ATSDR/DHAC_ASTM

NYC 9/11 Public Portal Document

equivalent to 0.08 (mg/n?)-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 not known, it is believed cancer incidence is linked to the development of other silica-induced lung disease (34). It is possible the mechanisms involved in the development of silicosis are linked to the carcinogenic process through the production of reactive oxygen species and tissue responses to immune signals and fibrotic changes (36,37). Some researchers have developed dose cancer risk estimates from epidemiologic studies by building off risk estimates for silicosis (34,38,39). A risk assessment produced for both silicosis and lung cancer on the basis of crystalline silica exposures reviews major studies and proposes that lung cancer incidence may be expected to increase by 14% for each additional 1 (mg/n?) work-year exposure above the baseline of 1 (mg/n?) work-year exposure (36).*

The maximum estimated detection of quartz (crystalline silica) in air in lower Manhattan was 19 gg/m?J. Using the above risk estimates, the cumulative exposure for 1-year continuous exposure 0.08 (mg/n?) work-year poses negligible increased cancer risk. Although there are uncertainties in this evaluation, current scientific opinion would indicate short-term exposures to the measured quartz levels do not pose a health hazard.^

If the highest measured airborne levels are representative of long-term conditions, the cumulative exposure for a continuous 30-year residence would exceed the 1 (mg/n?) work- year cumulative exposure which is believed to be the departure point for adverse effects t In order to compare a continuous residential exposure to a workday exposure, the estimated air level was adjusted to be comparable to a shorter workday exposure across the entire year. J K should be noted that children might be at slightly higher rid: than adults in sensitive populations. However, the estimated 0.08 (mg/m’) wotk-year exposure is significantly below the departure threshold of 1 rrgZm? per work-year exposure, and therefore no adverse effects would be expected. * This estimate was derived by looking at cancer incidence studies and by relating the incidence of lung cancer to silicosis ride estimates. Both methods provided similar risk estimates. However it should be noted other authors caution about directly linking silicosis and tung cancer risk estimates (39). t There are marry uncertainties about whether this risk would be realized. The first is that even though the risk of cancer is not linear, the risk estirtrates treat it as linear (36). Therefore there may be an overestimate of risk at low exposures compared with the studied populatioais. The air levels of silica we are examining here are well below those studied in worker populations; therefore even this small increased risk may be overestimated. Conversely, the studied populations were adult workers. So risk estimates based on these studies may imderestimate risks to children and to adults with respiratory disorders.

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