Heat can be dangerous for anyone. But heat is not equally dangerous for everyone. 

For people living with a chronic condition, a hot day can do far more than make them uncomfortable — it can strain the organ or system already under pressure. 

Here’s who is most vulnerable and how to protect yourself and your loved ones.

Cardiovascular disease

To get rid of heat, your body sends more blood to your skin. This requires your heart to pump harder to keep blood pressure up, raising the workload on the left ventricle.

This can make things harder for those with cardiovascular disease (CVD) through dehydration, blood clots and electrolyte imbalance. High humidity and polluted, poor air quality — which is made worse by high heat — also add to the effects.

All of this combined can lead to heart failure, heart attacks, stroke, arrhythmias and trigger acute coronary syndrome (ACS) and acute myocardial infarction (AMI).

• What it feels like: Chest pain, shortness of breath, a racing or irregular heartbeat, dizziness or swelling.
• What helps: Check the U.S. Centers for Disease Control’s HeatRisk forecast daily and act when it hits orange or higher. Click on the Breathing link on The Weather Channel app to see your Air Quality Index and act when it is above 100.

Respiratory conditions (asthma and COPD)

Breathing hot, humid air can promote already-inflamed airways and then constrict them on top of that, according to the American Lung Association. 

Heat and sunlight also form ground-level ozone. This pollution similarly affects airway muscles and inflames the lung lining, almost like a sunburn. 

(MORE: 'Like sunburn for your lungs:' How heat degrades the air you breathe)

• What it feels like: Worse wheezing, coughing, chest tightness and shortness of breath, sometimes at lower pollution levels than others feel it at.
• What helps: Limit time outdoors when ozone and heat are high and keep rescue medication on hand. Check the AQI before heading out — click on the Breathing link on The Weather Channel app to get that info.

Kidney disease

As blood is diverted to your skin and muscles to cool the body, blood flow to the kidneys — and often other organs — drops. 

(MORE: What heat waves and climate change mean for kidney stones)

Combined with dehydration, this can cause renal hypoperfusion, ischemia and a higher risk of acute kidney injury. Recurrent heat-related injury can progress to chronic kidney disease.

• What it feels like: Signs of dehydration and loss of kidney function — reduced or dark urine and fatigue.
• What helps: Rehydrate with water, not sugary drinks — fructose-containing fluids can actually worsen dehydration-related kidney damage. 

Diabetes

Heat changes how the body uses insulin, and diabetes-related nerve and blood-vessel damage can impair sweat glands, so the body cools less effectively — raising heat-exhaustion and heat-stroke risk.

Sweating is your body’s way of cooling itself, but it also drives dehydration. This raises your blood sugar, which causes more urination, and that causes more dehydration — a terrible cycle if you’re not replenishing your fluids.

(MORE: Traveling with diabetes in hot weather: Tips for staying safe and healthy)

• What it feels like: Feeling shaky, fatigue, nausea, lightheadedness and heavy sweating — which overlap with low blood sugar.
• What helps: Check your blood sugar with a glucose meter if you start feeling symptoms. It’s a good idea to test more often in intense heat. Heat also damages insulin, medicine, test strips and devices — so keep your supplies cool and never leave them in a hot car.

Mental health and neurological conditions

Studies show many psychiatric drugs disrupt temperature control. Many antipsychotic, anticholinergic, antidepressant, sedative, mood-stabilizing and nervous system medicines — including some to treat Parkinson's disease — can increase heat vulnerability by inhibiting the natural thermoregulatory processes of your body.

Some can impair sweating, reducing your body’s ability to get rid of heat and increasing the vulnerability of those on these types of medication in heatwaves. Thirst perception may also be affected, increasing the risk of dehydration since you’re less naturally inclined to drink water.

(MORE: Heat anxiety is real: How high temperatures affect your mood)

Rising temperatures can impact your mental health by increasing stress hormones, disrupting sleep and making it harder to stay emotionally regulated.

• What it feels like: Overheating without the usual warning of sweating. Irritability, impulsivity, trouble concentrating and poor sleep can also be the psychological consequences of extreme heat, according to the American Psychological Association.
• What helps: Stay hydrated even if you’re not thirsty and talk with a provider about which medications you’re being prescribed. Don't stop a psychiatric drug on your own.

Pregnancy

Heat in any trimester is linked to preterm birth, stillbirth and low birth weight. And in the first trimester, heat may raise the risk of some birth defects. Extreme heat can cut placental blood flow and increase dehydration.

(MORE: How to keep cool during a summer pregnancy)

As little as one day of high heat may increase risk, according to the CDC.

• What it feels like: Pregnancy already raises body temperature and fluid needs, so overheating and dehydration come on faster. 
• What helps: Put in place increased breaks to cool down, hydrate and use a bathroom. If you work in a hot environment, you may be able to request reasonable accommodations.

Multiple sclerosis

As much as 80% of individuals with multiple sclerosis (MS) experience heat sensitivity.

Uhthoff phenomenon is a temporary worsening of neurological function for those with MS due to increases in core body temperature. The flare-up is temporary, usually under 24 hours, reverses with cooling and does not mean there is new nerve damage, according to MS Society.

• What it feels like: Blurred vision, fatigue, weakness, slurred speech and balance or coordination trouble when overheated — from weather, exercise, a hot shower or stress. 
• What helps: Active cooling — cold showers, ice packs, cold drinks and cooling vests, plus pre-cooling before activity. The Multiple Sclerosis Association of America helps cover cooling-equipment costs.

Migraines

While the science is limited, more than one-third of people with migraines report weather triggers, including hot or humid conditions, according to the American Migraine Foundation.

(MORE: Understanding the weather-migraine connection)

On top of that, heat effects often work through dehydration, which about a third of people with migraines name as a trigger on its own. 

• What it feels like: Throbbing head pain, with triggers that stack — hot weather plus stress, hunger or missed sleep raises the odds.
• What helps: Carry water and stay ahead of dehydration, run errands before peak heat and keep rescue medication handy.

Staying safe in the heat: Remedies that help almost anyone

• Drink water regularly, before you feel thirsty — about a cup every 20 minutes in the heat. Skip sugary, caffeinated and alcoholic drinks. Dark yellow urine means drink more.
• Find air conditioning, even if you're away from home — a mall, library or a car with AC.
• Plan for power outages if you depend on refrigerated medicine or powered medical devices.
• Check on at-risk people twice a day — older adults, people who live alone, pregnant women and children.
• Never leave a child or pet in a parked car — interior heat turns life-threatening within minutes.
• Know heat exhaustion vs. heat stroke. Cramps, heavy sweating, headache, nausea, dizziness and cool, clammy skin signal exhaustion; if it doesn't improve within an hour of rest, shade and fluids, get help. Heat stroke is a medical emergency.

Content writer Ada Wood enjoys exploring the stories that science and climate teach us about our natural world and how it influences the way we live in it.