Cold exposure and cold-related stress can also interfere with the normal function of the airways, particularly in people with sensitive respiratory systems.
Together, these effects can suppress the body’s first lines of defence in the nose and throat. Cold air does not create viruses, but it can make it easier for viruses to gain a foothold once exposure occurs.
Crowds and close contact
Seasonal changes in human behaviour and indoor environments also play a major role. Cold weather encourages people to spend more time indoors, often in close contact with others. Crowded spaces with poor ventilation allow virus-containing droplets to build up in the air, making transmission more likely.
During winter, reduced sunlight exposure leads to lower production of vitamin D in the skin. Vitamin D plays a role in regulating immune function, and low levels are associated with weaker immune responses. Indoor heating, while essential for comfort, also dries out the air.
Dry air can dry the lining of the nose and throat, reducing the effectiveness of mucus. Mucus normally traps viruses and helps move them out of the airways, a process known as mucociliary clearance. When this system is impaired, viruses have an easier time infecting cells.
Cold weather can be especially challenging for people with existing respiratory conditions such as asthma or allergic rhinitis, commonly known as hay fever.
Epidemiological studies — research that examines patterns of disease in populations — show that cold conditions can worsen symptoms and increase functional impairment in these individuals. This can intensify the effects of respiratory infections when they occur.
Taken together, the evidence paints a clear picture of what cold weather does and does not do. Cold temperatures are linked to higher rates of respiratory infections, including influenza and coronaviruses, particularly in temperate regions during winter. Laboratory and environmental studies show that viruses survive longer and spread more easily in cold, dry air.
Cold exposure can also weaken immune defences in the nose and airways, including reduced mucus movement and decreased antiviral activity in nasal tissues. Behavioural and environmental factors typical of winter — such as indoor crowding, poor ventilation, and reduced sunlight leading to lower vitamin D levels — further increase the risk of viral spread.
