Q: I heard you talking on the radio about finding diatomaceous earth to use for bugs in gardens. I bought the stuff from a pool supply store, where it is cheap!

A: Unfortunately, diatomaceous earth (D.E.) for pools is not the same as diatomaceous earth for insects. Diatomaceous earth originally comes from the “shells” of tiny sea creatures. The “shells” were deposited at the bottom of extinct shallow seas and the mineral is now mined for use in many products.

The pool product has been heated, turning much of the silicon dioxide in the mineral into crystalline silica. This makes it great for filtration but very dangerous for human exposure. Breathing the dust can lead to silicosis of the lungs and possibly cancer.

The particles of unheated insecticidal diatomaceous earth have very sharp edges. When an insect walks over them, the particles cut the insect’s skin, causing it to dry out and die. Insecticidal D.E. can be effective if applied where insects crawl or congregate. Though dusty, it contains a very small amount of crystalline silica. I recommend you stop using the pool-grade product.

See section 11 of this DE label

Crystalline silica inhaled as respirable dust, has been classified as carcinogenic to humans over prolonged and sustained exposure. Long term inhalation of respirable crystalline silica may contribute to the respiratory disease silicosis (non-cancerous lung disease). In a 1997 monograph (Volume 68, “Silica”), the International Agency for Research on Cancer (IARC) concluded that overall the epidemiological findings support increased risk of lung cancer from inhaled crystalline silica resulting from occupational exposure.