Approach to heat-related illness

Sources of information

The MeSH terms heat-related illness and primary care were searched in PubMed. Clinical trials, practice reviews, and systematic reviews were included. All reference lists were reviewed for additional articles.

Main message

According to the World Health Organization¹ and the Public Health Agency of Canada,² climate change is the greatest health threat of this century, already affecting the health of people in Canada.³ The frequency, intensity, and duration of extreme heat events are increasing as a result of climate change, with many regions in Canada projected to see 3- to 4-fold increases in number of days with temperatures hotter than 30°C.⁴ In fact, the heat dome that engulfed western Canada in the summer of 2021 was the deadliest weather event in Canadian history.⁵

Since climate change is a threat multiplier that worsens existing inequities,^6-8 people who are structurally marginalized face the greatest health risk from climate change and extreme heat.

Pathophysiology of heat stroke. Internal thermoregulation maintains a person’s core temperature between 36°C and 37°C. The body’s thermoregulatory centre, the hypothalamus, senses changes in body temperature and sends signals to the cardiovascular and respiratory systems to restore equilibrium. In the case of excess heat, either from external elements or internal heat generated from exercise, these signals increase heart rate, respiratory rate, and blood flow to the periphery so that heat can be dissipated. Under normal conditions the dominant mechanism by which this occurs is infrared radiation, which accounts for about 65% of body heat loss. As ambient temperatures rise to match normal body temperature, radiation is less effective, so humans rely on sweating and evaporation. However, as humidity increases, the effectiveness of sweating decreases, and at 90% humidity it stops working entirely.^9,10

While protective behaviour such as seeking shade, resting during the hottest time of day, or soaking in a cool bath can play a role in cooling, normal thermoregulation is challenged by extreme heat. Thermal balance is vulnerable to failure when the body’s normal physiologic response is undermined or when normal cooling and heat-dissipating mechanisms are unavailable. When the body is unable to rid itself of excess heat, several pathologic changes take place. These changes can affect circulatory volume, cardiac output, organ perfusion, salt and water balance, and, in extreme cases, can lead to direct cytotoxic damage with systemic inflammatory response and consequent shock and multiorgan failure.^11,12

While exertional heat stroke (EHS) tends to occur sporadically among young athletes and labourers in activities where excess body heat is produced without effective cooling, classic heat stroke (CHS) occurs in an epidemic pattern in concert with extreme heat events, typically affecting the most vulnerable, such as elderly populations. The clinical presentation of heat stroke includes confusion, cramps, nausea and vomiting, dizziness, tachycardia, tachypnea, and syncope. However, if CHS is not treated quickly, death is possible. Aggressive resuscitation with rapid cooling and supportive management of vital organ systems is required.

Unlike EHS, with a mortality rate less than 5%, CHS has a mortality rate greater than 50%. Of those who survive CHS, many have permanent severe decline in function that leads to institutionalization.¹³ Long-term neurologic sequelae of patients can include ataxia, amnesia, and dementia.

Both CHS and EHS are medical emergencies. Typically, patients suspected of having EHS because of confusion and elevated core temperature should undergo immediate aggressive cooling before transfer to an emergency department (ED). In CHS, given that the typical patient (ie, individuals who are elderly, those with comorbid conditions) is not as robust and will have a larger differential for elevated temperature and confusion, efforts should be made to cool or at least not further exacerbate heat, but definitive cooling is typically not achieved until assessment in the ED.

How else does heat affect health? Extreme heat events have both direct and indirect effects on health. In Canada excess heat has been associated with a 2.5% increase in excess deaths.¹⁴ Heat events have also been associated with acute worsening of mental health and increased risk of violence and suicide.¹⁵ Heat can exacerbate chronic cardiovascular, respiratory, and kidney disease.^16-19 Extreme heat also contributes to air pollution in the form of ground-level ozone and particulate matter,¹⁵ which can aggravate chronic medical conditions.

Access to health services is also affected by extreme heat. Emergency services use can increase substantially throughout heat events. During the 2021 British Columbia heat dome event, emergency services call volumes increased threefold, with wait times of up to 11 hours for ambulances.²⁰ A study published in 2023 modelling possible health system impacts of a multi-day blackout co-occurring with a heat wave in several American cities showed that nearly half the population of Phoenix, Ariz, would require ED care for heat stroke or other heat-related illnesses.²¹

Who is at increased risk of heat-related illness? People may be at higher risk of heat-related illness due to increased sensitivity or exposure to heat or due to insufficient access to information and resources.²²

Increased sensitivity to heat: Intrinsic factors that blunt the body’s physiologic response to heat or extrinsic factors that exacerbate heat susceptibility can increase an individual’s risk of heat-related illness.

Intrinsic factors include the following: decreased physiologic capacity (usually found in older individuals); pregnancy; obesity; dementia; schizophrenia; and heart, lung, kidney, and endocrine diseases that interfere with the normal physiologic or behavioural response to heat.^16,17

Extrinsic factors that increase heat vulnerability include medications and substances that act on the cardiovascular or nervous systems (Box 1) and affect cardiac output, sweating, fluid balance, perception of heat,²³ and behavioural response.

Increased exposure to heat: Exposure to heat can substantially increase heat-related illness. Some groups are at greater risk than others, such as labourers in construction, landscaping, and delivery services, as well as those who work around heavy machinery or those who are required to wear personal protective equipment. Young athletes and military personnel are susceptible to EHS due to rigorous conditions and pressure of top performance; yet, despite education around heat exposure, the incidence of EHS has been shown to be on the rise among high school football players and armed forces personnel in the United States.^24,25 Meanwhile, people whose work is precarious, such as people who work in the gig economy and migrant workers, may not have access to heat protections afforded to those with more stable employment.²⁶

People who live in congregate settings such as prisons, those experiencing homelessness, and older adults living in long-term care or unregulated apartment complexes may not have access to mechanical cooling due to cost. Cities are prone to having urban heat islands²⁷ that can have temperatures up to 12°C hotter than in nearby neighbourhoods with adequate tree cover.²⁶ Of note, populations in urban heat islands tend to have low incomes and be racialized.²⁸

Returning to the 2021 heat dome event example, 98% of deaths occurred indoors. People living in neighbourhoods without adequate tree cover were at increased risk of death, as were those living in poverty.²⁹

Barriers to accessing resources: People who lack adequate access to information and other resources face higher risks of heat-related illness. This includes people with disabilities or language barriers as well as those experiencing homelessness or who are socially isolated (this was an important risk factor during the British Columbia heat dome event).²⁹

For some the risks are multifactorial. People living with schizophrenia during the heat dome event had an odds ratio of death of 3.07, likely a combination of physical vulnerability, increased exposure to heat, and barriers to accessing resources.³⁰

How can family physicians help protect vulnerable patients? Family physicians are well positioned to help patients avoid heat-related illness due to their ongoing relationships with them and their holistic approach to care. In general, family physicians are trusted by patients as a source of reliable health information. Since many patients do not see themselves as being vulnerable to extreme heat,³¹ family physicians can reinforce important messages about heat safety from sources such as public health agencies.

Step 1. Identify individuals at increased risk: Family physicians can flag individuals at increased risk in electronic medical records or simply post a list of risk factors in patient examination rooms.

Step 2. Counsel at-risk patients: Counselling should include written recommendations to send home with patients. PreparedBC has a plain language extreme heat guide that can be emailed or printed and given to patients.³² Box 2 outlines strategies family physicians can suggest to patients to decrease their risk of heat-related illness. Family physicians should reinforce that announcements about heat events on the television, radio, or weather applications are relevant, regardless of whether the individual has access to mechanical cooling.

Counselling may also be directed toward caregivers or family members of individuals at risk. There are resources that can be provided to caregivers, such as the National Collaborating Centre for Environmental Health’s guide to performing a heat check (Figure 1).³³

• Download figure
• Open in new tab

Figure 1.

Rapid risk assessment checklist for use during extreme heat events

Step 3. Advocate: Advocacy can take many forms, from advocating for individual patients to advocating on a regional, national, or global scale. At the individual level, family physicians can advocate for patients with disabilities to receive funding for purchasing an air-conditioning unit from disability support programs. For example, in Ontario, patients enrolled in the Ontario Disability Support Program are eligible for funding for an air conditioner for health reasons.³⁴ Some patients who receive benefits from Ontario Works, the province’s non-disability social welfare program, are also eligible.³⁵

Family physicians can also take action at a regional level by supporting community organizations calling for maximum indoor temperature regulations³⁶ or for the protection of green spaces in urban areas. Tree-planting efforts in urban spaces can help decrease peak neighbourhood temperatures, thereby providing nearby immediate health benefits to these neighbourhoods and their inhabitants.

At the highest level, clinicians should advocate for systems-level action regarding the climate crisis. Burning of fossil fuels should be named as an upstream determinant of poor health, and health workers should join calls for the rapid decarbonization of the economy.