Viruses are microscopic infectious agents that can only replicate inside the living cells of an organism. They are not considered living organisms in the strictest sense because they lack the ability to reproduce or carry out metabolic processes independently. A virus consists of genetic material, either DNA or RNA, encased within a protective protein shell called a capsid. Some viruses also possess an outer lipid envelope. Viruses infect all forms of life, from animals and plants to bacteria, and are responsible for a wide range of diseases.
How Heat Inactivates Viruses
High temperatures inactivate viruses primarily by denaturing their proteins and damaging their genetic material. Denaturation is a process where the complex, folded structures of proteins, such as those forming the viral capsid or enzymes necessary for replication, unravel and lose their functional shape. This structural damage prevents the virus from attaching to host cells, injecting its genetic material, or replicating once inside a cell.
The genetic material, whether DNA or RNA, is also susceptible to heat. While viral RNA can be relatively heat-stable, temperatures above 60°C can denature it, further hindering the virus’s ability to infect. This combined assault renders the virus unable to cause infection. The process is more accurately termed inactivation rather than “killing.”
Temperature Thresholds for Disinfection
Heat effectively disinfects surfaces and objects from viruses. For general disinfection, temperatures from 60°C (140°F) to 100°C (212°F) or higher are used, with times varying by virus and surface. For example, moist heat is recommended for SARS-CoV-2 inactivation. Steam cleaning, generating hot water steam at 100°C (212°F) or higher, inactivates viruses on surfaces like metal, plastic, and fabric within minutes.
Dishwashers and laundry cycles use elevated temperatures to inactivate viruses. Hot water cycles in dishwashers, reaching temperatures above 60°C (140°F), break down viral structures. Similarly, hot water wash cycles in laundry, especially those above 60°C, are effective for disinfecting fabrics. Maintaining temperatures above 70°C (158°F) is effective for thermal disinfection on temperature-resistant surfaces.
Heat and Foodborne Viruses
Cooking food to appropriate internal temperatures inactivates foodborne viruses. Norovirus, for instance, can be inactivated at temperatures exceeding 63°C (145°F). Hepatitis A virus is also susceptible to heat; proper cooking inactivates it in contaminated food.
For poultry, an internal temperature of at least 74°C (165°F) is recommended to eliminate viruses and bacteria. Ground meat, such as ground beef, should be cooked to an internal temperature of 71°C (160°F). Leftovers should also be reheated to 74°C (165°F) to ensure any potential viral contaminants are inactivated. Use a food thermometer to confirm these safe internal temperatures are reached throughout the food.
Factors Influencing Viral Inactivation
Beyond temperature, several other factors influence viral inactivation by heat. Exposure time is one factor; a virus needs sufficient duration at a specific temperature to be inactivated. Higher temperatures allow shorter exposure times. Lower temperatures can still inactivate viruses but require longer periods.
Initial viral load, or virus concentration, also plays a role; higher concentrations require more rigorous conditions for complete inactivation. Organic matter, like proteins or bodily fluids, can protect viruses, requiring higher temperatures or longer exposure. Humidity is another factor; moist heat is more effective than dry heat for viral inactivation, as water aids denaturation.
Common Misconceptions About Temperature and Viruses
A common misunderstanding is that cold temperatures “kill” viruses. Freezing does not destroy viruses; it preserves them, allowing them to remain infectious once thawed.
Another misconception is that internal body temperature can be raised enough to “kill” a virus. While the body’s immune system raises its temperature during a fever to create a less hospitable environment for pathogens, this increase is not high enough to directly destroy viruses. Fevers help the immune system function and can slow down viral replication, but they do not eliminate the virus entirely.