O.K., I'll take a crack at this again.
FOOD GRADE is an FDA designation, but it was vetted by the EPA ( AMORPHOUS NOT
CRYSTALLINE). It is used (as has been mentioned) primarily as an anticaking agent and for insect control. Its designation by the FDA is that it is GRAS (Generally Regarded As Safe).
Please download and read the EPA Report:
A:SILICA2.PDF06-14-2000
... are limited to the naturally mined silicon dioxide-containing product diatomaceous earth... revise the exemptions in 40 CFR 185.1700 and 186.1700 for diatomaceous earth to include silica gel used in food and feed handling ...
http://www.epa.gov/oppsrrd1/REDs/old_reds/4081red.pdf (PDF)
Amorphous Silicon Dioxide does not kill insects by slashing them to shreds (that is what POOL GRADE DE - CRYSTALLINE does). As a matter of fact, the more finely it is ground, the better it works. It kills by
adsorption that results in desiccation.
To go back to the source. This is from Popular Mechanics Magazine, Aug/1958 (pg. 103) describing Walter Ebeling's research:
he applied an insecticide in the form of a mist to a group of wooden test blocks, in the form of a vapor to another group of blocks, and as a dust to a third group. Seventeen months later Robert Wagner placed each group of blocks in separate test chambers containing Dry Wood Termites and watched the results.
Strangely, none of the insects that crawled over the blocks treated with the mist or with the vapor was affected by the poison applied to the wooden surfaces. Yet every insect that moved across one of the dusty blocks died soon afterward. More study showed that in all three cases the insecticide itself had decomposed with age and was no longer lethal. There was only one possible conclusion: In the case of the dusted blocks it was the inert diluent, the nontoxic powder used to give body to the insecticide, that was doing the killing.
The investigators found that dust is a killer because it adsorbs or disrupts the waxy film that covers an insects body and normally prevents loss of moisture by evaporation. Without the protection of this lipid layer an insect becomes dehydrated and perishes. In many cases enough dust is picked up by its feet and transferred to its body to break down the protective layer. In several minutes or several hours, depending on the type, the insects lose a fifth or more of its weight by evaporation and thats the end of it.
Of the many diluents that are used in insecticide dusts, the research group found that several highly sportive natural clays and diatomaceous earths are quite effective killers.
Walter Ebeling's paper (if you read the above don't spend the $28.00)
http://www.ingentaconnect.com/content/esa/jee/1959/00000052/00000002/art00003
If it is FOOD GRADE it will Say FOOD/FEED GRADE AMORPHOUS SiO₂
Amorphous DE is no panacea (for bugs). The more humid it is, the less effective it is. Some insects are less susceptible to the effects than others (those that prefer humid locations and sport thicker, nasty, chitinous exteriors - turn loose the chooks!).
Amorphous DE is NOT associated with Silicosis. However, breathing any large quantity of most dusts is not recommended. I feel safer standing downwind from dirt bathing turkeys than I do emptying the vacuum cleaner.
ed:sp
ED: Sorry, forgot to add the Bentonite (also GRAS):
http://www.accessdata.fda.gov/scripts/fcn/fcnDetailNavigation.cfm?rpt=scogsListing&id=35 I assume (an ugly practice but I'm going there) that adding Bentonite to Amorphous Silica results in a `feed' grade product (good for livestock).
