I was reading Cresson H. Kearny's Nuclear War Survival Skills this evening, and there's an interesting discussion of a method for protecting bulk grain and beans from "weevils, other insects, and rodents." Remember that you wouldn't want to use an insecticide on food, and it's really hard to get all the teeny, tiny little critters out of bulk grains or beans. So the method is astonishingly elegant.
Place about 4 ounces of dry ice on top of the grain in a 5-gallon metal container. Put the lid on somewhat loosely, so that air in the grain can be driven out of the can. (This will happen as the dry ice vaporizes and the heavy carbon dioxide gas sinks into the grain and displaces the air around the kernels.) After an hour or two, tighten the lid and seal it with tape. After one month, all insects in this carbon-dioxide atmosphere will have died from lack of oxygen.We just finished setting off a couple of bug bombs in the crawlspace under the house, because we were getting too many wolf spiders here in our rooms--and I had a couple of nasty bites in my sleep last year from something that decided to crawl into bed with me.
The bug bombs work reasonably well--but setting up the bug bombs is a nuisance, since they almost never stay vertical (as they are supposed to) when I am putting in place through the rather limited access openings. I also prefer not to have more insecticide floating through my environment than necessary, especially because I'm concerned that we could be breeding an insecticide-resistant strain of superbug down there. But what about carbon dioxide?
It turns out that that both carbon dioxide and nitrogen atmospheres are used for fumigating food. It also appears that raising the partial pressure of oxygen to 10 atmospheres (so about 50 times normal air pressure) not only kills a number of bugs because of oxygen poisoning, but the enhanced pressure may increase carbon dioxide poisoning as well. (Interesting, but not practical in our crawlspace.) A 60% carbon dioxide atmosphere is more lethal to some bugs (the ones that are considered a problem in food) than a 100% carbon dioxide atmosphere. It also appears that raising the temperature accelerates the killing effects of all these fumigant approaches. I'm guessing that the increased temperature accelerates biochemical reactions that cause high concentrations of carbon dioxide to kill the insects.
Anyway: my thought is to figure out how many pounds of dry ice it would take to get a 20% carbon dioxide atmosphere in the crawlspace. Because carbon dioxide is much heavier than air (molecular weight is 44, compared to about 29 for air), it will leak out through the crawlspace vents, rather than rising. And with the warm temperatures that we have been having lately, a few pounds will go gaseous during one long day, and likely produce a continuous stream of insect death under the house. (I don't think that there would be hazards in the house above, but we'll have all the windows open, and let the continual winds up here keep the house itself fresh.)
Carbon dioxide's weight is 44 grams per mole (which is 22.4 liters at standard temperature and pressure). There's about 6000 cubic feet in the crawlspace, or about 170,000 liters, or 7589 moles. To completely fill that space would require 334 kg of carbon dioxide; to get a 20% atmosphere (assuming instant sublimation to gas) would require about 150 pounds of dry ice. This might be an interesting experiment, since dry ice isn't hideously expensive, and doesn't leave any nasty toxic residue after it has all drifted out from under the house. Unlike all other gases, your body recognizes a dangerous carbon dioxide concentration immediately, and lets you know about it.
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