What does a heatwave look like in the health data?
A heatwave is usually described in temperature. UKHSA's health data for England shows what those days look like from the other side.
By Sonia Leung
A heatwave is usually described in temperature.
But temperature is only one part of the story.
I looked at UKHSA syndromic surveillance data for England and compared heat-related health activity with daily temperatures between July 2025 and July 2026.
The result was clear: heat-related NHS 111 calls were concentrated around a small number of hot periods rather than spread evenly through the year.
Three significant peaks occurred when maximum temperatures went above 30 degrees. The largest was on the hottest day in the data, when the temperature reached 32.8 degrees.
The health data therefore gives us another way to think about heatwaves.
We are not just experiencing a warmer day.
We are seeing a change in the number of people asking for help.
UKHSA explains that the body loses heat by sweating and by moving more blood to the skin. High temperatures can also put additional pressure on the cardiovascular and respiratory systems.
But there is an important gap in the data.
We know what happened outside. We do not know what happened inside.
A temperature recorded at a weather station does not tell us whether somebody’s bedroom stayed cool overnight, or whether a living room became unbearable during the afternoon.
That depends partly on the building.
The hidden temperature inside our homes
The way a home responds to heat depends on many things.
A large south-facing window can let in a great deal of solar heat. External shading can stop much of it before it arrives. Ventilation can remove heat that has already built up, but only while the air outside is cooler than the air inside.
Insulation adds another layer.
It helps keep a home warm in winter, and a well-insulated home can hold on to unwanted heat during a hot spell if there is no effective way to get rid of it.
That is a difficult design problem.
We want our homes to hold heat when it is cold and release it when it is hot.
Heat and cold are not the same problem
The cold data in the same analysis shows a different pattern.
Cold-related A&E activity is more persistent across the seasons and includes fractures, hypothermia and cold injuries.
Where heat produced three sharp spikes against a flat year, cold sits close to its expected level for almost the whole period. One day stands out: 377 attendances on 7 January 2026, the day after the winter’s lowest minimum temperature, at minus 5.8 degrees.
So the challenge is not to trade winter protection for summer protection.
It is to find ways for homes to do well in both.
The next question
The health data can tell us when weather-related activity rises above what would normally be expected.
It cannot tell us which homes became too hot, which interventions worked, or how much difference shading, ventilation or insulation made.
Those are building questions.
They may matter more and more as extreme heat becomes a familiar part of a British summer.
Where these figures come from
Daily readings from UK Health Security Agency climate and environment syndromic surveillance for England, compared against UKHSA’s published expected-activity baselines. Temperatures are Met Office Hadley Centre Central England Temperature daily figures, a central England series used as the standard national proxy for England.
Three of the 365 days carry no published expected figure, so the share of days above the expected level is calculated across the 362 that do. The cold measure referenced above covers female attendances only, and is not the cold-related impact across the whole population.
Questions
- What is syndromic surveillance?
- A count of how many people contacted a health service with a particular kind of complaint. It counts people seeking care, not confirmed cases, so it moves with how worried people are as well as with how ill they are. That is a feature rather than a flaw. It is fast, and it shows pressure building days before a diagnosis would.
- Does a hot day always mean more calls?
- No. Calls rose above the expected level on just under half the days that had an expected figure published, and the year's three big peaks all landed on spells above 30 degrees rather than on any warm day. The peaks are concentrated, not spread.
- Why does the chart not show indoor temperature?
- Because nobody publishes it. A weather station records the air outside. Whether a bedroom stayed cool overnight depends on the building, and there is no national record of that at all, which is the gap this piece is about.