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Using Heat to Kill the Coronavirus
Question: Does heat kill coronavirus (COVID-19)? Can it disinfect face masks and packages?
Answer: If used properly, moderate or high heat can be used to "kill" coronavirus, inactivating the virus so that it is no longer infectious. However, in many cases this may be more time-consuming than other methods such as chemical disinfection (with disinfecting wipes or sprays) and there is often no need to disinfect packaging at all if you can just remove the packaging, dispose of it (preferably by recycling), and then disinfect your hands by washing with soap and warm water or using another disinfection method. Furthermore, the FDA has stated that "...there is currently no evidence of human or animal food or food packaging being associated with transmission of the coronavirus that causes COVID-19." Although the agency notes "...if you wish, you can wipe down product packaging and allow it to air dry, as an extra precaution."
Heating kills coronavirus
Heat is very effective at sanitizing and disinfecting objects from coronavirus. If anyone tells you that coronavirus is resistant to heat, they're wrong. In fact, the SARS-CoV-2 virus that causes COVID-19 may be even more sensitive to heat than the earlier SARS-CoV virus. Experiments done in China in 2002 with SARS-CoV in culture medium (Duan, Biomed Env Sci 2003) showed that coronavirus became undetectable after 30 minutes when heated to a temperature of 167°F, but recent tests with SARS-CoV-2 in Hong Kong showed that it became undetectable after just five minutes at only 158°F (70°C). The time required to kill SARS-CoV-2 increased as the temperature was reduced, such that the time by which it was undetectable increased to 30 minutes at 132°F (56°C), two days at 98.6°F (37°C), and two weeks at 71.6°F (22°C). At 39°F (4°C) the virus remained detectable at two weeks when the experiment ended (Chin, Lancet 2020).
This suggests, for example, that if you purchase take-out food and wish to disinfect the container itself of coronavirus (as well as keep your food warm), you can simply place the container in a warm oven or warming drawer for a period of time, such as at 150°F (65°C) for 60 minutes (giving it ample time to heat up) to disinfect it. Just be sure it is not directly exposed to a heating element so as not to pose a fire hazard. Most plastic and paper containers are stable for short periods at up to 200°F (93°C).
Theoretically, heating a face mask this way may also disinfect it. Moist heat may be better than dry heat, so, if done in an oven, placing a pan of water in the oven may be best. A study by researchers at Stanford University found that heating N95 masks (known as respirators) at 167°F (75°C) for 30 minutes (at 85% relative humidity) did not compromise the masks, even after 20 cycles — although these researchers have subsequently cautioned that contaminated masks should not be brought into homes. (If you use an N95 respirator, you may want to watch a video from the New England Journal of Medicine that instructs health care workers on how to properly put on, take off, and test the fit of such masks and other personal protective equipment.)
A U.S. government laboratory heated SARS-CoV-2-contaminated N95 mask material in a dry oven at 158°F (70°C) and found that virus was undetectable at 50 minutes, but they also found two cycles of this dry heating caused the material to lose some integrity (possibly suggesting that some humidity during heating may be beneficial) (Fischer, medRxiv, 2020 — preprint).
These same government researchers found that ultraviolet light (UVC at 0.005 mW/cm2) could achieve the same antiviral effect as heat with less impact on mask integrity but required about 60 minutes and this did not factor in additional time to properly irradiate curved surfaces of the mask. They determined that vaporized hydrogen peroxide (requiring a special, enclosed incubator) was the fastest and least damaging method of decontamination, taking only 10 minutes. Spraying ethanol to saturate the mask material was a bit faster at deactivating virus but caused the greatest reduction in mask integrity (Fischer, medRxiv, 2020 — preprint).
You should not soak N95 or surgical masks in disinfectants such as alcohol or other liquids as this can compromise their electrostatic charge (reducing their filtering capability) and fit (Interview with R. Shaffer, JN Learning 2020). Researchers at Stanford University found that immersing N-95 mask material in 75% ethanol or spraying with a household chlorine bleach solution (2% sodium hypochlorite) decreased the filtration efficiency (due to loss of electrostatic charge) to unacceptable levels after just one treatment, from about 96% to 56% and 73%, respectively. Steam treatment (material placed 6 inches above a glass containing boiling water for 10 minutes) was effective if used once, but after five treatments filtration efficiency fell below acceptable levels (85%) (Liao, ACS Nano 2020).
Note that washing cloth face masks in a washing machine should suffice to disinfect them, according to the CDC. The agency also cautions not to touch your eyes, nose, and mouth when removing cloth coverings, and to wash hands immediately after removing.
Coronavirus lasts longer on certain surfaces, particularly surgical masks!
The Hong Kong researchers noted above also placed a small amount of the SARS-CoV-2 on a variety of surfaces at room temperature (at 65% relative humidity) to see how long the virus would last before becoming undetectable. On tissue and regular paper it became undetectable within just 3 hours. On cloth and on paper money, it lasted 2 days. Surprisingly and disturbingly, it lasted longest on the outer layer of a surgical mask: Virus was detectable on the mask at day 7 (although at only 0.1% of its original level), which was also how long it lasted on plastic and stainless steel.
Don't directly refrigerate or freeze — it keeps the virus infectious
Studies with coronaviruses noted above and by others (Kampf, J Hosp Infect 2020) indicate that cold temperatures help preserve it and keep it infectious. Consequently, you should not place a recently purchased food container directly into a refrigerator or freezer and you should not "quarantine" a recently received package in a cold cellar or cold garage, as this will preserve coronavirus and could keep it infectious for days. Freezing can preserve coronavirus for years (WHO, COVID-19 Situation Report 32, 2020). The good news is that the coronavirus won't linger quite as long on surfaces and in the air in warm summer weather as it does in the winter, although this won't make much of a difference should an infected person cough near you.
How effective are disinfectants against coronavirus?
The Hong Kong researchers also showed that common disinfectants were effective in killing SARS-CoV-2. The virus was undetectable after 5 minutes of exposure to household bleach (at a concentration of 1:49 or 1:99), ethanol (70%), povidone-iodine (7.5%), chloroxylenol (0.05%), chlorhexidine (0.05%) and benzalkonium chloride (0.1%). Fifteen minutes were required for the virus to be undetectable when exposed to a hand soap solution. The researchers only checked at 5 and 15 minutes, so it is possible that less time is necessary, but as we don't know the minimum time for disinfection, it would seem prudent to allow disinfectants a few minutes on surfaces before wiping them off.For the latest list of disinfectants that meet the U.S. EPA's criteria for use against SARS-CoV-2, see the EPA's "List N." There is a search box to quickly look up a product or ingredient and you should view the "Contact Time" to see how long each disinfectant must remain wet on a surface. The list includes ready-to-use liquids, dilutable liquids, and wipes. (Note that products on List N have not been tested specifically for SARS-CoV-2, but have demonstrated efficacy against a similar coronavirus or a harder to kill virus.)
Be careful when using and storing chemical disinfectants
Calls to poison control centers increased sharply in March 2020 due to disinfectants (CDC, 2020). Cases have included a woman who developed difficulty breathing after mixing 10% bleach with vinegar and water to wash her produce and a child was found unresponsive after consuming ethanol-based hand sanitizer. The CDC advises that "users should always read and follow directions on the label, only use water at room temperature for dilution (unless stated otherwise on the label), avoid mixing chemical products, wear eye and skin protection, ensure adequate ventilation, and store chemicals out of the reach of children."
If you use a disinfecting wipe on a surface that may come in contact with food or be placed in the mouth (like a baby bottle), be sure to rinse the surface with water and dry after wiping. A Clorox spokesperson told ConsumerLab that "Clorox Disinfecting Wipes can disinfect plastic packaging that is non-porous. Packaging should not have any holes that would allow the disinfectant to make direct contact with food. The wipes should never be used directly on food and should not be used on paper or cardboard packaging." As some consumers have wondered if the wipes can be used on microwaveable "steam" bags (as for steaming vegetables), it would seem that the wipes can be used on the front and back surfaces but perhaps not at the ends and seams where steam vents are placed.
Clorox wipes have a shelf life of one year from the date of manufacture, and Lysol indicates two years for its wipes.