The common belief that bringing cold air into a house kills germs is a misconception. Cold temperatures alone do not function as a sterilizer for common pathogens like bacteria, viruses, or mold spores. The perception that cold air improves indoor air quality is due to other atmospheric factors that affect how germs are physically removed from a space. The real controls over household germs are air movement, dilution, and the moisture level in the air.
Does Cold Temperature Inactivate Germs?
The effect of cold on microorganisms is generally to slow their processes, not to destroy them completely. Bacteria exposed to the temperature of typical winter air or even a household freezer enter a state of metabolic stasis or dormancy. This means they stop reproducing and growing, but they remain preserved, ready to become active again once temperatures rise to a comfortable range.
For laboratory purposes, scientists often store bacteria and other microbes at extremely cold temperatures, such as -80°C (-112°F), specifically because the deep cold prevents death and preserves them for later study. Viruses also maintain their structure well in cold conditions. Many respiratory viruses, including influenza and some coronaviruses, are actually more stable and survive for longer periods outside of a host body in cold, dry air compared to warm, humid conditions. Mold spores are similarly resilient, entering a dormant state when the air temperature drops. Cold air does not eliminate them, and they remain viable, waiting for the right combination of moisture and warmth to trigger renewed growth. True sterilization requires high heat or harsh chemicals, not the mild cold achievable by opening a window.
The Crucial Impact of Air Exchange
The relief often felt when introducing cold outdoor air is a result of physical dilution, not temperature-based killing. Stagnant indoor air allows fine airborne particles, known as aerosols, which may contain viruses or bacteria from breathing, talking, or coughing, to accumulate and reach higher concentrations. This concentration increases the risk of transmission within the space.
Opening a window introduces a high volume of fresh, clean air, which physically pushes the contaminated indoor air out, effectively lowering the overall concentration of airborne pathogens. The effectiveness of this process is measured by the Air Changes per Hour (ACH), which is the number of times the total volume of air in a room is replaced in one hour. A typical tightly-sealed home often has a low ACH.
Increasing the ACH is the most effective way to reduce the risk of inhaling infectious particles. Health organizations suggest aiming for five or more air changes per hour of clean air for significant virus mitigation. This physical movement of air, whether by opening windows, using exhaust fans, or employing air purifiers, is the true mechanism that helps reduce the spread of illness in a home during the colder months.
Humidity: The Silent Controller of Germs
The moisture content, or relative humidity (RH), of indoor air plays a significant role in the survival of airborne germs and the body’s ability to defend against them. The ideal range for both human health and virus inactivation is between 40% and 60% relative humidity. Within this range, many airborne viruses are rapidly deactivated, reducing their infectious time.
When cold outdoor air is brought inside and heated, its relative humidity drops sharply, often falling below 40%. This low moisture level is detrimental to human respiratory defenses, causing the nasal passages and mucous membranes to dry out. A dry respiratory tract is less effective at trapping and expelling inhaled pathogens, making the body more susceptible to infection.
Furthermore, low humidity extends the airborne life of many respiratory viruses, allowing them to remain infectious for longer periods outside the body. Conversely, letting humidity climb too high, generally above 60%, can also cause problems by creating a favorable environment for the growth of mold and dust mites, which are common allergy and asthma triggers. Maintaining the air’s moisture balance is therefore a key strategy for indoor health that is independent of the actual temperature.