medicine, University of South Florida College of Medicine Tampa, US, have reported 5 categories of heat related illnesses. The categories include heat oedema, heat cramps, heat exhaustion, heat syncope and heatstroke. The heat oedema causes blood vessels to expand causing the body fluids to flow downward by gravity. Heat cramps are caused by loss of large amounts of salt. Heat syncope results in physical exertion when blood flow to brain gets slower due to the cooling process of the body in a hot environment. Heat exhaustion may happen due to exposure to high temperatures while carrying out a strenuous physical activity. It leads to fatigue, nausea, vomiting and chills of head and neck. A heatstroke, which happens due to a high heat index, can cause hypertension, high body temperatures, altered behaviour, tachycardia (a condition that makes our heart beat more than 100 times per minute), coma, disseminated intra-vascular coagulation and acute renal failure etc. Reports show that a heat stroke can cause several organs to work less or stop functioning altogether. This may lead to a mortality rate of more than 10 per cent. It is reported that with a core body temperature of up to 40.6 degrees centigrade (which is 105 degrees Fahrenheit) one may be conscious runner. However, at 42-43 degrees centigrade one may collapse. Longer core body temperature exposure can cause sudden cardiac death, urgency of defibrillation, increased mortality rate and increased chances of organ damage. Dr Jen-Chun Wang from the Department of Emergency Medicine, Tri-Service General Hospital, National Defence Medical Centre, Taipei, Taiwan, and their colleagues from Taipei, Taiwan, reported a research on heat heatstroke patients, heat related illness patients and without heat related illness patients. Their research, The Association Between Heatstroke and Subsequent Cardiovascular Diseases was based on 150 heatstroke patients, 150 heat-related illness patients and 150 patients without heat-related illnesses. They found that heatstroke patients had a significantly higher incidence of developing cardiovascular diseases than the other heat related illness patients and control patients (without heat related illnesses). 32.67 per cent of patients with heatstroke had cardiovascular diseases; 23.33 per cent of patients with heat related illnesses had cardiovascular diseases, whereas only 16.67 per cent patients without heatstroke and heat related illness had cardiovascular diseases. Similarly, acute myocardial infarction (commonly known as a heart attack), acute ischemic stroke (occurs when the arteries to brain become narrowed or blocked) and an increased risk of chronic kidney disease followed the similar trend for heatstroke, heat related illnesses and controlled patients. The summer of 2019 is upon us. There is no way to stop the heat, particularly given the climate change and global warming, but it is possible to avoid the deaths and diseases caused by it. Most of the heat wave deaths in Karachi were reported from poor populations. The intensity of heat strokes varies with envelopes of population depending upon the quantity of heat trapped or stored. Places with concentrated structures and emissions of waste heat remain warmer and cause higher heatstroke incidents. This gets worst for the slum dwellers. Water and power shortages further to a people’s vulnerability to heatstroke. Industrial set-ups with high numbers of working hours and minimal facilitation also contribute to heatstroke cases. Lack of resources to counter emergency heatstroke incidents and cases cannot be ruled out. More importantly, lack of awareness about heat strokes and the proper response contribute to heat related deaths. Spreading the word can contribute to mitigation of heatstroke incidents. The writer is an assistant professor at Dawood University of Engineering and Technology Karachi. He can be reached at shaukat.mazari@duet.edu.pk