As winter approaches, the familiar chorus of coughs and sneezes begins. It’s a seasonal pattern we all recognise, but why does it happen? The answer is more complex than just spending more time indoors. It’s a perfect storm of biology and physics.
The Air Itself Becomes an Ally for Viruses
One
of the most significant factors is the change in the air we breathe. Cold winter air holds less moisture than warm summer air, resulting in lower humidity levels both outdoors and in our heated homes. This dry air is ideal for respiratory viruses like influenza. When an infected person coughs or sneezes, they release virus-containing droplets. In humid air, these droplets are larger and heavier, causing them to fall to the ground quickly. However, in dry winter air, the droplets evaporate faster, becoming smaller, lighter aerosol particles. These tiny particles can hang suspended in the air for much longer, travelling greater distances and increasing the chances of being inhaled by someone else. Studies have shown that influenza virus survival and transmission are significantly higher in low-humidity environments.
Your Nose's Defences Are Weakened
Our bodies have a remarkable first line of defence against inhaled pathogens, located right in the nose. The nasal passage is lined with a protective layer of mucus that traps viruses, and tiny hair-like structures called cilia that sweep them away. However, cold weather directly impairs this defence system. Breathing in cold, dry air can chill the inside of your nose. Research from Harvard Medical School found that even a modest drop in nasal temperature, by about 5°C, can cripple the immune response in the nose. This cold exposure was shown to reduce the secretion of 'extracellular vesicles'—tiny sacs released by nasal cells to swarm and trap viruses—by over 40%. The cold also thickens respiratory mucus, making it harder for the cilia to clear pathogens, and constricts blood vessels, reducing the circulation of infection-fighting immune cells to the area.
Viruses Become More Resilient
It’s not just about the environment and our bodies; the viruses themselves can become hardier in the cold. Some viruses, including influenza, are enclosed in a fatty outer layer called a lipid envelope. At warmer temperatures, this envelope is more fluid, like melted butter, making the virus less stable. But in the cold, this layer hardens into a more durable, gel-like state. This protective shield helps the virus survive for longer periods outside a host as it travels through the air from one person to another. Studies have confirmed that influenza viruses remain stable and infectious for longer periods at colder temperatures combined with low humidity.
The Indoor Crowding Factor Still Matters
While biological factors are key, the conventional wisdom about spending more time indoors isn't wrong—it's just part of the bigger picture. During winter, people naturally gather inside homes, offices, and public spaces to escape the cold. These indoor environments are often poorly ventilated, allowing those longer-lasting virus aerosols to accumulate in the air. Closer proximity to others simply increases the opportunity for an infected person to transmit the virus. Furthermore, reduced exposure to sunlight in winter can lower our Vitamin D levels, which play a role in regulating the immune system, potentially making us more vulnerable to infection.
How to Protect Yourself This Winter
Understanding these factors allows you to take targeted steps to stay healthy. Since dry air is a major culprit, consider using a humidifier at home to maintain indoor humidity between 40% and 60%, a range shown to deactivate airborne viruses more quickly. When you go out into the cold, loosely wrapping a scarf over your nose and mouth helps warm and humidify the air before you breathe it in, protecting your nasal immune defences. Continue to practice good hand hygiene, ensure good ventilation by opening windows periodically, and consider getting vaccinated against seasonal illnesses like the flu. These actions, combined with a healthy lifestyle, provide layers of protection against the winter surge in respiratory infections.
