Although respiratory tract infections can occur throughout the year, the cold temperature and low humidity typical of autumn and winter favor them to occur more frequently at these times of the year. As coronaviruses penetrate the respiratory system, it is reasonable for us to wonder if extreme cold will have an impact on the course of the pandemic.
Respiratory tract infections are the most common infections worldwide, responsible for 20% of excess winter mortality, as well as a considerable economic burden on public health . The incidence of the common cold varies by age, with the highest rates found in children under 5 years of age. They have three to eight viral respiratory illnesses per year, while adolescents and adults have about two to four, and those over 60 have an average of less than one. Despite these alarming numbers, keep in mind that most heal on their own and completely.
It is bacteria and viruses, not the weather, that cause these infections. So why do so many people get sick in the cold, wet months of fall and winter? I point out some reasons that justify this connection and that depend on both human behavior and our physiology.
- After the summer, the children go back to school. Close contact increases the chance that viruses and, less often, bacteria, will spread.
- Cold weather causes more time to be spent indoors. This promotes close exposure to other people, who may be infected.
- In addition, there are more factors related to air quality in closed environments , in homes or offices, where people spend more than 90% of their time.
- The most common cold-causing viruses survive best in cold, low-humidity environments, such as fall and winter.
- Inhalation of cold air, cooling of the body surface and cold stress provoke pathophysiological responses , such as the alteration of our immune system, which can contribute to an increased susceptibility to respiratory infections.
The nose protects us from the cold and microorganisms
The nose is the part of the upper respiratory system that is responsible for counteracting the cold and the lack of humidity. The air, as it passes through the nose, goes through a tortuous path between turbinates and meatus, which bring the inhaled air into intimate contact with the surface of the nasal mucosa, with three objectives:
- Heat the air until it reaches body temperature, thanks to the fact that the mucosa has many blood vessels.
- Moisten that dry air until it is saturated with water vapor through the secretion of mucus and water by the cells of the mucosa.
- Retain particles and microorganisms so that they do not pass into the respiratory tract, thanks to the ciliated epithelium.
If we breathe through the mouth, we lose all this protective, humidifying and heating role of the nose and we force other components of the pathway to make an extra expense , since the air has to reach the alveoli in an adequate way that guarantees gas exchange .
The pathophysiology of respiratory infections with cold
Let us now see which are the aspects that result in a greater susceptibility to infectious diseases and that affect the mucosa, smooth muscle and blood vessels of the respiratory tract.
Cold air can dry out and damage both the inner lining of the nose and other superficial epithelia, making them more susceptible to penetration by a virus. In addition, this epithelium will generate pro-inflammatory substances, which, if persistent, can cause epithelial lesions and favor the penetration of microorganisms.
Cold causes vasoconstriction to preserve body heat. This could make it difficult for leukocytes to reach the respiratory mucous membrane, making it difficult to protect against germs.
Air pollution forces phagocytic cells (macrophages) to work harder to remove particulate debris and reduces their ability to fight microorganisms.
Vitamin D plays an important role in maintaining a healthy immune system. Reduced exposure to the sun during winter can lead to lower levels of vitamin D.
In studies with mice, it has been seen that, as the temperature drops, they have decreased production of interferons, which is the intrinsic antiviral defense, which leads to the infection being more persistent and difficult to resolve.
Ultimately, the cold can aggravate the extent and severity of the pandemic.
We are in conditions and at a time of year in which exposure must be reduced as much as possible and containment mechanisms for the population increased. Hopefully in the northern hemisphere we will be able, not only to achieve what they achieved in the southern hemisphere, making the flu disappear almost completely, but also to reduce the painful tribute of SARS-CoV-2. An important part is in our hands. Let us act with knowledge and responsibility.
Rafael Sirera Pérez, Professor of Cell Biology in the Department of Biotechnology. I research in Immunology and Molecular Biology of Cancer in the Mixed Unit of the General Hospital and the Príncipe Felipe Center, Polytechnic University of Valencia
This article was originally published on The Conversation. Read the original.
