After testing air samples from 11 fracking sites in Pennsylvania, North Dakota, Colorado, Arkansas and Texas, CDC's National Institute of Occupational Safety and Health found silica levels above defined exposure limits, it said in a hazard alert posted on the Department of Labor's OSHA website.

Exposure to silica can lead to silicosis, a disease that causes inflammation and scarring of lung tissues and can impair breathing, according to the study.

U.S. energy companies have employed a combination of horizontal drilling and fracking technologies to tap into oil and gas reserves in shale and tight deposits previously viewed as inaccessible.

They inject a slurry of water, sand and chemicals into the sideways wells to free trapped hydrocarbons.

The report recommended the use of protective gear and alternative proppants like ceramics, resin-coated sand and bauxite, where feasible.

Fracking has also drawn fire from environmental groups and public health advocates who say it pollutes drinking water.

(Reporting by Selam Gebrekidan; Editing by David Gregorio)

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