It is a common belief passed down through generations that stepping outside into cold weather with wet hair will directly lead to sickness. This idea often centers on the notion that the chill itself, or the rapid cooling of the head, somehow causes a cold or the flu. This assumption creates a direct, but scientifically incorrect, link between temperature exposure and infectious illness. The myth suggests that a person’s physical state of being cold is the primary trigger for catching a respiratory infection.
Addressing the Myth Directly
The definitive answer to whether wet hair in the cold makes you sick is no; temperature alone does not cause an infectious illness. Neither cold air nor the chilling effect of water evaporating from your hair introduces a pathogen into your system. Feeling cold may cause temporary discomfort, but it cannot spontaneously generate the viruses responsible for a cold or the flu. Getting sick requires exposure to a foreign infectious agent that successfully invades the body. However, temperature can influence the body’s physical ability to resist pathogens that are already present or encountered.
The True Cause of Colds and Flu
Infectious illnesses like the common cold and the flu are exclusively caused by viruses, not by environmental temperature. The common cold is predominantly caused by Rhinoviruses, though over 200 different virus strains can be responsible, including various coronaviruses and adenoviruses. The flu, or influenza, is caused specifically by the influenza virus, which typically results in more severe symptoms than a cold, such as higher fever and body aches. These viral agents are transmitted when an infected person expels respiratory droplets into the air through coughing, sneezing, or talking.
These tiny, virus-laden aerosols can then be inhaled by a healthy person, or the virus can be transferred indirectly through contaminated surfaces. When the virus enters the body, usually through the nose or mouth, it begins to replicate and causes inflammation in the membranes of the upper respiratory tract. The symptoms experienced, such as a runny nose and sneezing, are largely the result of the body’s immune response attempting to clear the infection. The presence and action of the specific virus are the necessary conditions for illness to occur, completely independent of whether a person’s hair is wet or dry.
How Cold Exposure Impacts the Immune System
Acute cold exposure can temporarily compromise the body’s localized defenses against viruses. When a person inhales cold air, the temperature inside the nasal passages drops significantly, which can impair the effectiveness of the body’s first line of defense. A decrease of just 9 degrees Fahrenheit (about 5 degrees Celsius) in the nasal tissue temperature has been shown to reduce the local immune response dramatically. Nasal epithelial cells normally release microscopic structures called extracellular vesicles (EVs), which are filled with antiviral molecules to neutralize pathogens.
This drop in temperature can reduce the number of these protective EVs released by over 40% and compromise their quality, weakening the innate immune response. Furthermore, cooler temperatures can slow the movement of the cilia, which are small, hair-like projections that normally sweep mucus and foreign particles out of the respiratory tract. This impaired clearance allows viruses that have already been inhaled to attach to and infect nasal cells more easily and for a longer duration. This physiological response explains why cold weather is associated with increased susceptibility.
Why Illness Spreads More in Winter
The correlation between cold weather and increased sickness is largely explained by a combination of environmental and behavioral factors. As temperatures drop, people spend more time indoors in closer proximity to one another, which facilitates the transmission of respiratory viruses. Crowded indoor spaces, especially those with poor ventilation, allow airborne droplets containing viruses to concentrate and linger for longer periods. Low humidity in the winter air, often exacerbated by indoor heating systems, also plays a significant role in viral survival and spread.
In drier conditions, the water content of virus-containing droplets evaporates quickly, leaving behind smaller, lighter particles that can remain suspended in the air and travel farther. This low-humidity environment also dries out the mucous membranes in the nose, making them less effective as a physical barrier against invading pathogens. Additionally, some enveloped viruses, like Influenza and COVID-19, are structurally more stable and can survive longer on surfaces and in the air when the temperature is cold and the air is dry. These external conditions and changes in human behavior create an environment where viruses thrive and transmission is maximized.