WTC environmental health assessment draft, May 2002

NYC 9/11 Public Portal Document

Crystalline silica - respirable fraction of alpha-quartz

Tier Action Crystalline Silica (settled Crystalline Silica (indoor air) dust)

Level Basis Levet Basis I Aggressive cleaning. If 50 /zg/m^ Occupational levels persist, take' exposure limit additional action to for respirable reduce exposure. silica in air is 50 /zg/m^ as a time Evaluate conditions in weighted average the area including for an 8 hour representativeness of exposure/ air samples, asbestos dust levels and loading.

n Maintain recommended above 50 /zg/m’ to The occupational cleaning methods. background 5 *; exposure limit Consider additional (see above) is montoring. adjusted for a 24- hour exposure Evaluate conditions in and a non-workef the area including population by representativeness of using an air samples, fibrous uncertainty glass dust levels and factor of 10. loading.

in No further action background Level is set at :S 5jLZg/mI See above required. that found in background, or unaffected, areas. In this way, no increase in risk due to the silica dust would be expected. * Silica measured in airborne respirable dust, byNIOSH 7500.

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CONFIDENTIAL, CITY_21MC100_003423805 SUBJECT TO PROTECTIVE ORDER NO. 2, SEPT. 2008

NYC-WTC_000176176