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SARS-CoV-2 does not appear to be particularly affected by seasonality like other respiratory viruses (a recent study on Nature), in fact, you can get sick even in summer (it happened in various parts of the world), but we are that we are weaker in winter: during the cold season, in fact, we are more exposed to viruses in the air, such as SARS-CoV-2. This is why.
Cold paralyzes hair cells
It is easier to get sick in winter from a virus that attacks the respiratory tract. 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 (particularly temperature and humidity) and in human behavior (increased attendance in enclosed spaces). Absolute humidity and relative humidity influence the important defense mechanism of our respiratory system called mucociliary clearance– The “hairy” cells found in the windpipe that move mucus outward, incorporating dust and tiny foreign bodies, including viruses and bacteria, that 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 environment (but also vice versa).
The Yale Study
Not only the cold to block the movement that protects us: thehumidity has a great impact because when it goes too low (or go too high) blocks the action of “hair” cells, as mentioned in a review of studies published in September in Annual Virology Review. The study, conducted at Yale, summarizes the factors that drive the seasonality of infectious viruses, focusing onabsolute humidity (AH) and relative humidity (RH). Absolute humidity values express the amount of water vapor present in the air, generally in grams per cubic meter. Relative humidity (the relationship 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% relative humidity that for us it represents a dry, dry heat that is usually pleasant.
Dry air is not good for you
That is to say that in winter the outside temperature influences that of the closed spaces: therefore, an absolutely low outside humidity translates into a low inside relative humidity and a low humidity blocks our defenses: the classic expression of “dry throat” of the blocking of the mechanism of the “hairy” cells and, therefore, increased susceptibility of people who attack respiratory viruses. The influenza virus, the human coronavirus and the human respiratory syncytial virus (RSV) clearly show the incidence peaks in the winter months, the US study reminds us, as well as the parainfluenza virus (PIV). Several investigations on this point indicate that high relative humidity (> 60%) and low (<40%) promote the vitality of influenza viruses in respiratory droplets emitted by infected subjects, while in conditions of intermediate relative humidity (40% to 60%) the viruses are inactivated. Viral transmission was generally more efficient at 5C than at 20C regardless of other factors. Unlike temperate regions, respiratory infections have little seasonality in tropical regions. A study focused on this aspect showed that no aerosol transmission observed at 30 ° C in any humidity, although contact transmission was more widespread and comparable at 30 and 20 C. Therefore, elevated ambient temperatures probably nullify the effect of humidity on influenza transmission in tropical areas, according to the study.
Be careful to breathe cold air
Thus, ideal humidity to prevent Respiratory viral transmission from aerosols at room temperature appears to be between 40% and 60% RH. Inhalation of dry air causes immediate loss of epithelial cilia in the airways, shedding of epithelial cells, and inflammation of the trachea in guinea pigs. Inhaling cold air, which always dries up. due to the limited water storage capacity of cold air, causes deterioration of the defense mechanism mucociliary. A study on the effect of room temperature on the ciliary beat rate of nasal and tracheal hair cells isolated from human subjects showed that the mucociliary beat begins to decrease when the temperature drops below 20 C and is no longer observed at 5 C, why cold alters antiviral defense mechanisms. In winter and in dry indoor environments, the airways are less defensive.
Scarf and humidifiers
How can we help the airways respond better? Basically, although it seems trivial, protect (and warm) the nose in the open air (with a scarf, but also in this period, a mask) and use air humidifiers in the house, without keeping the heat too high. These types of interventions with humidifiers have been performed 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 keep airways warm, humidifiers of environments are necessary do not dry too much indoor air (even radiator humidifiers are good). Finally, opening the windows and ventilating the rooms (in addition to promoting air exchange) also helps to stabilize the relative humidity conditions in the home or in the offices.
Covid-19 non-seasonal
Obviously, in the case of the recent Covid-19, not only humidity and temperature are at stake (as mentioned at the beginning): transmission between people occurs at any time of the year and rather also depends on the duration of contact with a positive person, because of the proximity, because of the spaciousness of the spaces and how much people protect themselves with masks and other measures that we know. Winter for, by the mechanisms described, can facilitate the entry of viruses and weaken our defense mechanisms.
October 27, 2020 (change October 27, 2020 | 12:00)
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