Heat and Dehydration in Older People: Why London Summers Can Be Dangerous
By Dr Daniel Gordon • 27, Jun 2026
Each summer, I see older and vulnerable patients in clinic who had been managing well suddenly begin to struggle in the heat.
They may feel dizzy when they stand, become unusually tired or confused, lose their appetite, or drink far less than they need. Someone living with a stable heart or lung condition may also notice worsening breathlessness, swelling or general weakness.
These changes can develop gradually and are easily mistaken for an unavoidable effect of hot weather. But older people are less able to regulate their body temperature and may not feel thirsty until they are already dehydrated. Heat can also place additional strain on the heart, lungs and kidneys.
Here is a practical guide to help you recognise the warning signs and know what to do.
Why heatwaves are dangerous for older people
Our bodies cool themselves mainly through sweating and by sending more blood towards the skin.
As we get older, these cooling mechanisms can become less effective. Older adults may sweat less and feel thirsty much later, so they may already be dehydrated before realising that they need a drink.
Many vulnerable patients also live with conditions that reduce their ability to cope with heat, including:
Heart disease
Chronic obstructive pulmonary disease (COPD)
Asthma
Kidney disease
Diabetes
Dementia
Parkinson’s disease
Reduced mobility
Certain medicines can also affect hydration, blood pressure, sweating or kidney function.
UK guidance identifies people aged 65 and over, those with heart or breathing problems, people taking certain medicines and those unable to adapt their environment as being at greater risk during hot weather.
Why London heat can be particularly difficult
London buildings and roads absorb heat during the day and release it slowly overnight. Upper-floor flats and poorly ventilated homes can therefore remain very hot, even after temperatures outside begin to fall.
Older people who live alone or have limited mobility may find it harder to cool their home, move to a cooler room or get outside safely.
Even lower-level heat alerts can pose a serious risk, particularly for older adults.
Signs of dehydration in older people
Dehydration happens when the body loses more fluid than it takes in.
Early signs can include:
Dry mouth or lips
Headache
Tiredness
Dizziness
Dark, strong-smelling urine
Passing urine less often
Weakness
Poor appetite
Constipation
In older or vulnerable people, dehydration may also cause changes in behaviour.
They may become confused, sleepy, irritable or unusually quiet. They may struggle to stand, stop managing their normal routine or become more likely to fall.
A sudden change in confusion should not simply be blamed on age or dementia. Heat and dehydration can cause it, but other problems such as infection or medication side effects may also be responsible.
Why dizziness is more common in hot weather
Dizziness is one of the most common problems during a heatwave.
When you become dehydrated, your blood pressure can fall, especially when you stand up. You may feel light-headed, unsteady or even faint.
For older people, the biggest risk is falling.
Although heat and dehydration are common causes, ongoing or severe dizziness should not simply be blamed on the weather. It can also be caused by medication, heart problems, infection or other medical conditions. For that reason unless you are certain the symptoms are due to dehydration, dizziness should always be assessed by a healthcare professional.
Signs of heat exhaustion
Heat exhaustion happens when the body becomes too hot and loses too much fluid.
Common symptoms include:
Heavy sweating
Intense thirst
Headache
Dizziness
Feeling very weak or tired
Nausea or vomiting
Muscle cramps
Pale, clammy skin
Fast breathing or a fast pulse
Confusion or irritability
Older people may not describe these symptoms clearly. Instead, they may become sleepy, withdrawn or unsteady on their feet.
Help them move to a cool place, remove extra clothing and cool their skin with water, damp towels or a fan. Encourage small amounts of cool water if they are awake and able to drink.
Most people should start to feel better within 30 minutes.
When heat becomes an emergency
Heatstroke is a medical emergency.
Call your doctor immediately if someone:
Is still unwell after 30 minutes of cooling down
Has a very high temperature
Has hot, dry skin
Becomes confused or behaves unusually
Has rapid breathing or a very fast heartbeat
Has a seizure
Collapses or loses consciousness
You should also seek emergency help for severe breathlessness, chest pain or signs of a stroke.
How heat affects heart and lung conditions
Hot weather makes the heart work harder and can make breathing conditions worse.
People with heart disease may notice:
More tiredness
Dizziness
Palpitations
Swollen ankles
Increased breathlessness
People with asthma or COPD may also find their symptoms become worse, especially when heat is combined with high pollen or air pollution.
Do not assume worsening breathlessness is "just the heat". Chest infections, heart failure and blood clots can all develop during hot weather.
Medicines and hot weather
Some medicines can make people more vulnerable to heat and dehydration.
These include some:
Water tablets (diuretics)
Blood pressure medicines
Heart medicines
Antidepressants
Antipsychotics
Sedatives
Antihistamines
Parkinson's medicines
Do not stop taking prescribed medication without medical advice.
Instead, look out for dizziness, confusion, reduced urine, weakness or worsening symptoms during hot weather.
Simple ways to stay safe during a heatwave
A few simple steps can make a big difference:
Drink regularly, even if you are not thirsty
Keep water within easy reach
Close curtains or blinds during the hottest part of the day
Open windows when it becomes cooler outside
Stay in the coolest room in the house
Avoid going outside between 11am and 3pm
Wear light, loose clothing
Use a fan or cool damp towels
Eat light meals and foods with a high water content
Limit alcohol
Checking on an older person
If you are checking on an older relative, friend or neighbour, ask more than "Are you OK?"
Try asking:
Have you had enough to drink today?
Have you been to the toilet today?
Do you feel dizzy when you stand up?
Is your breathing worse than usual?
Have you eaten today?
Have you taken your medication?
Also look for changes. Are they more confused than usual? More sleepy? Less steady on their feet? More breathless?
A short daily visit or phone call during a heatwave can make a real difference, especially for someone who lives alone.
The key message
Older people do not always say they are too hot or thirsty.
Instead, they may become tired, confused, unsteady or more breathless than usual.
During a heatwave, pay attention to changes from their normal routine. Spotting these early, encouraging regular fluids and keeping them cool can often prevent a much more serious illness.
Disclaimer
This blog post provides general information only. It is not intended to provide instruction and you should not rely on this information to determine a diagnosis, prognosis or course of treatment. It should not be used in place of a professional consultation with a doctor.
The medical information is the personal opinion of the stated author(s). It is based on available evidence or, where no published evidence is available, on current medical opinion and practice.
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You should not disregard the advice of your physician or other qualified healthcare provider because of any information you read on this website. If you have any health care questions, please consult a relevant medical practitioner.
