It SARS-CoV-2 it does not seem to be particularly influenced by seasonality like other respiratory viruses (a recent study on Nature), in fact you can get sick even in summer (happened in various parts of the world), but we are us to be weaker in winter: during the cold season, in fact, we are more exposed to airborne viruses, such as SARS-CoV-2. Here’s why.
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The cold paralyzes the hair cells
Getting sick in winter from a virus that attacks the airways is easier. And the same for the coronavirus. The winter environment promotes the spread of a variety of respiratory virus infections. The two main contributing factors are changes in environmental parameters (in particular temperature and humidity) and in human behavior (increased attendance of enclosed spaces). Absolute humidity and relative humidity influence the important defense mechanism of our respiratory system called clearance mucociliare: the “ciliate” cells found in the trachea and which are responsible for moving the mucus outwards, which incorporates dust and tiny foreign bodies, including viruses and bacteria, which have entered the airways. The cold paralyzes the movement of these “cilia” and consequently the mucus stagnates with all its viral or bacterial load. The negative effect of cold on mucociliary clearance amplified by the thermal shock that occurs in the passage from the very hot internal environment to the particularly cold external one (but also vice versa).
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The study from Yale
Not only the cold to block the movement that protects us: thehumidity has a major impact because when it goes down too much (or it goes up too much) blocks the action of the “hair” cells, as mentioned in a review of studies published in September on Annual Review of Virology. In the study, conducted at Yale, we summarize the factors that push the seasonality of infectious viruses, focusing onabsolute (AH) and relative (RH) humidity. Absolute humidity values express the amount of water vapor present in the air, usually in grams per cubic meter. Relative humidity (the ratio between the density of the vapor contained in the volume of air and the maximum density that this volume can contain) affects the perception of dry or humid breathed air. We can have the same absolute humidity value, but at zero degrees the environment can have 100% relative humidity (fog), or, at a temperature of 30 degrees, centigrade the same absolute humidity makes us reach 15% of relative humidity which for us represents a dry, dry heat that is usually pleasant.
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Dry air is not good for you
This is to say that in winter the external temperature influences that of enclosed spaces: therefore an absolute low humidity outside translates into low internal relative humidity and low humidity blocks our defenses: the classic “dry throat” expression of the blocking of the mechanism of the “hair” cells and therefore of the greater susceptibility of people attacking respiratory viruses. The influenza virus, the human coronavirus and the human respiratory syncytial virus (RSV) clearly show the peaks of incidence in the winter months, the US study reminds us, as well as the parainfluenza virus (PIV). Various researches on this point indicate that high relative humidity (> 60%) and low (<40%) promote the vitality of influenza viruses in the respiratory droplets emitted by infected subjects, while in conditions of intermediate relative humidity (from 40% to 60%) the viruses are inactivated. Viral transmission was generally more efficient at 5 C than at 20 C regardless of other factors. Unlike temperate regions, respiratory infections have little seasonality in tropical regions. A study focusing on this aspect showed that no aerosol transmission was observed at 30 C in any humidity, although contact transmission was more widespread and comparable at 30 and 20 C. Thus, elevated ambient temperatures likely negate the effect of humidity on influenza transmission in tropical areas, the study reads.
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Be careful to breathe cold air
Therefore, an ideal humidity to prevent respiratory viral transmission from aerosols at room temperature appears to be between 40% and 60% relative humidity. Inhalation of dry air causes immediate loss of airway epithelial cilia, detachment of epithelial cells and inflammation of the trachea of guinea pigs. The inhalation of cold air, which always dries up due to the limited water storage capacity of the cold air, causes deterioration of the defense mechanism mucociliary. A study on the effect of ambient temperature on the rate of ciliary beating of nasal and tracheal hair cells isolated from human subjects showed that the mucociliary beating begins to decrease when the temperature falls below 20 C and is no longer observed at 5 C, why the cold alters the antiviral defense mechanisms. In winter and in dry indoor environments, the airways are less defensive.
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Scarf and humidifiers
How can we help the airways to respond better? Basically, even if it seems trivial, protecting (and warming) the nose outdoors (with a scarf, but also in this period, a mask) and using air humidifiers in the house, without keeping the heating very high. Such interventions with humidifiers have been carried out since the 1960s with promising results, specifies the Yale study. More recently, he adds, a study in Minnesota found that humidifying preschool classrooms from January to March to about 45% relative humidity results in a significant reduction in the total number of influenza viruses and copies of the viral genome. found in the air and on objects. The scarf therefore it serves to guarantee the optimal functioning of the mucociliary apparatus keeping the airways warm, humidifiers of environments are needed not to dry out too much internal air (even radiator humidifiers are good). Finally, opening the windows and ventilating the rooms (as well as favoring the exchange of air) also helps stabilize the conditions of relative humidity in the home or offices.
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Non-seasonal Covid-19
Obviously in the case of the recent Covid-19 it is not just humidity and temperature that are at stake (as mentioned at the beginning): transmission between people takes place at any time of the year and rather also depends on the duration of contact with a positive person, by the proximity, by the width of the spaces and by how much people protect themselves with masks and other measures we know. Winter for, for the mechanisms described, it can facilitate the entry of viruses and weaken our defense mechanisms.
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27 October 2020 (change October 27, 2020 | 12:00)
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