The influenza virus is an enveloped RNA virus that causes a contagious respiratory illness. Understanding the temperature at which this virus is inactivated is a practical necessity for cleaning and environmental safety. High temperatures destroy the virus’s ability to infect, a process called inactivation, while cold temperatures merely preserve its infectious structure. The virus’s viability outside of a host is heavily influenced by its surrounding environment, especially temperature and moisture levels.
The High Temperature Threshold for Inactivation
High heat is the most effective and rapid method for guaranteeing the destruction of the influenza virus. Laboratory studies demonstrate that the virus can be rapidly inactivated at temperatures starting around 56°C (133°F). For instance, an effective reduction in viral infectivity can be achieved in about 30 minutes at 60°C (140°F).
Raising the temperature even slightly above this threshold drastically reduces the required exposure time for complete inactivation. At 70°C (158°F), certain influenza strains can be completely inactivated in as little as five minutes, showing a strong inverse relationship between temperature and survival time. This principle is utilized in processes like pasteurization, where heating to 72°C (161°F) for only 15 seconds is sufficient to eliminate viral infectivity in liquids.
Practical applications of this thermal threshold are seen in household disinfection, such as laundering and sterilization. To disinfect clothing and linens contaminated with the flu virus, washing in water that reaches at least 60°C (140°F) is recommended. Using a high-heat drying cycle provides an additional layer of protection. The application of steam or boiling water achieves reliable destruction of the virus on surfaces and materials.
Survival and Preservation in Cold Environments
Cold temperatures do not destroy the influenza virus but instead act as a preservative for its infectious structure. The virus thrives in lower temperatures because its protective lipid envelope remains stable, extending its period of infectivity. This stability is a significant factor in the seasonal pattern of influenza, which typically peaks during the colder winter months.
Temperatures around 5°C (41°F) have been shown to be highly favorable for the stability and transmission of the virus. This extended survival time is why viral samples in research laboratories are stored under freezing or cryogenic conditions, often at -70°C (-94°F) or below, to maintain their viability indefinitely. At common freezer temperatures, such as -20°C (-4°F), the virus can still lose infectivity over time, but its lifespan is significantly longer than at warmer temperatures.
The improved stability in the cold means that the virus remains infectious for longer periods on environmental surfaces and in the air. This extended viability, combined with people spending more time indoors in close proximity, contributes to the enhanced spread of the flu during winter.
The Role of Room Temperature and Humidity in Viral Longevity
The longevity of the influenza virus at typical room temperature is governed by the type of surface and, critically, the humidity level. On non-porous, hard surfaces like stainless steel, plastic, or doorknobs, the virus can remain infectious for 24 hours or even longer under certain conditions. However, on porous surfaces like clothing or tissues, the survival time is generally shorter, typically a few hours to a day.
The most important environmental factor influencing survival at room temperature is relative humidity (RH). The influenza virus is known to survive longest in dry air, such as below 40% RH. As heated indoor air often results in very low humidity, this contributes to viral survival.
Conversely, the virus’s infectivity is often lowest at intermediate humidity levels, around 50% RH. At this mid-range, the salt concentration within the microscopic respiratory droplets that carry the virus becomes most damaging to its structure, leading to rapid inactivation. Therefore, maintaining indoor humidity between 40% and 60% is a recommended strategy to reduce the airborne survival time of the virus.