How does heat affect our health?

This BBC podcast is supported by ads outside the UK. Hello and welcome to the Evidence from the BBC World Service. I'm Claudia Hammond and in this series we take a single health issue and we delve deep into the research behind it. And we've come to the Sofia Science Festival in beautiful... Sofia, Bulgaria, where I'm joined by a live audience. Do say hello, audience.

Thank you for that. Now, the weather has been really nice and sunny since I've been here. It's been a lovely temperature because it's not too hot. But we have seen hundreds of air conditioning units on the back of buildings, and I know that it actually can get very hot here. in Bulgaria, to the point that the country experienced the second highest number of deaths related to the heat in Europe in 2023. Only Greece had more. Now of all the extreme weather that we see, heat...

is the most deadly. In the US, for example, it kills more people in an average year than hurricanes, tornadoes and floods combined. And it's not just in Europe and the US. As a result of climate change in many parts of the world, the number of of people dying from the heat is on the rise.

So what we want to know is why our bodies sometimes react so seriously to high temperatures and what the best ways are to stay safe and healthy when the heat hits. Well, I'm joined on stage in Sofia by a panel of experts. who have some answers for us. Zornitsa Spasova is a climatologist at the National Centre of Public Health and Analysis here in Sofia.

Andreas Flores is professor of physiology at the University of Thessaly in Greece, and he's director of a laboratory which investigates the health effects of environmental factors, particularly heat. Petya Dimitrova is an associate professor of immunotherapy at the Stefan Angelov Institute of Microbiology in Sofia. And later on, I want to come to you, Petya, to talk about work specifically on how heat affects our immune systems.

And joining us from Varna on the Black Sea coast of Bulgaria is Elisaveta Petkova, an assistant professor of public health at Drew University in the United States. Now, thank you all so much for coming along. And actually, I want to start with...

audience first of all a poll can you shout yes if you like the heat Oh, not many people there. A few people like the heat. And can you say no if you're someone who doesn't like the heat and you prefer it when it's cooler and you like the shade?

Oh, that's very clear there. The people do not like the heat here. So I'd like to know, has anyone here had a negative experience with heat? Is there anyone who has overheated or had a heat-related illness? Or maybe you know someone who has. Is there anyone who'd be... willing to tell us about that, any experience that's happened. Oh, we have somebody, yes. Hello. Hello. I have been overheated. I think I haven't had a heat stroke, but what I had experienced and actually...

Recently I had an episode when we had a heat wave. I'm outside and I'm doing tasks, but everything is hard to do. I don't really think straight and I feel hot, although I'm not sweating. So those are starting going into heatstroke territory. So I go back home, cold shower.

Cold room, but still the heat is persisting and I feel it for some time. So I think I've dodged the bullet, but just barely. So that's interesting because you realised that something was going on. You realised you weren't well. Yes. Yeah. Is there anybody else who's had an experience? Yes, there's somebody just here. Yeah, a very sad moment was when... My father's grandmother, she actually died of a heat stroke. Yeah, so the heat is a very serious problem.

These countries, Greece, Bulgaria, around the seas. Yeah, I'm so sorry to hear about what happened and thank you so much for sharing that with us. And Zenitza, I know this is something you've experienced yourself as well. Did these descriptions from the audience sound familiar? Absolutely familiar and unfortunately also my grandfather died in this heat wave. I haven't shared this with you. And for me, I was, I think, very close to that because...

I have never felt so badly. My big mistake was that I took a coffee. I had a headache and I thought that maybe I have a low blood pressure, but unfortunately I was wrong. And I took a coffee and then this headache was even stronger and I felt difficult to breathe. I felt very dizzy. And my skin was so hot. I remember my skin was so hot. And what helped me to survive was that I came back home, but in the car I put water on my body.

So this evaporation of the water took the heat out from me. And that's how maybe I survived. And maybe a few months after this, I had dizziness every day. Every day? A few months later? Absolutely. Like all the summer, every day I had dizziness. And then every summer, when it comes, I feel more vulnerable to heat than before, after this experience.

So Andreas, what is happening in our bodies when we overheat? Why would we have these symptoms that we've heard about? Well, the main system that we use to reduce our temperature is sweat. And when the environment gets hot, and also when it becomes humid, sweat evaporation doesn't work. So the sweat accumulates on our clothes, it drips on the floor.

And it doesn't cool us anymore. We keep sweating, we keep it dehydrating, but it does not work to cool us down. As a result, to address this, our body diverts more blood to the skin to cool us down. But this is not as strong of cooling effect. And also it robs our brain, our gut, and our muscles from very valuable blood and nutrients and oxygen.

And as a result, our brain starts to not work so well as before. We start having dizziness. We can't think very well. We can't analyze and strategize how to solve difficult situations. And often we get into accidents. And soon later, our court temperature starts rising, and we collapse and have a heat stroke.

And can we get some definitions straight, Andreas? Is there a difference between heat exhaustion, sunstroke, and heat stroke? You hear all those three phrases. Well, heat exhaustion is the more mild form. Typically, our temperature there is around 38, 39 degrees Celsius. Still, our body functions more or less well. It's trying to cool us down.

So we have a lot of sweating, and our brain functions well, our blood system works well, our nervous system works well. But if the conditions are really hard and it... Our thermoregulatory system doesn't help to cool us down, then core temperature will increase. And then we will start having symptoms of heat stroke. Heat stroke is a condition where temperature rises beyond 39 or 40 degrees Celsius.

Our sweat stops functioning. We often have very hot but very dry skin. And most importantly, the Achilles tendon, as we say in Greece, so the most vulnerable link. is our brain. Our central nervous system collapses. We fall on the floor. We have coma. We have seizures. We cannot communicate with the environment, and that's what we call heat stroke.

So Elisa Vetter, what are the warning signs that we could look out for that would indicate we're dangerously overheating? Thank you for asking this question. So in the beginning, when the body starts overheating... A person may feel a little tired or dizzy, and they may also start feeling thirsty. And then I was very sad to hear some of the experiences that the audience... But once a person already experiences confusion...

dizziness, inability to think very well. This is already an extremely dangerous situation and this is where a heat stroke may be indicated. And Petya, what should we do if we start feeling those symptoms? Well, what we can do is first to drink water and to try to decrease our activity. So to sit down in a quiet space, under the shadow, in order to calm down and to turn on our thermal regulation. because the term regulation is this system which controls and synchronizes all other systems.

It synchronizes body temperature with the cardiovascular fitness, with the blood flow. It also affects our respiratory system and even immune system. And why is it, Andreas, that people seem to be more vulnerable if they've already experienced some heat-related illness once, that it then might happen again, or even for a longer period, because it's feeling dizzy a few months later? Well, having heatstroke means that you had...

serious or mild form of organ dysfunction. This means that your gut blood barrier has been damaged a bit. Possibly there's also slight damage in the hypothalamus, the part of the brain that regulates temperature. And as a result, you're more sensitive to heat. So if you're exposed to even less dangerous conditions, it's more likely that you will have it again.

And Petya, there is research showing that heat can have an effect on our immune system as well, and this is very much your area. What does heat do to the immune system? Well... There is a very complex interplay between the body temperature and immune system. Actually, if the heat is coming from endogenous source like inflammation, then the heat is still related because...

we would like to eliminate the pathogen by the heat. So the temperature between 38 and 39 degrees is tolerated by immune system, immune cells are activated, and they... like this temperature, actually. But if the temperature is very high, over 39, about 40 degrees Celsius, then... there is a threat of some immune cells that are very important for our body, the lymphocytes. So we see that in every kind of stroke when we have a very good stress on the body.

acute stress on the body can induce dying of immune cells. And this is a problem with the heat. You can lose immune cells if you have a heat stroke. And I guess the problem is then you can have too much inflammation, too big a response from the body, and then that's not good for the body. Yes, yes. And also it's the case that, for example, one...

Part of the innate immune system, which is a soluble element in the immune system, is very close to the coagulation system. They regulate each other. And when there is a very high temperature, the coagulation system changes. and this changes and activates the innate immune system. That's why, for example, always the people said... that the heat can activate actually innate immunity and also acquired immunity.

And so it sounds as though heat can affect us in lots and lots of different ways. I've got a question for our audience, actually. I'd like to know where you spend your time in the heat. Where were you when you last got really hot? I don't know, maybe you work out. or maybe you make sure you're always at home with aircon. I'd be interested to know what you do in the heat, where you might be. Can anyone tell us where they last were when they got really hot?

Yes, there's somebody at the back first, and then we'll come to here. So when it's really hot outside, I try to stay indoors. where there is air conditioning. And of course, I try to stay hydrated and not to go outside very much. Yeah, that's interesting that you stay indoors. Anybody else? yes there's somebody just there the last time I was really hot was on a bus public transport was there air conditioning no open windows

Open windows, no air con, and it was really hot. I actually loved the heat, and that was too much. Was it one of those situations where you get the hot air coming in the windows of the bus? Yes. Not very nice. Anybody else? want to tell us where they were when they were hot? Yes, there's somebody just there. It bothers me the most when I'm outside and the air is not moving and there is not. a proper shade, like three shade.

A few times I have experienced, so there is some shade, but it's shade from a building. So you go there and it doesn't do much to you. So without trees and without... Wind is the place where I have felt the hottest. Yes, it's not nice when you're in those situations like being on a bus or if you can't get any shade where you just can't get cool.

Now, Andreas, weirdly, it seemed that more people died on the days where the heat was moderate rather than where the heat was really high, which just seems really odd. Well, first of all, because the moderate heat days are more. In fact, in a recent International Labour Organization report that we published, and I had the privilege to be the lead author, we found that 90% of heat exposure happens...

in non-heatwave days. So, in fact, if a policy addresses only heatwaves, it basically addresses 10% of the problem. So the non-heatwave days are more. That's why more people die during those days. Also, during heatwave days, there are policies to protect people. People get sensitive to this. They get a lot of information and advice.

And as a result, they're more likely to protect themselves. And also during these non-heatwave days, it's a lot more likely to... to have a person who has suffered an issue in their health the days before, during the heat wave, but they will actually pass.

They will die in the days after the heat wave, and that's why their death will be recorded later, but in fact the problem happened a lot earlier than that. I see what you mean. So it will seem as though it was happening on a day when the heat was more moderate, but actually it was caused by what happened.

happened when it was when the heat was extreme and so when we're looking at groups in Bulgaria who is the most exposed to heat where where are they the people who are most exposed I would say in Bulgaria, people who are living in the big cities, because of the effect of the heat island, that means in the city centre we have higher temperature than compared with the surroundings.

because of the concrete anthropogenic sources of heat. And a big part of the Bulgarian population, they don't have air conditioner at home. In the European Union, we are the country with the lowest number. of air conditioning per homes, like 46% it was the data. Also the old people, and we have compared with the European Union a higher percent of old people, so we have more people who are vulnerable because of the age.

Also, we have many people with chronically diseases. This is 42% of the population. And many of these diseases are so-called climate-sensitive diseases, like cardiovascular disease, for example. Also children, pregnant women and people who are homeless people, they have much higher risk to get hit stroke, of course. And the problem is we care about them.

when it's cold, but so far not when it's too hot. So this is another vulnerable group. And Elisa Vetter, what about globally? Who is most exposed to heat globally? So Zornitzel already provided a very good overview of groups that are exposed and there is an overlap with those groups globally. I would also add agricultural workers and people involved with manual labor.

who in many cases don't have control over their schedule and don't have access to breaks or going into a cool or shaded environment. So definitely outdoor workers. And also the homeless and people living in places where they live in very cramped housing complexes and they're not able to find any relief from the heat.

So in addition to what she mentioned, I would say those two groups are of particular importance globally as well. And workers were mentioned there. And I know, Andreas, you've looked at the effects of longer term heat exposure on people working outside. found that.

The most dangerous parts of heat exposure is the first few days and also the long-term effects, as you said correctly. The first few days you're a lot more vulnerable because you're not acclimatized, and that's why a heat wave in the beginning of the summer is much more dangerous than the same heat wave.

way towards August or towards the end of the heat season, then we get a little bit acclimatized, so we're not as sensitive to heat. But then if this happens for 20, 30, 40, 50 days or even more... because we become chronically dehydrated, when our blood passes through the kidneys to get filtered, because of dehydration, the blood is thicker, so it produces, it generates damage to the kidneys, and as a result, we're more likely...

to have chronic kidney disease or acute kidney injury. In fact, there's many sources of evidence showing that there's hundreds of thousands of people around the world who have chronic kidney disease and they are treated... in neuropathy clinics because of being exposed to heat for a prolonged period of time because they were working in a hot environment.

Yeah, so it's partly about adaptations, and we'll talk a bit more about that later on as well. Now, Zenitza, we've been mostly talking about heat in the daytime here, but how important are nighttime temperatures when it comes to how our bodies cope? Because sometimes if it's really hot, you just... can't get cool at night can you it has been found that you sleep best when the temperature is 18 19 degrees but over 20 and this is called tropical night it doesn't give your body to rest

to recover from the heat wave. Actually, the heat wave is when we have at least three days consecutive heat. So this increases the effect of the heat wave. If the heat wave is only with high daily temperatures, it's one thing, but if we have high night temperatures... Absolutely another thing. Yeah, so it sounds as though the night is a really important time for the body to recover. And Elisa Vetter, does it tend to be harder to keep cool at night in the city or in the countryside?

Definitely in the city, because the nature of the heat island effect is that the built environment tends to absorb more heat. And then this makes it very difficult to... to cool down during the evening hours. And when there is no relief from the heat after a very hot day, this is where physiological mechanisms for thermoregulation in... humans make it also difficult to restart the process in the next day. So definitely we are seeing much more of this during

heat waste in urban environments. Now, Andreas, I know you began your research career by studying how elite athletes perform in the heat. Do they deal with it better than us regular mortals? Are they fine and not at risk? Well, yes they do. And they do because they are selective. It means that if you are an elite athlete, it means that you can withstand very high temperatures. You cannot become an elite marathoner if you cannot cope with having a core temperature of 40, 41, 41.

42 sometimes degrees Celsius. If you can't, it means that you will not be able to perform as a result. Soon you will stop. training so much because you realize that you cannot cope with the others. So yes, they are selective and they are much better to cope with the heat also because exercise itself is a way to acclimatize to being hot all the time.

So it's both selective but also the impact of exercise. But studies on people in the military have shown they can still experience heat stroke, can't they? Absolutely. Also because in the military you have people who have a lot of masks and they carry a lot of equipment and they also wear protective clothing. So these are three factors that make them a lot more susceptible to having a heat stroke.

And of course, in some parts of the world, having extremely high temperatures is nothing new. And humans, in a way, have survived this for centuries. So does that suggest, Andreas, that the human body can cope, really? Yes, it does. We actually can tolerate, can acclimatize to heat a lot. We have very high physiological adaptation to heat, but of course there's limits.

That's why we see, for example, mortality rising in the Gulf countries at very high temperatures, beyond 30 degrees Celsius, whereas if you check mortality in Helsinki, for example, you'll see that it rises. at 20 degrees Celsius. However, at 20 degrees Celsius in Riyadh, in Saudi Arabia, people are actually quite cool and there's no increase in mortality. Thank you for that. So that is how the heat affects us as individuals.

But what can each of us and the societies we live in do about it? We are breaking for the BBC News now, but after that we'll be discussing how we can best deal with the heat to make sure it doesn't affect our health. I'm Claudia Hammond and I'll be back soon with Zunitsa. Spasova, Andreas Flores, Elisaveta Petkova and Petya Dimitrova for the evidence from the BBC.

The engines ignited, and it was like something being born. We had so much riding on something we'd never done before. 13 Minutes presents the Space Shuttle. Roger, copy that, Bruce. Coming soon. Bye.

Hello, I'm Claudia Hammond. Welcome back to The Evidence from the BBC. I'm at the Sofia Science Festival in Bulgaria with a live audience. And today we're talking about how heat can affect our health and why it can sometimes kill. On my panel, I have Zonitsa Spasova from the National Center of Public Health and Analysis here in Sofia, Andreas Flores, who's director of the FAME Laboratory in Greece,

Petya Dimitrova from the Stephen Angeloff Institute for Microbiology in Sofia. And from Varna on the Black Sea coast of Bulgaria, Elisaveta Petkova of Drew University in the U.S. is joining us. was one of the hottest summers there's ever been in Europe, and there were 70,000 excess deaths across 12 countries. Now, it is predicted that what was a rare event could happen every other year. Thank you.

saw more excess deaths due to heat than anywhere else in Europe. And Elisa Vetter, I'm interested to know, how do we measure that? Are we tallying up the deaths from heat that are recorded in hospital? How do we know how many people died? We have several different approaches when we're trying to capture the impacts of heat. Certainly, one way to do that is to obtain data where we have deaths that are directly attributable to heat.

It is often very difficult to actually obtain such data because those deaths are now specific and they're not documented properly very often. So when we try to analyze data that has been classified. as heat-related mortality or morbidity, we're usually underestimating the impacts of heat. So another approach is to just look at all-cause mortality and morbidity and estimate what the heat-related morbidity and mortality is over time in a specific location.

And Zuritsa, it's not only in Bulgaria, is it? Extreme heat is becoming more of a problem in many parts of the world. Yes, heat waste are increasing their frequency. They're becoming more long and more severe. That means the temperature, the maximum temperature is getting higher. But as you mentioned, in 2023, we have the second place of mortality here in Europe. I can say that last year was even hotter, much hotter actually than 2023.

And these results from mortality are not available yet, but I expect they will be quite bad. Actually, the mortality will be much higher. than this 2023. In Bulgaria, between 100 and 1,500 people die every year because of heatwaves, which compared with, let's say, mortality from car accidents is... two or three times higher, but it's not that much discussed in the society. One of the reasons because there is not very good statistics and because of this lack effect of the mortality.

And maybe because during the car crashes, more young people die. Well, here, more old people die. So maybe the society is more sensitive. And because it affects also children, also pregnant women, the society should be more aware of this problem. Yeah, that's amazing that it's two to three times as many people as die in car accidents. That's extraordinary.

So given that we're going to see more heat, what we need is some ways to stay cool and to stay healthy when the temperature rises. And I wonder in the audience if you've heard of some. What pieces of advice have you heard about how to stay safe in a heat wave?

then we'll break those down with our panel has anybody got any tips they've heard and we will we can find out whether they work or not so don't worry about what you suggest but has anybody heard any tips about how to stay cool yes we have somebody in the front row thank you One of the best ways to keep away from heat is to be in the natural world.

with natural shade and natural stream, river stream, I mean, or the best way, Black Sea, of course, or some other sea. Nice, very nice. Yes, and a couple of rows behind. Yeah, drinking a lot of liquids, given it's water, juices, but no alcohol. Thank you. So, Andreas, what do you make of some of these tips? Well, the three best strategies to protect yourself from the heat if you're starting to feel hot is, as we say, rest, shade, hydration. So get away from the heat, reduce your metabolic rate.

So rest. Shade. Try to find shelter from the sun because the sun we see more and more in recent research. that it's a lot more dangerous being under the sun than being hot in the shade. The sun affects our head and our central nervous system a lot more, so it's very dangerous.

And then hydrate, as many of you mentioned, is really important. And then if you want to add two more, I would say clothing, which means to regulate your clothing appropriately. So wear long-sleeved but light-colored, breathable fabrics. And then also schedule when you do physical activity. So if you need to be active in the day, schedule it early in the day or later in the afternoon. So avoid exposure.

Now, we've talked a lot about what individuals can do, but there are things that whole societies can do as well. And Zonica, many countries are putting heat warning systems in place. Do we know how good those are? How much of a difference do they make? For old people, they can double-diminish the mortality that has been found if you have this heat action plan. So they could halve the number of people who die?

Yeah, absolutely. So they're very effective. And one of the reasons Bulgaria has this high number of people who are dying during the heat is that we don't have a heat action plan, unfortunately. But we are involved in this heat. in this early warning system, also for heat. European called metal alarm. So we show these warnings, this yellow... orange-red coats. But for me, the trouble is that the treasure is higher than compared with, let's say, neighboring countries.

And this is important because when we have issued orange and red coat, it's forbidden to work outside. So for workers outside, it's very important what is this threshold. We have to know maybe with studies. As Elisaveta said, which is this treasure? Because it has been found that after a certain temperature, the temperature is increasing, the mortality is increasing very rapidly.

And for every geographical region, it's a different temperature. So for Bulgaria, if it's 32, we should not say we can issue this orange coat at 38, for example. So this is very important and this should be scientifically based. Yeah, Elisa Vetter, it's interesting how these threshold values vary. So it has to get to a certain temperature before an alert is issued. And I was looking at some of them. So it's 30 degrees in Belarus. It's 38 degrees Celsius in Greece.

degrees in Phoenix, USA. How are these worked out? And I guess it depends on how hot it is. You don't want a heat alert every day, do you? Absolutely. So as Andres mentioned earlier, those optimal temperatures are really location dependent because our bodies have a real capacity to adapt to the temperature. during a specific timeframe. So it would really depend on the specific location. And this is why we need to establish the optimal temperature threshold.

for each location that we are investigating. And we also need to establish those thresholds for alerts for each location where we are planning to develop a heat health warning system.

Now, one of the things we've talked about, Andreas, is finding shade or going to a cool place when it's too warm or to use air conditioning. So one solution might be to install lots, lots more air conditioning everywhere the warmer countries get. But, of course, then that will contribute to... emissions and that will increase the warming of the planet. Are there other ways to keep cool or to keep, say, whole cities cool that don't involve more air conditioning? Oh, absolutely.

Greening the city is by far the best strategy, so plant more trees. There's a lot of studies showing very big increases in temperature. in houses and offices where there's no trees around. So creating parks will definitely help reduce temperatures of cities. Also... If possible, whenever we expand cities, it's wonderful to align the roads with the local winds, changing the surface.

the color of the surface on the top of the buildings, making it more white. And especially nowadays, there's more and more very effective reflective paints being developed that can reflect 99% of the sun's rays, which may... the building much cooler indoors. So these are ways that we can use to make the cities a lot more climate friendly and reduce the urban heat island. And I've heard cooling centers mentioned, particularly in the US. Elizaveta, what are cooling centers? How do those work?

Yes, cooling centers are extremely important and they have been shown to be very, very effective in reducing heat-related morbidity and mortality. So cooling centers are a part of the heat health warning systems in many. cities such as Philadelphia, where actually the first heat health warning system was established in the 90s. And they're part of the response at the community level once a heat warning has been issued.

And once this happens, there are different locations in the city that are designated as cooling centers, which can be libraries. It can be various community centers where anybody can come in. And use the space to just get some relief from the heat. And their communications strategies to make sure that.

Everybody who is at risk knows where to find the closest cooling center. And there is also sometimes transportation that can be provided for vulnerable individuals to get there. So certainly cooling centers.

make a big difference and it will be very beneficial to create something like this in bulgaria as well so it's a bit like having a formal system of a place where you can go and get cool i have done cutting through shopping centers before air-conditioned shopping miles in order to get cool but um this sounds like a

more formal way of doing that. And of course, it's not just heat that is the worry as the world gets warmer, but infections and the bugs that can carry them also start to move to warmer parts of the world. Now, Petya, is heat going to have an effect? on how many different diseases we're exposed to? Yes. Unfortunately, there are data showing that... the heat can increase with 58% certain diseases that are vector-borne diseases, food-borne diseases, and also water-borne diseases.

And one problem in that is, first, because the population does not have a very healthy collective immunity. This is a problem because... Heating can compromise our immune system. And if more people experience the heat, then we will compromise the total collective immunity. This is the first thing. Second thing is that a lot of, for example, vector-borne diseases, they are transmitted much easily. And the reason...

To have so many vectorial diseases nowadays, and even in the Mediterranean region, there's a lot of cases, for example, of West Nile fever that is not... typical for this region because mosquitoes are not typical for this region. So one thing is that it is very important to know that also insects

can bear more pathogens. And this is a very interesting issue nowadays to investigate why this has happened. And one hypothesis is that insects, they have innate immune system, but this innate immune system... system can be compromised by the heat because it is pushing to becoming aged. So the insects actually cannot react with elimination of the pathogen, and they can tolerate the pathogen, and then you have more.

insects bearing actually the pathogens. So that's amazing. The heat is actually changing the immune systems of the insects, which means they're more likely to carry things, which we then get if they bite us. Yes. And this is also another problem might be that the heat and all the climate changes can...

change the pathogenicity of different bacteria and viruses. So they'll change the bacteria themselves and the diseases you get? Yes, because sometimes they can mutate and adapt, for example, in the individuals that experience... more heat, and then they will tolerate the heat inside of our body. So the immune system will attack by the heat, but will be not successful. So I would like also to mark something very interesting. There was an article in 2023 by the Chinese researchers showing that...

COVID-19 spreaded in the regions where there were heat waves. So they investigated how many new cases of COVID-19. are in June 2022 in different continents. And what they found out is that there's... correlation between the cases of COVID-19, new cases of COVID-19 and the heat waves. And this was particularly... visible in the countries like Morocco, Tunisia, where they have increased with 100 to 200 percent.

of the cases of COVID-19. And at that time, at this country, the temperature were very high. And when I read this article, I have decided to check if this is true for Bulgaria. And I find out that in July 2022, they were... many cases of COVID-19 because of the new variant. But at that time also, we had temperatures, for example, in Sofia between 35 and 36 degrees.

Do they have any theories about why this might happen? Why should a heat wave make a difference to the transmission of COVID-19? Well, they were wondering because... The heat and the UV light might destroy the virus. but instead we have a very fast transmission. So one explanation was that too much people were gathering together during the summertime, and also there's a lot of people traveling in different countries.

countries for vacation so this was the hypothesis but behind that maybe again that for example during the summertime our Immune system likes the warm conditions, so it's sensing the danger much easier than during the winter. So maybe, maybe, it's my hypothesis. The transmission of the disease is related with the fact that heat compromises exactly the dispreparedness of the immune system to react to pathogens. I see what you mean. And then that means you might get...

More on Will and Ela. Thank you for that. So it sounds as though heat can affect our health in more ways than we might imagine. I want to turn to our audience now to see what questions you might have for our panel. I know you've all experienced the heat, but what would you like to know from our panellists?

hand is a question already just there we already discussed the need to stay hydrated but i wanted to ask you at what temperature the drinks should be because we know that culturally in countries like the northern part of Africa. Thank you. make the hydration effective. That's such a good question, Andrea. Sometimes in the UK, sometimes in England, you hear people say, oh, a nice cup of tea, that'll cool you down. But is that true, or should we be drinking cold things, or very cold things?

Well, it can actually trigger sweating. So if you drink something that's really hot, it may trigger sweating, and this may in turn cool you down a little bit. However, most of the research evidence suggests that, especially if you're doing something like exercise or work in a hot environment,

that will last for a prolonged period of time, you should drink as cool drinks and fluids as possible. Even the best one you can do is actually, we call them ice slurries. So a crushed ice, so a drink that has... has a little bit of ice in it, that is the best way to cool yourself down if you're doing something that is prolonged in a hot environment. Thank you for that. Let's take another question. Yes.

Thank you. What is the more effective way for heating an apartment where a young person lives with climate or with a ventilator? So the question is, what's more effective to call an apartment where elderly are living, an apartment that has an AC or an apartment that has a ventilator? So for where older people are living? Did you say? Yeah. So in an apartment where older people are living, what's the best kind of...

way of cooling things. I can take this question, yes. Over 35 degrees, it has been found that only air conditioner can help. If you want to use ventilator, you should use this... icy, let's say, bottles, put ice in a cup. in a mug and put it in front so it can dry cold air. But after 35 degrees it has been found that only the air conditioner can help and it's my advice to have at least one room in your home.

which is air-conditioned. And to stay there at least three, four hours per day, this will be your goal, to cool yourself, because otherwise the heat is accumulating. So if you have a fan at over 35 degrees, does that just push the hot air around the room and then create more heat because of the motor of the fan? Yes, it will not help. It will be even worse, as you said, because it is put your...

To yourself, hot air. Thank you. Does anybody else have a question? Yes, there's one just there and there's one over here. Thank you. So I remember the summers of my childhood were nowhere as hot as they are now. So that, I mean, I'm not a scientist, but...

and I'm curious to know what the data is, but that to me suggests that we're not just experiencing heat, but generally hotter summer months. So I'm wondering, are there things that we can learn and borrow from countries that traditionally have? several sustained months of hot weather. And if there are any good behaviors or tips you can share with us that we can adopt for the summer months.

That's a good question. Does anybody have any tips on the panel from other countries, from what other places have done to adapt? Andres? I've been doing a lot of work with occupational groups around the world, and specifically we've done a lot of work in Arab countries and also in Sub-Saharan Africa and Southeast Asia.

There, people have been dealing with heat for centuries, really. And this is something that in Europe we just see the last few decades. So as you said, it's really important to learn from them. What we see is that they have one of the main cultural changes that has happened over there is when the day starts. It starts a lot earlier than in Europe. It starts at 6 a.m. And in the middle of the day, you have a prolonged break. There are countries that have...

prohibited working hours for outdoor activity for workers, so the entire economy shuts down for a few hours. In my mind, this is a little bit extreme. But it certainly is something that shows you how important it is to protect yourself in the middle of the day and be active in the beginning and in the end of the day. Also, clothing, if you consider the clothing that they wear in Arab countries and sub-Saharan Africa, is always...

loose fitting breathable light colored fabrics so this is also something that we can learn from them to protect ourselves because this is something that we'll see more and more of the heat aspect in Europe in the coming years.

Thank you for that. Now I want to end by asking each of you one very quick question, which is there are plenty of gaps in the research in this field. You've mentioned a few things where we don't quite know enough. What research would you most like to see done into heat and health? Well, the heat and immune system is something that has not been investigated enough, so we don't know exactly how the body reacts to the heat.

and the immune system, can we deal with the heat stroke by supporting the immune system? So we don't know so much about the heat and immune system. We only know that immune system is compromised and something is going wrong. But there's no enough study. And also there's no enough study showing... how the heat can affect the transmission of different infectious diseases. This is a key important point, and we have to figure out that. Andreas, what would you most like to know? Well, I think we...

We need to increase our efforts in studying how heat impacts workers. This is because workers are one of the largest population groups that are affected by heat, and also workers often cannot do much to avoid it. For example, we can build cooling centers for the public, but you cannot ask a worker to stop, or at least it's not easy to ask a worker to stop their work and go to a cooling center for a few hours. I think this is important because in...

Previous years, a few decades ago, we thought that heat impacts workers and their productivity when it's getting too hot, so when it's around 28, 29, maybe 30 degrees Celsius. But more and more research evidence is showing that... the optimal temperature for somebody to work is at 18 degrees Celsius. And beyond that, for every degree Celsius, we lose about 2.3% productivity. This means that...

not only some very hot areas of the world are affected. It means that UK, Sweden, Bulgaria certainly, Greece certainly are very much affected. We lose 20 to 30% productivity sometimes in the summer. It affects almost all countries in the world, even Canada and Siberia sometimes. So it's not something that is targeting only certain countries. We need to address it right now because it's also impacting our bottom line, our economy.

And Zornitsa, what research would you like to see done? I think one of the interesting research is the effect of heat waves on car accidents. traffic safety, because it has been found for Bulgaria that summer is the season with highest number of car crashes. And very briefly, Elisa Vetter, what research would you like to see done?

One area of research that would be very interesting would be integrating our response from the acute heat stress to the long-term climate adaptation continuum, because we're still... treating those two areas of research and also practical work as very separate. And it would actually be extremely beneficial if we combine them to enhance the capacity of our long-term planning.

Thank you very much for that. Well, it sounds as if there is a lot to be done, but thank you so much to our panel, Zenitza Spasova, Andreas Flores, Petra Dimitrova, and Elisa Vetter Petkova. And thank you to the lovely Sofia Science Festival. for having us, and to our studio engineers today, Emma Hart and Bob Nettles, and to the production team, Margaret Cesar Hawkins and Siobhan Maguire. And thank you to all of you in the audience for coming along to The Evidence and for joining in.

And to hear more on global health, do join me every week on the BBC World Service for Health Check. Bye for now. Dovijane. I'm Andini, and I'm looking back on the life of a Hollywood icon. whose legacy lives on through more than just her film roles. She was someone who was interested in invention all her life. She wasn't that interested in the film that she was supposed to be starring in. She was much more interested in the latest invention.

It's in GPS, it's in Wi-Fi, it's in Bluetooth. From the BBC World Service. Untold Legends, Hedy Lamarr. Available now wherever you get your BBC podcasts.