Diatomaceous earth is an inert dust and type of rock composed of skeletons of fossilized diatoms. Diatoms are tiny, one-celled marine and freshwater plants that are composed mainly of silica, a naturally occurring mineral. When crushed, these skeletons break up into tiny pieces of a glass-like material, so tiny that it feels like talcum powder — to us. Easily picked up by the hairy bodies of most insects, DE scratches through their protective waxy layers, resulting in the insect losing water rapidly, drying up and dying.

DE is mined for industrial filtration applications but its oldest and best-known use is as a very mild abrasive in metal polishes and toothpaste. It has also been used as an exfoliant in skin products, a source of detoxification and mineral additive in pet food and an extremely effective reduced-risk pesticide.

Large deposits occur in California, Nevada, Washington and Oregon, but it is harvested around the globe.

There is no evidence that DE or the silica that it is composed of is toxic to humans or other large animals, including experiments with cancer research. That being said, there is a particular form of silica that could pose a health hazard but the amount occurring in natural DE is so small that any health hazard has been ruled out.

When DE is heated to high temperatures, the silica is changed chemically and becomes a potentially harmful form, but only if inhaled at high levels for a long time (years) under occupational conditions. It should be noted that “pool-filter” grade DE is processed differently and not recommended for garden pest control.

The natural DE sold in garden centers and other retail outlets labeled “food grade” poses no more of a health hazard than inhaling dust in general. Food-grade DE is safe for human consumption. It is allowable on foods labeled “organic” under USDA rules. Still, it is recommended that DE be applied with a bulb duster and protective equipment such as a dust mask be used.

DE is in no way harmful to fish or other aquatic life. In water, it is basically the same as plain sand. Remember, it was actually created by living marine organisms.

Natural DE used as an insecticide works in a purely physical manner, not through any chemical toxicity. The abrasive diatom skeletons act like tiny razor blades and rub holes in an insects’ waxy layers, causing a loss of bodily fluids resulting in the death of the insect. It is effective on anything with an exoskeleton; any insect (with six legs) and non-insect arthropods (eight or more legs).

DE is not in any way harmful to plants (although it is probably not good to smother them with the dust). 

Insect dusts such as DE have been around for thousands of years. Ancient cultures used them to protect stored grains against pantry pests like moths and beetles. The same mode of action is used by birds and other animals that take dust baths — they are ridding themselves of their insect parasites.

DE is commonly used in animal production facilities to control external parasites and flies. This control is achieved by dusting the animals and the litter or bedding area. It has also been included in the diet (2 percent in the grain ratio) to control certain internal parasites; this practice is said to result in lower fly populations in the resulting manure.

According to Professor Stuart B. Hill of McGill University in Montreal, “It is perhaps the safest effective pesticide for use in the home and has a valuable place in the protection of stored food and control of insects in animal production units.”