- Welcome to the Huberman Lab Podcast, where we discuss science and science-based tools for everyday life. I'm Andrew Huberman, and I'm a professor of neurobiology and ophthalmology at Stanford School of Medicine. Today is an Ask Me Anything episode or AMA. This is part of our premium subscriber channel. Our premium subscriber channel was started in order to provide support for the standard Huberman Lab Podcast, which comes out every Monday and is available at zero cost to everybody on all standard feeds, YouTube, Apple, Spotify, and elsewhere.

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So without further ado, let's get to answering your questions. The first question is about colds and flus. And the question specifically is, why is it that we get more colds and flus in the winter months? Is it the temperature outside? Is it the fact that people are spending more time indoors?

Or is it a myth that we get more colds and flus in the winter months? Well, the first thing is that the research very clearly shows that in winter months, there's a greater prevalence of colds and flus. Now, of course, the words in the winter months means many things.

For instance, in most areas of the world, unless you live directly near the equator, in the winter months means that days are going to be shorter and nights are longer than in the summer months. In general, that's true. It's of course especially true if you live very far from the equator.

So imagine somebody living up in Tromsø, Norway, which is very close to the North Pole. In winter, days are very, very short in Tromsø. And conversely, in the summer months, days are very, very long in Tromsø, Norway. Now, Tromsø, Norway represents an extreme of day length variation according to time of year.

The closer you get to the equator, the less variation there is in day length and therefore night length across the year. However, in most locations on Earth, days will be significantly shorter by about an hour or two at least for certain parts of the year. And in general, shorter days correlate with colder temperatures.

That's what's really important to know, which is that shorter days generally correlate with colder temperatures, regardless of where you live on the planet Earth. Okay, so in the winter months, as we're calling it, involves at least two things. Shorter days, which means what? It means less sunlight and either more artificial light or more darkness, depending on how much artificial light you rely on.

It also means, as we mentioned, colder temperatures. Colder temperatures, of course, doesn't necessarily mean that you are exposed to those colder temperatures because what's the third thing? Shorter days and colder temperatures outside mean that people are spending more time indoors. So what the research clearly shows is that one of the reasons, not the only, but one of the reasons for the greater prevalence of colds and flus in the short days, AKA winter months of the year, is that people are spending more time indoors and therefore at closer proximity, which raises a whole bunch of really interesting questions that have been explored in the peer-reviewed research about proximity, cold, and flu transmission.

There have been studies, for instance, where people come into the laboratory who are suffering from cold and flu and are sneezing, having them sneeze at different proximity to other people. Obviously, people are doing this under consent. They're agreeing to participate in these particular experiments. And there does seem to be a very nice relationship between physical distance between you, the uninfected person, and the sneezing, coughing, nose wiping, eye wiping, we'll get into why I mention all of those things in a moment, person that is suffering from the flu.

In other words, the closer you are to somebody who's sneezing or coughing, the higher probability that you will contract that cold or flu. Okay, so we've already established two things. First of all, physical proximity is a key variable in terms of transmission of cold and flu. And at some level, that's sort of a duh.

I mean, if you think about it, if you're in the next room or down the hallway from or across the street from somebody with a cold or flu, you don't expect to contract that cold or flu from them. Whereas if you're in the same vehicle with them or you're sitting next to them on an airplane or you are on the same bus or in the same classroom as them and they are sneezing and coughing, well, then the probability that you would get that cold or flu from them is increased.

That's just obvious. But the studies that have explored the relationship between physical proximity and transmission of cold and flu have actually analyzed things down to the range of, well, if you're standing one foot apart or you're standing three feet apart or six feet apart, what's the probability that you will contract that cold or flu?

And it follows a very direct relationship where the closer the proximity, the more likely that you're going to contract the cold or flu from that person. And of course, anything like exchange of skin contact or exchange of saliva, because obviously in cases of romantic relationships where people will kiss or in familial relationships or friendships where people hug, any kind of contact also further increases the transmission of cold and flu, not just from stuff emitted from the nasal passages and from the mouth when people sneeze and cough, but also when people wipe their eyes, even when people wipe their skin, because oftentimes they have cold or flu virus on their skin, it doesn't always survive terribly long on the skin, but if they wipe their face or wipe their nose or are using tissues and then don't wash their hands thoroughly afterwards, indeed they can transmit cold and flu simply through handshake, okay?

So for all you hypochondriacs out there, I'm arming you with a lot of useful knowledge that will justify staying away from people who perhaps have cold and flu. Now, this particular point about proximity and people being indoors more during the winter months, and that's why there's more cold and flu transmission, raises a whole bunch of interesting questions and in fact, protocols that I plan to cover in a Huberman Lab podcast episode solely devoted to colds and flus and how to avoid getting colds and flus.

But one brief mention or telegraph of what I plan to cover in that episode is that if people are telling you that they are sneezing and coughing due to a cold or flu, but that they are quote unquote not contagious, either because they are early in the cold or flu or late in the cold or flu, and they have these theories or claims that, oh, you know, I'm no longer contagious.

Keep in mind that anytime someone is sneezing or coughing, they are in fact contagious. So I'll get back to that in that full length episode about colds and flus, but I felt it was too important not to mention right now that people's theories about when they are quote unquote contagious or not contagious are rarely substantiated by the actual data.

The actual data point to the fact that when people are sneezing and coughing, if it's due to a cold or flu, they are still contagious. Okay, so we've established that cold outside means people tend to be indoors more, which increases physical proximity, which is one of the reasons why there's more cold and flu transmission.

One of the other reasons why being indoors more tends to increase cold and flu transmission is that in the cold months of winter, when people are indoors, they tend to be not under air conditioning, not at neutral temperatures, but rather they tend to be in heated rooms. And depending on the type of heating that's used, but in general, due to all forms of heating, the heated air tends to be drier air.

Now this is a little bit counterintuitive because if you ever go outside on a really cold winter day, you'll realize that the cold weather outdoors is extremely dry. You can almost quote unquote feel the dryness of the air. And if you can't quote unquote feel the dryness of the air or imagine what that's like, just imagine this.

Think of yourself outdoors on a very cold winter day, taking a brisk walk. Then think about yourself taking that same brisk walk on a very humid summer day. Humidity is of course the concentration of water in the ambient environment, the air, whereas the dryness is the lack of humidity.

And just that little simple gedanken or thought experiment will remind you just how dry the cold air is out of doors during the winter months. Whereas indoors, we're heating that air. And indeed the heating of that air does have the property of drying the nasal and oral passages, but especially the nasal passages.

So another key reason why there's more transmission of colds and flus in the winter months is because people are spending more time indoors. And oftentimes the way those indoor environments are being heated is drying out the nasal passages. And the nasal passages represent a primary site of defense for viral infections like colds and flus, but also bacterial infections and fungal infections for that matter.

Now, I've talked before on this podcast about the importance of using nasal breathing under any conditions where you don't have to breathe through your mouth. So if you are not eating, if you're not speaking, or if you're not exercising hard, which by the way, oftentimes requires that you breathe through your mouth, there's nothing wrong with breathing through your mouth.

If you're exercising hard and you need to breathe through your mouth in order to bring in enough oxygen, there are conditions under which that's entirely appropriate. You can learn more about that in the podcast episode I did all about breathing and breath work. But for the most part, it's best to be a nasal breather except under the conditions I just mentioned.

Why? Well, the nasal passages contain a number of physical barriers, including the hairs within your nose. I know that hairs in the nose, especially if they are super numerous, are not considered aesthetically nice. People will trim them, et cetera. But those hairs in your nose actually serve as a barrier toward infection.

This is well-established. The lining of the nose, the mucosal lining of the nose, contains a lot of things. First of all, it acts as its own physical barrier and physical trap for incoming viruses, fungal infections, and bacterial infections. They literally get trapped in the nasal passages and therefore can't enter deeper into your physiology.

And right now is not the time to go into the whole anatomy and physiology of the nasal passages, but keep in mind, if you saw the episode that I did with Noam Sobel, or you listened to the episode that I did on olfaction, both of those episodes highlight the fact that your brain sits not far behind your nasal passages.

There's a bony barrier there called the cribriform plate, and there's some other things as well, but it's not far from your nostrils to your brain. And it is not far from your nostrils to the rest of your respiratory pathway, of course. And so the hairs of your nostrils, the mucus itself, and the microbiome, the trillions of little micro bacteria that thrive in the mucosal lining of your nasal passages actually serve to protect against many of the incoming infections.

This is why it's so important to keep the mucosal lining of your nasal passages thriving and intact. How do you do that? Well, one of the best ways to do that is to make sure that the air that you're breathing is sufficiently humidified. So while there are many different claims out there about how to avoid colds and flus, as long as we're having a discussion about why there are more colds and flus in the winter months, because indeed there are, it's worth mentioning that if you suffer from colds and flus and you're going to spend a lot of time in a particular indoor environment, you might be wise to find a air humidifier, something that brings more moisture into the air that you're breathing, especially at night while you're sleeping.

Now, that's not going to prevent the person in your environment who happens to have a cold or flu and is sneezing actively from transmitting that cold or flu, but it will keep your nasal passages and the rest of your respiratory pathways as healthy as they can be and as resistant as they can be to any colds or flus that you might be fighting off.

And this is another key point, which is regardless of whether or not it's the winter months or the other months of the year, all day long, you're combating different types of infections. Different types of viral, fungal, and bacterial infections are bombarding your system, and your immune system counters that both through physical and through neurochemical and hormonal and pure immune mechanisms, okay?

There's a whole discussion of this in the episode I did on the immune system. We'll do more on this. But keeping the air that you breathe, especially at night, sufficiently humidified is one great way to try and offset colds and flus that you might be combating because you brought that cold or flu home from work and your immune system needs to ward it off, okay?

If you are having a hard time grasping how it is that you could bring home a cold or flu and then ward it off, do you have it, do you not have it? Is it sitting there trying to get into your system or do you have it a little bit?

Well, this is some of the gray area around cold and flu and viral transmission generally. I mean, we don't know, for instance, if you got a mild sniffle as opposed to a really bad cold or flu, whether or not it would have been a really bad cold or flu.

Had you slept less, had you not humidified your air better, et cetera, probably it would be. And if you've ever not slept well for a little bit and you're sick, you experience just how much worse that sickness feels. The symptoms of that sickness go from very mild to moderate or from mild to moderate to very severe when you are sleep deprived.

That said, it's also clear that some strains of the cold or flu can be more mild than other strains of the cold or flu. So this can be pretty nuanced and we'll go deeper into this in the episode that I do on cold and flu and how to avoid cold and flu.

But nasal breathing, whenever possible, day and nighttime is extremely important for encouraging the overall health and resistance of your respiratory pathways to incoming cold and flu and other types of viruses. Similarly, humidifying the air that you breathe, especially at night, but perhaps also during the day in your office environment or home environment can also be very beneficial for warding off colds and flus.

And if you'd like to read more about the nasal versus mouth breathing components of colds or flus, I'll just mention one paper now. And again, I'll go into this in more depth in the future episode on colds and flus. But I want to mention that this is a bi-directional effect, meaning people who deliberately breathe through their nose tend to get fewer colds and flus.

Also people who tend to breathe through their mouth more tend to get more colds and flus. Now you might think that's the same thing or just two sides of the same coin, but actually it's not. There's what we call a double dissociation whereby if people are mouth breathers, they are far more susceptible to colds and flus and other types of respiratory illnesses.

I'll put a link to one particular paper that I like a lot in the show note captions entitled, Association of Oral Breathing with Dental Malocclusions and General Health in Children. This paper explores a lot of different things about nasal versus mouth breathing. All of which by the way, point to the fact that nasal breathing whenever possible is better for us health-wise, aesthetically, dental health-wise than is mouth breathing.

But one of the key components here, and this is why I bring it up in the context of this discussion is quote, oral breathing is related to a significantly higher prevalence of allergies and a significantly more likely getting sick and taking medication for being sick, okay? So lots in this study, lots in related studies, but nonetheless, being a nasal breather, humidifying your air, paying attention to whether or not you're indoors with people who are coughing and sneezing because it's cold outside, all of those things are going to be relevant, perhaps keeping a little bit of distance, maybe a lot of distance from those people or encouraging them to wash their hands after they wipe their nose, maybe even sending them to a little bit of a, not necessarily isolation, although that might be necessary, but keeping them at a distance, right?

I don't think it's too much to ask somebody who's sick to not get you sick. Personally, I think that's the polite thing to try and do. And there's actually not many things ruder than someone who knows that they're sick, showing up to something because they didn't want to miss that particular event and getting everybody else sick, frankly.

That's my opinion. I think that opinion is shared with many of you out there as well. Now, as I mentioned earlier, when it's cold outside, the air tends to be drier, not always. I mean, you could live in an environment that's very cold and very rainy. Obviously very rainy is very humid.

It's maximum humidity if you think about it. But if you are going to spend time out of doors in cold days of winter, which by the way is a really good thing for other reasons, and I'll get into those reasons in a moment because they relate directly to why there's such a prevalence of colds and flus because in general, people are not doing what I'm about to tell you to do.

But there are two things to keep in mind if you're going to be out of doors. First of all, all the things about proximity still hold. There have been studies of people who have colds and flus sneezing in indoor environments versus outdoor environments and how much transmission there is.

Again, it boils down to proximity. Yes, you can get a cold or flu from somebody sneezing out of doors next to you just as well as you can if they sneeze next to you indoors. There's a slightly higher probability that you'll get it from them if they're sneezing at that same distance, but you're indoors as opposed to outdoors.

Obviously, sneezers, coffers, sneeze and cough in the other direction, cover your mouth and nose. If you don't have a tissue or something, we are told, and I subscribe to the idea that sneezing into your elbow or coughing into your elbow is probably the next best thing to not covering up at all as opposed to into your hand, which then you touch other things and yourself.

Anyway, without getting into the mechanics of sneezing and coughing too deeply, if you're going to be outside in the cold winter months and you'd like to avoid getting colds and flus, when you run, when you exercise, when you walk, when you're in conversation, try to nasal breathe unless you have to mouth breathe.

And if you can sense your nasal passages or your mouth drying out because of that hard breathing, and by the way, exercise during the winter months is still important, so I'm not discouraging people from exercising outdoors during the winter months as long as you can do it safely, but if you feel those air passages drying out, just be aware that when you take those dried out air passages indoors and you're around other people that might have colds and flus, you are going to be more susceptible.

So that's cause for taking a hot shower, perhaps if you have access to a steam room or humidifier and rehydrating those nasal and oral passages. Now, if you're outside and running and you're breathing hard and your nasal passages are drying out, does that mean that you're more susceptible to colds and flus that are just out in that environment if no one else is around?

Chances are no. However, what we were all told when we were kids, which is if you go outside and you get a chill, it's a good idea when you come back inside to take a hot shower or a hot bath, or if you have access to it, a sauna to heat back up, that's also true, why?

Well, it does seem that there's a relationship between core body temperature and susceptibility to cold and flu. And this is something I'm going to go deeply into in the episode on colds and flus. Keep in mind that fevers that are the response to exposure to a cold or flu or being full-blown sick with a cold and flu are an attempt of your immune system to heat up that virus and destroy it.

Keep that in mind if you're considering taking any kind of medication to lower your fever. Of course, know that highly elevated body temperatures, AKA fever, can be very dangerous to the brain and body. There's a fairly limited upper range of temperatures that we can go into before we start damaging ourselves.

But also keep in mind that fever is an adaptation designed to destroy viruses, okay? So if you blunt the fever, you are actually letting a colder virus to proliferate more readily than it would if you allowed a slight fever to combat that, okay? So there's a whole discussion to be had there because obviously you don't want people cooking their brains due to elevated fever and not treating that fever, but you also don't want to reduce your core body temperature too much, which raises the question of things like cold plunges.

Should you be taking cold showers and doing cold plunges in the winter months when it's already cold and you're susceptible to colds and maybe you're coming down with a cold? Should you do that? Ah, well, that segues into the next question that I'm going to answer for this AMA, which relates directly to cold exposure and so-called cyclic hyperventilation breathing, sometimes associated with so-called Wim Hof breathing, but other forms of deliberate breathing as well, and how specific forms of deliberate breathing and how deliberate cold exposure can indeed be used to offset or even prevent entirely infection due to bacterial or viruses, but if you're already coming down with a bacterial or viral infection, why cyclic hyperventilation breathing or why deliberate cold exposure may actually be the worst thing if you are already contracting or have contracted, if you are contracting or have already contracted a cold or virus.

So we'll answer that question next. In the meantime, just know that, yes, indeed, in the winter months, you are more susceptible to colds and flus because there's more of them going around. We talked about some of the reasons why they're going around and some of the things you can do to protect yourself against those colds and flus.

There are a bunch of other things that you can do that are very valuable that we'll explore in the full length episode on colds and flus and how to avoid getting them, and I'm hoping you all stay clear of colds and flus these winter months, and now you have some tools to try and reduce the duration and severity of those colds and flus should you get one.

Thank you for joining for the beginning of this Ask Me Anything episode. To hear the full episode and to hear future episodes of these Ask Me Anything sessions, plus to receive transcripts of them and transcripts of the Huberman Lab Podcast standard channel and premium tools not released anywhere else, please go to HubermanLab.com/premium.

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