Heat-Related Deaths in Hot Cities: Estimates of Human Tolerance to High Temperature Thresholds

In this study we characterized the relationship between temperature and mortality in central Arizona desert cities that have an extremely hot climate. Relationships between daily maximum apparent temperature (ATmax) and mortality for eight condition-specific causes and all-cause deaths were modeled for all residents and separately for males and females ages <65 and ≥65 during the months May–October for years 2000–2008. The most robust relationship was between ATmax on day of death and mortality from direct exposure to high environmental heat. For this condition-specific cause of death, the heat thresholds in all gender and age groups (ATmax = 90–97 °F; 32.2‒36.1 °C) were below local median seasonal temperatures in the study period (ATmax = 99.5 °F; 37.5 °C). Heat threshold was defined as ATmax at which the mortality ratio begins an exponential upward trend. Thresholds were identified in younger and older females for cardiac disease/stroke mortality (ATmax = 106 and 108 °F; 41.1 and 42.2 °C) with a one-day lag. Thresholds were also identified for mortality from respiratory diseases in older people (ATmax = 109 °F; 42.8 °C) and for all-cause mortality in females (ATmax = 107 °F; 41.7 °C) and males <65 years (ATmax = 102 °F; 38.9 °C). Heat-related mortality in a region that has already made some adaptations to predictable periods of extremely high temperatures suggests that more extensive and targeted heat-adaptation plans for climate change are needed in cities worldwide.


Direct Exposure to Environmental Heat
In the human, exposure to high environmental heat is often accompanied by exposure to the sun. Thus, the first set of ICD-10 codes are for conditions that include the terms "sunburn", "sun exposure", and/or "ultraviolet light". These were located by doing a text search of the online version of the ICD-10. The second set of ICD-10 codes identified as being due to the direct effect of heat are those that include the term "heat." These were also located by doing a text search of the online version of the ICD-10.

Dehydration
In the human, exposure to high environmental heat is accompanied by sweating and fluid loss. The consequence of fluid loss is dehydration, which is followed by volume depletion and hypovolemia. ICD-10 codes that included the term "dehydration" were identified by doing a text search of the online version of the ICD-10.

Possible Consequences of Heat and Dehydration
In the human, exposure to high environmental heat is accompanied by sweating and fluid loss. The consequence of fluid loss is dehydration, which is followed by volume depletion and hypovolemia. Supplemental Figure S1 is a graphical depiction of the model that explains our approach to selecting codes considered likely to be due to the physiologic and pathophysiologic consequences of heat and dehydration/volume depletion/hypovolemia. Volume depletion and hypovolemia can cause hypotension. Hypotension can lead to shock, coma and sudden death. Dehydration/volume depletion/hypovolemia can lead to the formation of kidney stones and thus to renal colic.
Hypovolemia can lead to hemoconcentration and cause disturbances in serum electrolytes (sodium-hypernatremia-and potassium-hyperkalemia) and osmolality (hyperosmolality). The disturbances of serum electrolytes can lead to arrythymias, coma, and sudden death.
Hypovolemia can lead to acute renal failure.
Thirst is a physiologic response to dehydration. When water is consumed in response to thirst without adequate electrolytes, the consequence can be disturbances in serum electrolytes (sodiumhyponatremia-and potassium-hypokalemia) and osmolality (hypo-osmolality).
Disturbances in serum electrolytes can lead to arrythymias, coma, and sudden death. One consequence of heat and dehydration is hyperthermia, which can directly cause arrhythmias, and can lead to shock, coma, and sudden death. Hyperthermia also can lead to fever and then febrile convulsions. Hyperthermia may cause disseminated intravascular coagulation (DIC).
A number of the consequences of heat and dehydration (e.g., hypotension) can cause malaise and fatigue.
Malaise and fatigue are also symptoms of hyperthermia and also of hypovolemia. "Dizziness and giddiness" is a condition with a specific ICD-10 code that has been identified in other studies as having a relationship to ED visits and hospitalization during heat waves.
We selected the ICD-10 codes that corresponded to the other named conditions that are a consequence of heat and dehydration by reading the ICD-10 code book and using information in the publications of others.

Cardiac Disease/Stroke
Studies of excess mortality and morbidity in heat waves consistently find an association with atherosclerotic heart disease and stroke. The description of the physiologic and pathophysiologic consequences of heat and dehydration provide ample explanation for an effect of heat on mortality and morbidity due to atherosclerotic heart disease and stroke. Hypotension, hemoconcentration, and electrolyte disorders would all be expected to exacerbate the severity of underlying vascular disease and/or lead to coronary or cerebral thrombosis.
We selected the set of ICD-10 codes for atherosclerotic vascular disease that have been used by other researchers noting that the link between hemorrhagic stroke (including subarachnoid hemorrhage) and heat/dehydration and its consequences seems tenuous.

Chronic Renal Failure
Chronic renal failure is associated with a multiplicity of underlying abnormalities of fluid and electrolyte metabolism. Many people with chronic renal failure are being dialyzed. An effect of heat and dehydration and their consequences in exacerbating the severity of chronic renal failure and even perhaps causing death in a person with chronic renal failure has high biologic plausibility. We selected all of the ICD-10 codes for chronic renal failure.

Heart Failure
Heart failure is associated with a multiplicity of underlying abnormalities in handling fluids. People with heart failure are at increased risk of atrial fibrillation and are vulnerable to other cardiac arrhythmias. They are also at increased risk of stroke. An effect of heat and dehydration and their consequences in exacerbating the severity of heart failure or causing death in a person with heart failure has high biologic plausibility.
We selected the ICD-10 codes for heart failure for this category. We also selected the ICD-10 codes for unspecified cardiovascular disease, other and ill-defined heart disease and unspecified heart disease for this category based on the belief that many people who have these non-specific codes listed as a cause of death would have heart failure.

Other Respiratory Diseases
Some prior research on deaths and hospitalizations in the context of extreme heat has found strong associations with respiratory illnesses. Following Semenza et al. [1] and Schwartz [2], we examined chronic obstructive pulmonary disease (COPD) and asthma separately from infections, influenza, and other respiratory diseases. Subsequent myocardial infarction of unspecified site I24 Other acute ischemic heart diseases I24. 1 Dressler's syndrome I24.8 Other forms of acute ischemic heart disease I24.9 Acute ischemic heart disease, unspecified I25 Chronic ischemic heart disease I25.0 Atherosclerotic cardiovascular disease, so described I25. 1 Atherosclerotic heart disease I25. 2 Old myocardial infarction I25.3 Aneurysm of heart I25.4 Coronary artery aneurysm I25.5 Ischemic cardiomyopathy I25.6 Silent myocardial ischemia I25.8 Other forms of chronic ischemic heart disease I25.9 Chronic ischemic heart disease, unspecified I49.9 Cardiac arrhythmia, unspecified I60 Subarachnoid hemorrhage I60.0 Subarachnoid hemorrhage from carotid siphon and bifurcation I60. Intracranial hemorrhage (nontraumatic), unspecified I63 Cerebral infarction I63.0 Cerebral infarction due to thrombosis of precerebral arteries I63. 1 Cerebral infarction due to embolism of precerebral arteries I63. 2 Cerebral infarction due to unspecified occlusion or stenosis of precerebral arteries I63.3 Cerebral infarction due to thrombosis of cerebral arteries I63.4 Cerebral infarction due to embolism of cerebral arteries I63.5 Cerebral infarction due to unspecified occlusion or stenosis of cerebral arteries I63.6 Cerebral infarction due to cerebral venous thrombosis, nonpyogenic I63.8 Other cerebral infarction I63.9 Cerebral infarction, unspecified I64 Stroke, not specified as hemorrhage or infarction I67 Other cerebrovascular diseases I67.0 Dissection of cerebral arteries, nonruptured I67. Generalized and unspecified atherosclerosis Chronic Renal Failure ICD-10 a Condition N18 Chronic renal failure N18.0 End-stage renal disease N18.8 Other chronic renal failure N18.9 Chronic renal failure, unspecified N19 Unspecified renal failure Heart Failure ICD-10 a Condition I50 Heart failure I50.0 Congestive heart failure I50. 1 Left ventricular failure I50.9 Heart failure, unspecified I51.6 Cardiovascular disease, unspecified I51.8 Other ill-defined heart diseases I51.9 Heart disease, unspecified Figure S1. Approach to selection of ICD-10 codes for conditions that are likely due to physiologic and pathophysiologic consequences of heat and dehydration.