WTC environmental investigation report, Sep 2002

NYC 9/11 Public Portal Document

Final Technical Report of the Public Health Investigation Page 39 of 56 Silica is chemically inert for the most part, so it is the physical form of silica that imparts its toxicity. Small crystals of silica may be deposited in lung tissue. Once deposited, the crystals remain and fibrogenic lesions may appear over time, damaging the lung tissue and reducing lung capacity (26). Silica has been demonstrated to be a pulmonary toxin linked both to silicosis (a fibrotic lung disease) and lung cancer (27,28,29,30). These findings are supported by both human and animal studies. Chronic silica exposure may contribute to lung cancer; emphysema; obstructive airway, etiology, immune, or connective tissue disease; and lymph node fibrosis (27,30,31,32,33).

a. Silicosis

Generally silicosis is seen after years of high-level occupational exposures, although it can result from short-term acute exposure (26,30). It is not known if chronic ambient environmental exposure contributes to silicosis (26). Because the lung damage is proportional to the total amount of silica deposited in the lung, current evaluations are based on a measure of cumulative exposure of air concentration over work-years of exposure (28,34,35).

Short-term exposures to quartz, even for a continuous year of exposure at the highest estimated air concentration, is not expected to result in any adverse health effects.

Assuming worst-case theoretical assumptions, the estimated quartz levels measured cannot rule out adverse health effects from chronic exposures (i.e., 30 years). For individuals who conduct frequent cleaning of their residences, as recommended in this report, or participate in the U.S. Environmental Protection Agency cleaning/sampling program, it is unlikely that their exposure would resemble these worst-case conditions.

Exposures of less than 1 milligram per cubic meter 1 (mg/m^) of air work-year*of exposure are believed to not result in significant incidence of silicosis in workers (28,34,35). The peak respirable quartz level estimated in lower Manhattan of 19 pg/m^J would be equivalent to 0.08 (mg/m^)-work year.- Therefore, at this measurement even an entire year of exposure 24 hours a day 7 days a week would not be expected to result in silicosis when compared to work environments known to result in silicosis.-

b. Lung Cancer

The International Agency for Research on Cancer (lARC) has determined that there is sufficient evidence that crystalline silica in the form of quartz or cristobalite is carcinogenic in humans (28). The lARC cites only limited evidence for the carcinogenic potential of tridymite from animal studies and inadequate evidence in either humans or animals to classify amorphous silica. Respirable crystalline silica is listed as a known human carcinogen in the National Toxicology Program's ninth report to Congress (27).

Although the exact mechanism for silica-induced lung cancer is

http://www.atsdr.cdc.gOv/asbestos/lowennanhattan/ResSamp_Sep2002_Text.html 07/18/2003 NYC-WTC_000178426