Cold air alone does not make you sick. You need a virus or other pathogen for that. But cold air does create real, measurable changes in your body and environment that make infections more likely to take hold. So while your grandmother’s warning was technically wrong, it wasn’t entirely off base.
The 2024-2025 flu season saw an estimated 51 million illnesses and 710,000 hospitalizations in the U.S., the highest numbers since at least 2010. That surge didn’t happen because cold air contains germs. It happened because winter creates a perfect storm of biological and behavioral conditions that help viruses spread and weaken your defenses against them.
Why You Need a Virus, Not Just Cold Air
A cold or the flu is caused by a specific microorganism entering your body and replicating. No virus, no infection. You could stand outside in freezing temperatures for hours and, without exposure to a pathogen, you would not develop a respiratory infection. The same applies to going outside with wet hair. Despite the persistent warning, wet hair in cold weather does not cause pneumonia or the common cold. As Cleveland Clinic physicians have noted plainly: hair being wet is not the cause for catching a cold.
That said, cold conditions change the game once a virus is already in the picture. The real story is about what cold air does to your nose, your immune system, and the viruses themselves.
Cold Air Weakens Your Nose’s Defenses
Your nasal passages are more than an air tunnel. They warm, humidify, and filter every breath before it reaches your lungs, and they mount an active immune defense against incoming viruses. Cold air disrupts all of this.
When you breathe cold, dry air, the thin layer of liquid coating your airways evaporates faster than your body can replenish it. This dries out the mucus lining and slows the movement of cilia, the tiny hair-like structures that sweep trapped particles and pathogens out of your nose. Thicker, stickier mucus combined with sluggish cilia means viruses linger in your nasal passages longer instead of being cleared out.
There’s also a more recently discovered immune mechanism at work. Your nasal cells release tiny particles called extracellular vesicles that act as a frontline defense, swarming viruses and neutralizing them before they can infect cells. A 2022 study published in the Journal of Allergy and Clinical Immunology found that cold exposure reduces both the total number of these particles your nose releases and their individual ability to fight viruses. Cold air, in other words, doesn’t just slow the physical clearance system. It dials down a key layer of your innate immune response right at the point where most respiratory viruses enter your body.
Viruses Thrive in Cold Conditions
Rhinovirus, the most common cause of the common cold, replicates most efficiently at 33 to 35°C, which happens to be the temperature inside your nasal cavity. At core body temperature (37°C), its replication is significantly less robust. This is why cold viruses cause disease primarily in the nose and upper airway rather than deep in the lungs. When you breathe cold air, nasal temperatures can drop even further below core body temperature, potentially giving rhinovirus an even more favorable environment.
Influenza viruses benefit from cold weather for a different reason. Low absolute humidity, the total amount of moisture in the air, helps flu viruses survive longer on surfaces and in airborne droplets. Cold air holds far less moisture than warm air. Research has shown that absolute humidity is a better predictor of virus survival than relative humidity alone, and that increasing moisture in the air accelerates viral inactivation. Winter air, especially indoors where heating systems strip out even more moisture, creates conditions where flu particles remain infectious longer.
Indoor Crowding Completes the Picture
Cold weather drives people indoors, where they share air in enclosed, poorly ventilated spaces. Schools, offices, public transit, and homes with closed windows all concentrate respiratory droplets. This behavioral shift is one of the strongest drivers of winter illness spikes. You’re simply in closer, more prolonged contact with other people’s exhaled particles during the months when it’s too cold to open a window or gather outside.
Vitamin D Drops in Winter
Sunlight triggers vitamin D production in your skin, and during winter months, especially at higher latitudes, UVB radiation drops dramatically. Vitamin D plays a direct role in immune function, influencing how your body responds to both viral and bacterial infections. Deficiency has been linked to a higher risk of respiratory infections and more severe outcomes. Children, older adults, and people who spend most of their time indoors are particularly vulnerable to this seasonal dip. This isn’t about cold air hitting your skin. It’s about reduced sun exposure quietly lowering one of the inputs your immune system depends on.
Cold Air Can Cause Symptoms Without an Infection
Even without a virus, breathing cold air can produce symptoms that feel like illness. Cold air activates a receptor on the cells lining your airways called TRPM8, the same receptor that makes menthol feel cool. When triggered, it stimulates mucus production, which is why your nose runs the moment you step outside on a frigid day. In people with asthma or chronic lung conditions, this receptor appears to be overexpressed, making them more sensitive to cold-triggered symptoms.
Cold air also causes blood vessels in the airway lining to dilate, thickening the mucosa and potentially triggering bronchospasm, a tightening of the muscles around the airways. This is why some people wheeze or cough after breathing cold air during exercise. The effect is amplified by heavy breathing, since faster airflow cools and dries the airways more aggressively. For people prone to exercise-induced asthma, cold outdoor workouts can provoke an attack that looks and feels like an illness but involves no infection at all.
How to Protect Yourself
Covering your nose and mouth with a scarf or mask in cold weather does something measurable. Research on face coverings in cold conditions found that while intranasal air temperature stayed stable, the humidity of inhaled air and the temperature of nasal mucosa both increased significantly after wearing a covering. This helps maintain the moist environment your nasal defenses need to function properly. Your nose also responded by opening up, with increased cross-sectional area, improving airflow without sacrificing air conditioning.
Beyond that, the same basics apply year-round but matter more in winter: wash your hands frequently, since viruses survive longer on surfaces in dry conditions. Keep indoor air humidified, ideally above the low-humidity range where flu viruses thrive. Get enough vitamin D through diet, supplements, or whatever sun exposure is available. And when possible, improve ventilation in shared indoor spaces, even briefly cracking a window can dilute the concentration of airborne particles.
Cold air is not a pathogen. But it reshapes the battlefield in ways that favor viruses and disadvantage your body’s natural defenses. The cold itself isn’t what makes you sick. It’s what cold does to everything around the infection equation.