What is the minimum air temperature that can kill SARS-CoV-2 instantaneously?

Question

I found this paper that talked about an air filtration system that heats up air to 200°C that kills SARS-CoV-2 in a single pass. But they seem to not be so sure of how hot it should actually be.

I've read some other papers that talked about the virus's temperature rating and the highest temperature they checked was 80°C. At that heat the virus still takes about a minute to die. I would guess 100°C would kill it faster but how fast? Has there been a study that tested the virus on temperatures above 100°C?

Any help is appreciated.

Answer 1

Dry heat sterilization is fairly inefficient, time-wise, in general. According to Wikipedia, which cites the CDC for this:

The proper time and temperature for dry heat sterilization is 160 °C (320 °F) for 2 hours or 170 °C (340 °F) for 1 hour or in the case of High Velocity Hot Air sterilisers 190°C (375°F) for 6 to 12 minutes.

But this is for all mirco-organisms, which some of which are more hardy than coronaviruses. I'm not sure if there's a study just on the latter in re dry heat.

Also this is not exactly what you're asking here, as you want to sterilize the air, but it is somewhat relevant if you're trying to do that by heat transfer from a surface, e.g. a hot plate, coil etc. Because of inefficiency relative to other methods, this way of sterilizing air it might not have been studied too much, e.g.:

Many ways have been suggested for sterilizing air. These include destruction of microorganisms by: dry heat-gas fired or electrical; adiabatic compression; and irradiation or removal of microorganism. The removal of microorganisms involve: scrubbing, electrostatic precipitation, sieving, and filtration fibrous or granular beds. Of these, only adiabatic compression, filtration through beds of fibrous materials, and filtration through beds of granular materials have found widespread usage on an industrial scale.

(Albeit that's a 1960's article, but physics didn't change much since...)

The (1st) (2020) paper you've linked to uses a heated filter, so it's really a combination method. (The filter had non-zero effect even without it being heated, if you look at their graphs--fig 3.) That paper also mentions the air flow rate at which they've tested their system. Which of course is a factor in heat transfer too; e.g. they quantified the observed temperature drop with increased air flow, albeit only at couple of points.

See also sterility assurance level for what professionals mean when they say something is sterilized. Basically, it's a statistical measure/guarantee, not an absolute one. In the 2020 paper you've linked to, they set/measured that at 99.8%.

Answer 2

Viruses are not "alive" in the sense that larger organism is. They are like little machines. So, they do not "die", they simply function at different levels of efficiency. So, for example, an increase in temperature may damage a virus and impair its function. Even if a virus is impaired and unable to reproduce by itself, it can still act in various ways, such as loaning its DNA to other viruses.

In a recent study of inactivation of various shellfish viruses, it was found that a temperature of 80-degrees centigrade (176 Farenheit) for 12 seconds was sufficient to inactivate all the viruses under study and this appeared to be due to capsid disruption. Since SARS-COV2 and all coronaviridae have a similar capsid, they probably subject to the same rule.

The study found that a lesser temperature of 62-degrees centigrade was sufficient to inactivate some viruses, if it was applied for as long as 30 minutes, but not all of the test viruses were completely inactivated.

Note that the chemical behavior of a virus can differ depending on the media in which it exists. Respiratory viruses spend their lives adhering to various proteins in mucus. If they are removed from that environment in placed dry air or even water, they can become unstable and more fragile.

Also, it is worth noting that even small variations in temperature can signficantly affect the efficiency of viral reproduction. For example, animals will often raise body temperature (a fever) to defend against an infection, and this change in temperature in the case of humans is only from 98 degrees Farenheit to 104 degrees, but even so it is enough to impair viral reproduction significantly.