Why Do We Always Catch A Cold In Winter?

Every winter, I’d blame the chilly air and my sinus infection for my frequent colds, but it was actually the cold months which weaken nasal defenses, dry the air, and increase indoor gatherings, making viruses thrive. Now, with better habits and prevention, its easier to manage my symptoms through the frosty seasons.

Perhaps you have heard someone advise you to wrap up during winter to not catch a cold. You would have thought that this saying was entirely baseless because science, indeed, proved that, yes, it's true. Respiratory infections, the common cold among them, are more widespread in winter months. However, this does not mean that it is caused by cold weather. What happens is that it develops conditions that cause higher susceptibility to infections.

In the United States, flu season typically lasts from October till April. According to the estimation of the Centers for Disease Control and Prevention (CDC), in the 2022–2023 flu season, there were approximately 26–50 million influenza cases. But why are people falling sick during winter? Read to understand the reasons:

The simplest explanation arises from human behavior during colder months. As the temperature cools down, we tend to get inside, where we are often much closer to other individuals. Places such as schools, offices, malls, and theaters have poor circulation systems, which provide an easy target for viruses. Common viruses that cause cold and flu spread through droplets that erupt into the air when infected person sneezes or coughs or even talking. These droplets suspended in the air or settle upon highly touched surfaces, which contribute to the spread.

Cold-season air is drier outside as well as inside. Many people use heating systems during winter, which dries out the air in buildings. Researchers have shown that flu and other respiratory viruses remain more stable and transmit more readily in low humidity environments. In addition, dry air reduces resistance in the human body. For example, epithelial cells in our nostrils-the first line of defense against germs-are not so good at flushing out the viruses under such conditions. Cilia-very tiny hair-like structures in the respiratory tract-will also fail to wiggle and throw off germs when the air is too dry.

Recently it has emerged that exposure to cold may weaken the immunity of an individual. The key part which filters germs in an individual is his nose. When exposed to cold air, the nasal cells produce less virus-fighting extracellular vesicles; these are tiny sacs trapping viral particles before they can infect the person. This decreased immunity through the nose exposes an individual to infections more.

A groundbreaking study in 2019 demonstrated the role of humidity in immune function. Scientists found that mice exposed to dry air conditions (10–20% relative humidity) could not fight flu infections as effectively as those mice in moderate humidity (50%). The study showed that dry conditions affect the movement of cilia and the secretion of interferons, which are chemicals that prevent viral replication. Although this study was conducted on mice, similar effects have been seen in humans.

Winter coincides with the peak activity of most respiratory viruses, such as the flu, RSV, and even COVID-19. Researchers think that these viruses have evolved to flourish in the cold, dry climate of winter, hence increasing the possibility of transmitting the disease.

While one can't eliminate the possibility of falling sick completely, you can be prepared for winter illness in the following ways:

Key is proper hygiene practices such as hand washing, covering your mouth when coughing or sneezing, and proper disinfection of surfaces that decrease the spread of viruses.

A healthy lifestyle does go a long way in building up your immune system. Stay hydrated, take a balanced diet full of fruits and vegetables, and keep yourself sleeping well and get active through exercises.

Isolation when ill prevents you from spreading germs to others. If you have a cold or flu, keep resting and recovering at home.

A humidifier can help maintain optimal humidity levels in your home—around 40–50%—keeping nasal passages and other respiratory defenses in top condition.

Flu vaccines and COVID-19 boosters are important measures to prevent the severity of disease. The CDC advises that everyone six months and older should get vaccinated, as immunity starts about two weeks after vaccination.

Wearing a mask can decrease the transmission of respiratory viruses, especially in crowded indoor environments.

Beyond winter, it's essential to have regular check-ups with your primary care provider to maintain overall health. Together, you can work on a wellness plan that is tailored to your needs, making you better prepared to face flu season and beyond.

Winter itself does not make you sick, but it sets the stage for conditions that do. And by understanding the interplay of cold air, low humidity, and indoor crowding, you can take steps to protect yourself and stay healthy through the chilly months.

Common Cold. John Hopkins.

Low ambient humidity impairs barrier function and innate resistance against influenza infection. Proc Natl Acad Sci U S A. 2019