There are apparently different types, with the type used for filtration being the one you don't want to breath.
For your enjoyment:
http://en.wikipedia.org/wiki/Diatomaceous_earth
excerpts
Pest control
Diatomite is used as an
insecticide, due to its physico-
sorptive properties. The fine powder absorbs
lipids from the waxy outer layer of insects'
exoskeletons, causing them to dehydrate.
[9] Arthropods die as a result of the water pressure deficiency, based on
Fick's law of diffusion. This also works against
gastropods and is commonly employed in gardening to defeat
slugs. However, since slugs inhabit humid environments, efficacy is very low. It is sometimes mixed with an attractant or other additives to increase its effectiveness. Medical-grade diatomite is sometimes used to
de-worm both animals and humans, with questionable efficacy.
[10][11] It is most commonly used in lieu of
boric acid, and can be used to help control and eventually eliminate
cockroach and
flea infestations. This material has wide application for insect control in grain storage.
[12] It has also been used to control
bedbug infestations, but this method may take weeks to work.
[13]
Safety considerations
The absorbent qualities of diatomite can result in a significant drying of the hands if handled without gloves. The flux-calcined form contains a highly crystalline form of silica, resulting in sharp edges. The sharpness of this version of the material makes it dangerous to breathe and a
dust mask is recommended when working with it.
The type of hazard posed by inhalation depends on the form of the silica. Crystalline silica poses a serious inhalation hazard because it can cause
silicosis. Amorphous silica can cause dusty lungs, but does not carry the same degree of risk as crystalline silica. Natural or dried diatomite generally contains very low percentages of crystalline silica. Diatomite produced for pool filters is treated with high heat (calcining) and a fluxing agent (
soda ash), causing the formerly amorphous silicon dioxide to assume its crystalline form.
The crystalline silica content of the dust's particulate is regulated in the
United States by the
Occupational Safety and Health Administration (OSHA), and there are guidelines for the maximum amounts allowable in the product and in the air near the breathing zone of workers.
[19]