belden/Diatomaceous_earth#safety_considerations

## Diatomaceous_earth#safety_considerations
Inhalation of crystalline silica is harmful to the lungs, causing
silicosis. Amorphous silica is considered to be low toxicity, but prolonged
inhalation cause changes to the lungs.[24] Diatomaceous earth is mostly
amorphous silica, but contains some crystalline silica, especially in the
saltwater forms.[25] In a study of workers, those exposed to natural DE for
over 5 years had no significant lung changes, while 40% of those exposed
to the calcined form had developed pneumoconiosis.[26] Today's common
D.E. formulations are safer to use as they are predominantly made up of
amorphous silica and contain little or no crystalline silica.[27]

The crystalline silica content of D.E. 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 (1%) and in the
air near the breathing zone of workers (6 mg/m3).[27]

In the 1930s, long term occupational exposure among workers in the cristobalite
D.E. industry who were exposed to high levels of airborne crystalline silica
over decades were found to have an increased risk of silicosis.[28]

Today, workers are required to use respiratory-protection measures when
concentrations of silica exceed allowable levels.

Diatomite produced for pool filters is treated with high heat (calcination)
and a fluxing agent (soda ash), causing the formerly harmless amorphous
silicon dioxide to assume its crystalline form.[27]

References:
* 24 http://www.cdc.gov/niosh/pel88/68855-54.html
* 25 http://www.spca.bc.ca/assets/documents/welfare/professional-resources/farmer-resources/diatomaceous-earth-factsheet.pdf
* 26 http://www.cdc.gov/niosh/docs/81-123/pdfs/0552.pdf
* 27 http://www.oznet.ksu.edu/grsc_subi/Teaching/GRSC651/GRSC651_Courses_Material/lecture_slides/GRSC651_lect_20(1)_Inert_Dusts.pdf
* 28 Hughes, Janet M.; Weill, Hans; Checkoway, Harvey; Jones, Robert N.; Henry, Melanie M.; Heyer, Nicholas J.; Seixas, Noah S.; Demers, Paul A. (1998). "Radiographic Evidence of Silicosis Risk in the Diatomaceous Earth Industry". American Journal of Respiratory and Critical Care Medicine 158 (3): 807–814. doi:10.1164/ajrccm.158.3.9709103. ISSN 1073-449X.
	Inhalation of crystalline silica is harmful to the lungs, causing
	silicosis. Amorphous silica is considered to be low toxicity, but prolonged
	inhalation cause changes to the lungs.[24] Diatomaceous earth is mostly
	amorphous silica, but contains some crystalline silica, especially in the
	saltwater forms.[25] In a study of workers, those exposed to natural DE for
	over 5 years had no significant lung changes, while 40% of those exposed
	to the calcined form had developed pneumoconiosis.[26] Today's common
	D.E. formulations are safer to use as they are predominantly made up of
	amorphous silica and contain little or no crystalline silica.[27]

	The crystalline silica content of D.E. 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 (1%) and in the
	air near the breathing zone of workers (6 mg/m3).[27]

	In the 1930s, long term occupational exposure among workers in the cristobalite
	D.E. industry who were exposed to high levels of airborne crystalline silica
	over decades were found to have an increased risk of silicosis.[28]

	Today, workers are required to use respiratory-protection measures when
	concentrations of silica exceed allowable levels.

	Diatomite produced for pool filters is treated with high heat (calcination)
	and a fluxing agent (soda ash), causing the formerly harmless amorphous
	silicon dioxide to assume its crystalline form.[27]

	References:
	* 24 http://www.cdc.gov/niosh/pel88/68855-54.html
	* 25 http://www.spca.bc.ca/assets/documents/welfare/professional-resources/farmer-resources/diatomaceous-earth-factsheet.pdf
	* 26 http://www.cdc.gov/niosh/docs/81-123/pdfs/0552.pdf
	* 27 http://www.oznet.ksu.edu/grsc_subi/Teaching/GRSC651/GRSC651_Courses_Material/lecture_slides/GRSC651_lect_20(1)_Inert_Dusts.pdf
	* 28 Hughes, Janet M.; Weill, Hans; Checkoway, Harvey; Jones, Robert N.; Henry, Melanie M.; Heyer, Nicholas J.; Seixas, Noah S.; Demers, Paul A. (1998). "Radiographic Evidence of Silicosis Risk in the Diatomaceous Earth Industry". American Journal of Respiratory and Critical Care Medicine 158 (3): 807–814. doi:10.1164/ajrccm.158.3.9709103. ISSN 1073-449X.