Temperature significantly influences viruses, affecting their ability to cause disease. Understanding this impact helps control their spread and develop effective disinfection strategies. This article explores how temperature affects viral viability and its practical implications.
How Heat Damages Viruses
Elevated temperatures inactivate viruses by disrupting their structural components and genetic material. Viruses consist of genetic instructions, either DNA or RNA, encased in a protein shell called a capsid, and sometimes an outer lipid envelope. Heat primarily targets these protein structures. When exposed to sufficient heat, viral proteins undergo denaturation, a process where their complex three-dimensional shapes unravel. This denaturation affects capsid proteins, rendering the virus unable to attach to host cells or protect its genetic material.
Beyond capsid proteins, heat also denatures enzymes essential for viral replication once inside a host cell. The genetic material itself, whether DNA or RNA, can be damaged by high temperatures, preventing effective replication. This combined assault on viral proteins and genetic material renders the virus non-infectious.
Specific Temperatures for Viral Inactivation
The temperature required to inactivate viruses varies depending on the specific virus and exposure duration. Higher temperatures generally achieve inactivation more quickly. Many viruses, including those causing waterborne illnesses like hepatitis A and enteroviruses, are inactivated between 60°C (140°F) and 65°C (149°F). Some viruses can be inactivated at 50°C (122°F) or higher, though more slowly than bacteria.
Increasing the temperature to 70°C (158°F) or above can significantly reduce viral infectivity, often by more than 99.999%, in under a minute for viruses like poliovirus and hepatitis A. Boiling water at 100°C (212°F) is a highly effective method, rapidly inactivating most pathogenic organisms, including viruses and bacteria. A rolling boil for one minute is sufficient to inactivate most viruses.
Using Heat for Practical Disinfection
Heat serves as a practical and effective method for disinfection in various everyday scenarios. In laundry, washing clothes in hot water, ideally 60°C (140°F) or above, helps inactivate viruses. For items that can withstand higher temperatures, a full-length cycle with bleach-containing detergent further enhances inactivation. Drying clothes completely on a hot setting in a tumble dryer also contributes to killing germs.
Dishwashers utilize high temperatures to sanitize dishes. Commercial dishwashers often require rinse water temperatures of at least 82°C (180°F) to ensure a 99.999% reduction of microorganisms. Household dishwashers with a sanitize cycle can reach temperatures upwards of 77°C (170°F), effectively inactivating viruses and bacteria. Pasteurization, a heat treatment process for food and beverages, involves heating products to specific temperatures, such as 63°C (145°F) for 30 minutes or 72°C (161°F) for 15 seconds, to eliminate harmful pathogens, including viruses.
Beyond Temperature: Other Factors Influencing Viral Survival
While temperature is a significant determinant, other environmental factors also influence viral survival. Humidity plays a complex role; some viruses survive longer at very low or very high relative humidity, inactivating more rapidly at intermediate levels. For instance, the SARS coronavirus demonstrated better stability at low temperature and low humidity, but its viability rapidly decreased at higher temperatures and high relative humidity.
The presence of organic matter, such as mucus or bodily fluids, can shield viruses from disinfectants and environmental stresses, extending their survival on surfaces. pH levels also affect viral stability; extreme pH values, both acidic and alkaline, can induce structural changes that lead to inactivation. Ultraviolet (UV) light, particularly UVC radiation, damages viral genetic material and proteins, leading to inactivation.
The Role of Cold Temperatures
Cold temperatures generally do not kill viruses; instead, they preserve them. Freezing can inhibit viral replication and spread, but allows viruses to remain viable for extended periods. Some viruses can persist for as long as 28 days at 4°C (39°F).
This principle is utilized in laboratories for long-term virus preservation, storing viruses at low or ultra-low temperatures, often -70°C (-94°F) or in liquid nitrogen, to maintain infectivity. While freezing can make viruses dormant, slowing their activity, it does not destroy their infectious potential. Relying on cold alone is not an effective method for viral inactivation.