People spend more time indoor when its cold, which has a much higher chance of transmission (less airflow, closer contact with people, more shared surfaces that are touched)
many viruses have better transmission chances at lower air temperatures and less humidity
your immune system gets weakened if you get too cold
Not so much on the biological level, but behavioural level.
Social distancing is very hard when it’s winter and everyone is inside breathing the same air.
Easy for respiratory diseases to spread in a confined space.
> The exact mechanism behind the seasonal nature of influenza outbreaks is unknown. Some proposed explanations are:
> People are indoors more often during the winter, they are in close contact more often, and this promotes transmission from person to person.
> A seasonal decline in the amount of ultraviolet radiation may reduce the likelihood of the virus being damaged or killed by direct radiation damage or indirect effects (i. e. ozone concentration) increasing the probability of infection.
> Cold temperatures lead to drier air, which may dehydrate mucous membranes, preventing the body from effectively defending against respiratory virus infections.[3][4][5]
> Viruses are preserved in colder temperatures due to slower decomposition, so they linger longer on exposed surfaces (doorknobs, countertops, etc.).
> Vitamin D production from Ultraviolet-B in the skin changes with the seasons and affects the immune system.[6][7][8]
> The reason for the seasonality has not been conclusively determined.[97] Possible explanations may include cold temperature-induced changes in the respiratory system,[98] decreased immune response,[99] and low humidity causing an increase in viral transmission rates, perhaps due to dry air allowing small viral droplets to disperse farther and stay in the air longer.[100]
Cold air has less ability to hold moisture, this results in less humidity, which in turn results in thinner nasal mucus. Mucus in your upper respiratory tract presents a physical barrier to viruses, and the thinning of this mucus due to decreased humidity results in more viral access to the nasal and respiratory mucosa.
It is an area of active investigation. There is a nice readable summary of work on the common cold in a New Yorker article by Atul Gawande from a few years back. Gawande relates the story of the isolated town of Spitsbergen, Norway, which despite the cold winter temperatures was free of respiratory infections every year until early summer, when the resupply ships arrived (Paul and Freese, 1933). Also, there is no demonstrated relationship between getting chilled and catching a cold, as repeated controlled experiments have shown.
There is a real puzzle underneath this: “The annual prevalence of multiple respiratory viruses (flu, rhinovirus, coronavirus, respiratory syncytial virus) is consistently highest in summer, but its progression to clinical disease is highest in winter (Álvarez-Argüelles et al., 2018; Birger et al., 2018; Galanti et al., 2019; Lee et al., 2012; Shaman et al., 2018).” (Wyes et al 2022). This is true both in the northern and southern hemispheres. Near the equator, progression to disease is related to the rainy season.
Wyes et al hypothesize that seasonality is related not to temperature or humidity or being enclosed, but rather to the day-night cycle impacting circadian rhythms and the immune system. They do not offer a strong test of this hypothesis. Other research on potential pathways indicate that the immune response in nasal cells is lowered by colder ambient temperatures (Huang et al 2023), which may account for some infections but does not explain transmission and disease progression in the tropics (neither does the day-light cycle).
Likely there are multiple reasons, especially given the variety of viruses that cause the common cold and respiratory infections.
First, it is important to know that cold does not directly cause or provoke these diseases, but it favours their appearance by temporarily weakening the defences of the respiratory tract, drying mucous membranes and facilitating the survival and transmission of viruses in cold, dry air; moreover, in times of low temperatures, people tend to congregate in enclosed spaces, increasing the risk of contagion.
Cold weather doesn’t cause diseases, obviously, but cold air is dryer than warm air. Mucus is a great barrier to many infections and allergens. Add that to keeping closer to everyone, and you can be more easily infected.
Cold weather doesn’t directly cause illnesses like the cold, flu, or tonsillitis, but it creates the perfect conditions for viruses to spread. People tend to stay indoors more, with poor ventilation and close contact, which makes it easier for respiratory viruses to pass from one person to another. Also, cold air can dry out our nasal passages, weakening our natural defenses.
Out of the three, the flu is definitely the most serious. While the common cold and tonsillitis are usually mild and self-limiting, the flu can lead to severe complications, hospitalization, or even death, especially in vulnerable groups like children, the elderly, or those with chronic conditions.
Always keep some iHealth 3-in-1 test kits at home. They test for COVID, Flu A, and Flu B all in one go. Catching it early, within the first 24 to 48 hours of symptoms, can really make a difference in how fast and how well you recover.
Consider the plight of the grade school teacher. Every September she or he encounters a couple of dozen little disease vectors, every one of them coming from a different environment. It’s inevitable that these people, shut up in a common room during cold weather, will pass diseases among each other.
Comments
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Multiple factors at play here.
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Not so much on the biological level, but behavioural level.
Social distancing is very hard when it’s winter and everyone is inside breathing the same air.
Easy for respiratory diseases to spread in a confined space.
> The exact mechanism behind the seasonal nature of influenza outbreaks is unknown. Some proposed explanations are:
> People are indoors more often during the winter, they are in close contact more often, and this promotes transmission from person to person.
> A seasonal decline in the amount of ultraviolet radiation may reduce the likelihood of the virus being damaged or killed by direct radiation damage or indirect effects (i. e. ozone concentration) increasing the probability of infection.
> Cold temperatures lead to drier air, which may dehydrate mucous membranes, preventing the body from effectively defending against respiratory virus infections.[3][4][5]
> Viruses are preserved in colder temperatures due to slower decomposition, so they linger longer on exposed surfaces (doorknobs, countertops, etc.).
> Vitamin D production from Ultraviolet-B in the skin changes with the seasons and affects the immune system.[6][7][8]
https://en.wikipedia.org/wiki/Flu_season
> The reason for the seasonality has not been conclusively determined.[97] Possible explanations may include cold temperature-induced changes in the respiratory system,[98] decreased immune response,[99] and low humidity causing an increase in viral transmission rates, perhaps due to dry air allowing small viral droplets to disperse farther and stay in the air longer.[100]
https://en.wikipedia.org/wiki/Common_cold#Weather
A primary reason is relative humidity.
Cold air has less ability to hold moisture, this results in less humidity, which in turn results in thinner nasal mucus. Mucus in your upper respiratory tract presents a physical barrier to viruses, and the thinning of this mucus due to decreased humidity results in more viral access to the nasal and respiratory mucosa.
https://www.sciencedirect.com/science/article/pii/S0966842X21000731
It is an area of active investigation. There is a nice readable summary of work on the common cold in a New Yorker article by Atul Gawande from a few years back. Gawande relates the story of the isolated town of Spitsbergen, Norway, which despite the cold winter temperatures was free of respiratory infections every year until early summer, when the resupply ships arrived (Paul and Freese, 1933). Also, there is no demonstrated relationship between getting chilled and catching a cold, as repeated controlled experiments have shown.
There is a real puzzle underneath this: “The annual prevalence of multiple respiratory viruses (flu, rhinovirus, coronavirus, respiratory syncytial virus) is consistently highest in summer, but its progression to clinical disease is highest in winter (Álvarez-Argüelles et al., 2018; Birger et al., 2018; Galanti et al., 2019; Lee et al., 2012; Shaman et al., 2018).” (Wyes et al 2022). This is true both in the northern and southern hemispheres. Near the equator, progression to disease is related to the rainy season.
Wyes et al hypothesize that seasonality is related not to temperature or humidity or being enclosed, but rather to the day-night cycle impacting circadian rhythms and the immune system. They do not offer a strong test of this hypothesis. Other research on potential pathways indicate that the immune response in nasal cells is lowered by colder ambient temperatures (Huang et al 2023), which may account for some infections but does not explain transmission and disease progression in the tropics (neither does the day-light cycle).
Likely there are multiple reasons, especially given the variety of viruses that cause the common cold and respiratory infections.
First, it is important to know that cold does not directly cause or provoke these diseases, but it favours their appearance by temporarily weakening the defences of the respiratory tract, drying mucous membranes and facilitating the survival and transmission of viruses in cold, dry air; moreover, in times of low temperatures, people tend to congregate in enclosed spaces, increasing the risk of contagion.
[removed]
Cold weather doesn’t cause diseases, obviously, but cold air is dryer than warm air. Mucus is a great barrier to many infections and allergens. Add that to keeping closer to everyone, and you can be more easily infected.
Cold weather doesn’t directly cause illnesses like the cold, flu, or tonsillitis, but it creates the perfect conditions for viruses to spread. People tend to stay indoors more, with poor ventilation and close contact, which makes it easier for respiratory viruses to pass from one person to another. Also, cold air can dry out our nasal passages, weakening our natural defenses.
Out of the three, the flu is definitely the most serious. While the common cold and tonsillitis are usually mild and self-limiting, the flu can lead to severe complications, hospitalization, or even death, especially in vulnerable groups like children, the elderly, or those with chronic conditions.
Always keep some iHealth 3-in-1 test kits at home. They test for COVID, Flu A, and Flu B all in one go. Catching it early, within the first 24 to 48 hours of symptoms, can really make a difference in how fast and how well you recover.
Consider the plight of the grade school teacher. Every September she or he encounters a couple of dozen little disease vectors, every one of them coming from a different environment. It’s inevitable that these people, shut up in a common room during cold weather, will pass diseases among each other.