In conversation in the Smoky Drinky Bar last night (and with Legiron a week or so back), the subject of the bactericidal, fungicidal, and insecticidal properties of tobacco smoke (in fact more or less any wood smoke) came up. Unfortunately, smoke doesn’t kill off viruses as well – although viruses are actually just pieces of DNA, and not living things.
It set me wondering what it was in (tobacco) smoke which killed off bacteria and fungi and insects. Nicotine is a poison, after all. And there are probably any number of poisons in a great many plants.
But then it occurred to me that it was perhaps not so much what was in smoke that gave it these lethal properties, but rather what was not in smoke that did so. And what was not in smoke – or not present in such great quantities as usual – was oxygen.
After all, the combustion of plant material entails combining oxygen with combustible substance, and the oxygen required for combustion is drawn from air, in which it is present in a concentration of 20% by volume. So smoke that is the product of combustion must be low in oxygen. Maybe it’s simply this oxygen deficiency which kills off bacteria and fungi and insects? Perhaps a reduction of oxygen concentrations from 20% to 5% or less might be enough to do that.
Large animals (like humans) are able to store air for short periods in their lungs. And oxygen is carried in their bloodstream in protective packets of haemoglobin in red blood cells. But insects (and presumably bacteria and fungi) don’t have lungs. They simply have pores in their bodies into which air diffuses, carrying oxygen. Their (blue) blood (haemolymph) uses haemocyanin rather than haemoglobin to transport oxygen. But because they are such small animals, it’s unlikely that they can store much oxygen. And so it may only need a brief interruption in the supply of oxygen for them to exhaust their oxygen stores and die. Larger animals, carrying larger stores of oxygen in lungs and extensive blood systems, would take longer to die in the same oxygen-depleted environment.
Of course, fresh oxygen will normally rapidly diffuse into an oxygen-depleted air space, but only if there is good ventilation. If a room (or building) is sealed, before a fire is lit to fill it with smoke, it may take a long time for fresh, oxygenated air to diffuse into the building. And this will mean that any bacteria or insects – even those is cracks and crevasses – will eventually experience a lethal oxygen deficiency.
If it’s oxygen deficiency that’s killing them, the important thing would be to arrange for the most complete combustion of as much of the oxygen in the air entering the fumigated space. It would need an experiment of some sort to determine whether oxygen depletion was the cause of death, rather than something else (such as the chemical components of smoke).
This explanation would also explain why viruses aren’t killed by fumigation. Viruses don’t breathe or absorb oxygen. So viruses are going to survive fumigation.
But it’s unlikely that viruses would survive combustion. A virus passing through a flame or ember would probably be oxidised along with everything else being burned.
And this prompted the thought that smokers probably experience some degree of protection from viruses because some of the air that they breathe has been passed through the high temperatures (800º C) in the glowing tips of their cigarettes (or pipes or cigars), and any viruses in that air will have been destroyed. If, for example, smokers get 5% of the air they breathe drawn through the tips of their cigarettes, then they will have 5% better protection from viral infections than non-smokers.
And if it could be arranged that the air people breathed had all been passed through the flame of combustion, it would be entirely free of viruses, and they would be immune to airborne viral epidemics – which would only affect non-smokers.
In fact, it would not need any smoke or flame to destroy viruses. The same effect could be achieved simply by heating air to a high enough temperature.
In fact it appears that much lower temperatures may suffice. Simply storing one particular waterborne virus in water at 15º C for 50 days resulted in lower survival rates (by a factor of 10) than in water at 4º C. Others suggest that if air is raised to 75º C for some period of time, this alone may be enough to kill off most of the viral load carried in it. Interestingly, it seems that viruses have half-lives:
the half-life of free infectious [influenza A] virus is ∼3 h.
This suggests that anyone who lives far enough away from the centre of any viral epidemic will be less likely to be infected because viruses lose infectivity with time. But we knew that already.
Aside from this, it should be noted that alcohol is also bactericidal (and was one reason why humans have often drunk beer or wine), and may also be virucidal.
Viruses cannot be killed, since they are not alive. However, they can be inactivated. Alcohol, bleach, acids and heat can inactivate a virus. It all depends on the virus in question.
All of which suggests that if you want to avoid bacterial or viral infections, smoking and drinking (preferably in tandem, and in large quantities) will result in a much healthier and infection-free life than a smoke-free, alcohol-free, supposedly “healthy” lifestyle.
The third and final part of my conversation with Legiron below touches on some of the subjects just discussed: