Days of stifling heat and muggy weather can be unpleasant for anyone. But for those with certain medical conditions, they can pose a more serious threat.
Studies show that people with chronic illnesses—for example, cardiovascular, respiratory, metabolic, and neurological conditions—are more likely to go to the emergency department or die during a heat wave.
In some cases, this is because people with pre-existing health problems may be more susceptible to heat exhaustion or heat stroke because certain body systems, such as the heart, kidneys and nerves, are not functioning optimally, explained Robert Meade, a researcher at the Harvard TH Chan School of Public Health who studies the effect of heat on vulnerable groups.
But heat stroke can also cause relapses or aggravate existing symptoms, even if you’re not close to overheating, Meade explained. To illustrate with a swimming analogy: if the body functions as if it were floating, the heat is like someone hitting it with a brick.
In that case, each system works harder to stay afloat, which can trigger health problems ranging from serious discomfort to life-threatening situations.
We consulted experts about the conditions that research shows are most often aggravated by heat, and how to manage flare-ups this summer.
Arthritis
Research on osteoarthritis (a breakdown of articular cartilage) and gout (a form of inflammatory arthritis that often begins with severe pain in the big toe) suggests that extreme temperatures can aggravate symptoms. Heat, especially combined with humidity, can increase inflammation, leading to more painful and swollen joints in people with arthritis, according to Aerial Petty, medical director of primary care at Morris Heights Health Center in the Bronx.
Gout can get worse in the heat for a different reason: Dehydration allows uric acid, a natural cellular waste product filtered by the kidneys, to concentrate in the blood and form more uric acid crystals in the joints, causing the pain of gout.
Arthritis can also be worsened by humidity, which can cause tissues within the joints to expand and put pressure on nearby structures. A 2019 study showed that among various weather conditions, humidity had the greatest effect on pain intensity in more than 2,000 people with chronic pain conditions, primarily arthritis.
Kidney stones and kidney disease
Medical records suggest that higher-than-average temperatures are linked to an increase in kidney stone diagnoses, which has been primarily attributed to dehydration: fluid loss causes urine to be more concentrated in minerals that can crystallize and form stones. In people prone to them, the mineral buildup can clump with existing stones, increasing the chance that a small stone will grow, dislodge, and block the passage to the bladder, causing severe pain.
There is also evidence that greater heat exposure is linked to a more rapid decline in kidney function in people with chronic kidney disease (CKD). The cooling mechanism that sends more blood to the skin during heat exposure leaves less available for internal organs, particularly the kidneys, Meade explained. Therefore, if you have poor kidney function, decreased blood (and oxygen) supply can push your kidneys into a “risk zone,” he added.
In people with chronic kidney disease, the kidneys are also less able to regulate fluid balance, explained Lauren Siewny, medical director of the Emergency Department at Duke University Hospital. Therefore, sweating can accelerate dehydration or, on the contrary, cause fluid retention, which leads to a dangerous electrolyte imbalance.
Asthma and COPD
If you have asthma or chronic obstructive pulmonary disease (COPD), cold air may irritate your airways. However, warm, humid air can be just as problematic for some people with these conditions. Various studies have recorded an increase in emergency room visits, hospitalizations and exacerbations related to these diseases during high temperatures.
Breathing hot air, whether dry or humid, is thought to cause narrowing of the airways, increasing the risk of an asthma attack or COPD exacerbation.
Periods of intense heat also often coincide with poor air quality, Siewny added. The combination of sunlight, heat and stagnant air promotes the increase of ground-level air pollution such as ozone, and humidity creates the ideal environment for the proliferation of mold spores and dust mites. All of this can irritate the respiratory tract and cause an outbreak.
Lupus
Research on heat and autoimmune diseases is still in its early stages, but emerging evidence suggests that lupus, or systemic lupus erythematosus, a condition in which the immune system attacks healthy organs, may be particularly sensitive to heat.
A 2020 study found that lupus flares related to skin rashes, joints, and blood in 1,200 patients coincided with warmer days, and the incidence of joint flares also increased when humidity was higher.
One possible mechanism is inflammation, which research shows can increase throughout the body with high temperatures and humidity. In patients with lupus, this can lead to increased swelling and pain in the joints, as well as increased fatigue, Petty explained.
Petty added that many people with lupus also have increased sensitivity to sunlight, where even a small amount of exposure can cause rashes, making it especially crucial to be diligent about sun protection.
Diabetes
Hot days have been linked to a disproportionate number of hospital admissions for both type 1 and type 2 diabetes, and heat waves to increased diabetes morbidity and risk of mortality.
Heat can make it difficult to control blood sugar, as it can cause fluctuations in both senses, Siewny explained. On the one hand, faster circulation and dilation of blood vessels could speed up insulin absorption, causing hypoglycemia. On the other hand, dehydration could concentrate sugar in the blood, causing hyperglycemia.
Because of the increased unpredictability, Siewny said it’s critical for people with diabetes to monitor their blood sugar levels more frequently as temperatures rise.
multiple sclerosis
It has long been known that in people with multiple sclerosis — a disorder in which the immune system destroys the myelin sheath that covers nerve fibers — an increase in body temperature due to exercise can exacerbate symptoms. Recent research has shown that this sensitivity to heat, known as Uhthoff’s phenomenon, also tends to occur in hot climates.
Heat hinders the transmission of electrical signals along damaged nerves, which can cause a sudden increase in symptoms such as fatigue, muscle weakness, numbness, and cognitive problems. These symptoms usually disappear when the body cools and do not indicate a real relapse of the disease.
heart disease
In hot weather, the body sends more blood to the blood vessels in the skin, which dilate, allowing heat to escape and sweat to evaporate. This process also requires the heart to pump harder, Meade explained.
This can be problematic for someone with coronary artery disease, which occurs when plaque blocks the arteries that supply blood to the heart. “There can be a mismatch between what the heart needs when it works harder to withstand heat and what the body is able to provide,” Siewny said.
That increases the risk of suffering a heart attack or stroke, both of which are more common during heat waves.
People with heart failure, who often take diuretics to control fluid retention, are at increased risk of dehydration in the heat. According to Siewny, this, added to the increased burden on the heart, could aggravate heart failure or even cause kidney damage.
Migraine
While studies on temperature and migraine have yielded inconsistent results, research suggests that seasonal changes and fluctuations in temperature and atmospheric pressure, in any direction, they can trigger seizures, possibly due to the way blood vessels and nerves adapt to those changes.
However, summer can pose a particular threat to migraines; for example, because of how heat increases air pollution at ground level (a trigger in itself). The intense glare of the sun can also trigger an attack by activating the nerve involved in migraine pain. Additionally, there are the additional risks of dehydration and sleep disorders, which can also contribute to exacerbations.
Depression and anxiety
Hot days lead to higher rates of mental health-related emergency room visits and even suicides, putting people with depression or anxiety at particular risk.
Since a wide variety of factors can influence mood, there are several possible explanations. Physiologically, activating the body’s stress response in the heat can increase anxiety. Psychologically, the feeling of not being able to control or escape the heat could play a role, according to Meade. Heat waves can also cause sleep disorders, which is known to aggravate depression and anxiety, said Lorenzo Norris, an associate professor of psychiatry and behavioral health at George Washington University School of Medicine and Health Sciences.
Additionally, people taking antidepressants, such as selective serotonin reuptake inhibitors, may be more susceptible to dehydration and overheating because these drugs impair temperature regulation, Norris added.
How to control heat symptoms
Regular measures to stay cool are even more important for people with chronic illnesses. Stay hydrated. Turn on the air conditioning at home, if you have it, or use fans or spray water bottles, Meade said. Petty also suggested taking a portable fan outdoors, wrapping a cool cloth around your neck, and wearing breathable clothing.
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It is also advisable to make an appointment with your doctor now, in anticipation of a possible outbreak, especially if you have not visited your doctor in a while. This way you can prepare for the heat wave and review your medications, as some may have a different effect in high temperatures, Meade explained.
Also, avoid others triggers that may be more under your control, such as skipping meals or getting too little sleep, so they don’t build up, Petty said.