Toxicologic Assessment of the Army's Zinc Cadmium Sulfide Dispersion Tests (1997)

THIS CHAPTER CONTAINS the subcommittee's exposure assessment for zinc cadmium sulfide (ZnCdS) and cadmium.

The subcommittee reviewed the Army's sampling and analytic methods for assessing exposures to ZnCdS and the Army's documents on exposure to ZnCdS in various locations. The Army measured the concentration of ZnCdS particles in its dispersion studies with impingement and filtration methods. The methods are described in Appendix F. The subcommittee concludes that those are accurate and appropriate methods for measuring ZnCdS.

The subcommittee also reviewed the Army's documents and estimated the concentrations and potential exposures (time-integrated concentrations) of ZnCdS that were achieved during the use of the compound in the Army's air dispersion tests. Table 5-1 provides information on location, dates, numbers, and quantities of ZnCdS releases. It also provides the maximum concentrations and exposure doses (concentration x time). The

four highest ZnCdS exposures were in Minneapolis (44 µg), Winnipeg (93 µg), St. Louis (156 µg), and Biltmore Beach, FL (2,500 µg). To obtain the exposure to cadmium, the maximal exposure of ZnCdS (expressed as µg-min/m³ as shown in Table 5-1) is multiplied by 0.0166 m³/min (the volume of air inhaled by an active person in 1 min). The product is then multiplied by 0.156 (the mass fraction of cadmium in ZnCdS). For example, the corresponding amounts of cadmium in the ZnCdS doses are 6.8 µg in Minnesota, 14.5 µg in Winnipeg, 24.4 µg in St. Louis, and 390 µg in Biltmore Beach.

The purpose of this section is to provide estimates of the magnitude of potential cadmium doses from human contact with cadmium compounds as a result of the dispersion of ZnCdS by the Army and to compare them with the estimated doses from environmental and industrial sources. In the United States, mean concentrations of cadmium in ambient air range from less than 0.001 µg/m³ in remote areas to 0.005-0.04 µg/m³ in urban areas (Davidson and others 1985; Elinder 1985; EPA 1981; Saltzman and others 1985). Atmospheric concentrations of cadmium are generally highest in the vicinity of cadmium-emitting industries, such as smelters, municipal incinerators, and fossil-fuel combustion facilities. Measurements of atmospheric cadmium up to 7 µg/m³ have been reported in these industrial types of areas in the United States (Schroeder and others 1987). Cadmium intake from air is estimated to be 0.1-0.8 µg per day in typical U.S. urban areas and less than 0.02 µg in rural areas.

Food is the largest potential source of cadmium exposure for the general population. There are several estimates of the daily adult intake of cadmium from food in the United States, but there is considerable variation among those estimates. Schroeder and Balassa (1961) reported a range of 4-60 µg and Nriagu (1981) a range of 38-92 µg, whereas estimated daily averages have been reported to be 30 µg (Gartrell and others 1986), 38 µg (Duggan and Corneliussen 1972), 50 µg (Duggan and Corneliussen 1972), 51 µg (Mahaffey and others 1975), and 92 µg (Murthy and others 1971). A more recent estimate based on a total diet

study shows the daily dietary intake to be about 15 µg (Gunderson, 1995). Analysis of the earlier data shows that these discrepancies are probably due to different analytic methods. Cadmium contamination of food has been reduced over the years, presumably because of better technology. However, the cadmium contamination encountered in the 1950s and 1960s, when the Army's dispersion tests were conducted, are more relevant for risk assessment. On the basis of the U.S. data and data from other industrial nations in the Northern Hemisphere, the subcommittee believes that the daily cadmium intake from food ranges from 10 to 60 µg.

Except in the vicinity of cadmium-emitting industries, the cadmium in most U.S. drinking- water supplies is less than 1 µg/L (Konz and Walker 1979). However, concentrations of up to 10 µg/L have been reported in some water supplies (EPA 1981). Thus, daily cadmium intake from drinking water is about 2-20 µg, assuming that a person drinks 2 L of water per day.

Cigarettes are also an important source of cadmium exposure. The amount of cadmium that can be inhaled from smoking one cigarette is 0.10.2 µg (Elinder and others 1983; Friberg and others 1974). Thus, it can be estimated that someone smoking one pack per day will take in 2-4 µg of cadmium per day. Environmental tobacco smoke or passive smoking is another source of human exposure to cadmium. It has been reported that the amount of cadmium (presumably as CdO) released in the mainstream smoke (the smoke that the smoker inhales) from smoking one cigarette is about 100 mg and the cadmium released in the sidestream smoke (the smoke that originates from the smoldering end of a cigarette in between puffs) is about 720 mg (NRC 1986; DHHS 1986; EPA 1992). The cigarette-smoker is subjected to the sidestream as is anyone near the smoker while the cigarette is burning. The amount of cadmium inhaled from passive smoking depends on several factors, such as number of cigarettes smoked, ventilation, dilution factor (e.g., size of the room), and duration of exposure.

Thus, the total daily human cadmium intake in industrial countries from environmental and industrial sources is 12-84 µg/d for an adult. For a 70-kg person, that corresponds to a potential intake of 0.2-1.2 µg/kg per day. Figure 5-1 shows the typical ranges of daily cadmium intake by exposure pathway. Food products and water contribute almost all the typical daily

FIGURE 5-1. Typical ranges of daily cadmium intake by exposure pathway.

human exposure; inhalation contributes a very small fraction. Not all the inhaled or ingested soluble cadmium is absorbed into the body. The subcommittee used the information on water-soluble cadmium compounds as the worst case. Only 25-50% of the inhaled cadmium is absorbed by the

lungs (Elinder and others 1976; Friberg and others 1986; Henderson and others 1979), and only 5% of the ingested cadmium is absorbed in the gastrointestinal tract (ATSDR 1992; Friberg and others 1974; IARC 1993; Nriagu 1980). Figure 5-2 presents a comparison of typical daily and annual inhalation intake of cadmium in urban areas with the time-integrated (total) potential inhalation doses of cadmium in the form of ZnCdS in 19 locations where it was released.

To consider both direct inhalation exposures and indirect exposures (ingestion and through skin contact) in its risk assessment, the subcommittee developed potential dose ratios that relate the direct-inhalation potential dose to the indirect potential doses associated with the same air concentration. The ratios are developed in Appendix I and summarized in Table 5-2, and were used to develop the comparisons in Figure 5-2. The exposure of a population to a cloud of ZnCdS is expressed as the product of time and concentration with units of µg-h/m³. Because the concentration varies in time, we believe that the cumulative exposure-time product, rather than peak concentration, is the most appropriate way to express exposure. Because the population doses that result from this exposure accumulate by multiple pathways, we developed a dose-to-exposure ratio for each pathway. These ratios are shown in Table 5-2. The potential doses can be interpreted as follows. Consider the inhalation pathway, listed at the top of the table. The entry of 1 µg per µg-h/m³ of exposure means that someone exposed for 1 h to a concentration of 1 µg/m³ will get a cumulative dose of 1 µg. It also means that someone exposed to 0.5 µg/m³ for 2 h, which is equivalent to an exposure of 1 µg-h/m³, will receive the same dose, 1 µg. A similar approach applies to each table entry.

Appendix I provides a detailed analysis of sources of environmental and industrial exposure, transport and environmental fate, sinks (reservoirs) of cadmium, and exposure pathways.

Estimated average total daily intake from environmental and industrial exposures to cadmium from all media (soil, water, food, and air) in urban areas are greater than any daily exposures to cadmium resulting from the ZnCdS particles in the Army tests, with the exception of 1 test at Biltmore Beach, an isolated beach area in Florida. It should be noted that the cadmium intake directly from air (that is, via inhalation) contributes very little to total cadmium intake in urban and industrial areas. Although on the

days of the Army tests, the sites of the highest monitored ZnCdS concentrations had concentrations of airborne cadmium (in the form of ZnCdS) that were above the estimated urban average daily airborne cadmium in about half the test sites, these short-term high concentrations had minimal impact on indirect cadmium exposures (from water, food, and soil).

The subcommittee concludes that the four highest potential inhalation doses of ZnCdS that humans were exposed to during the Army's dispersion tests occurred in Minneapolis (44 µg), Winnipeg (93 µg), St. Louis (156 µg), and Biltmore Beach (2,500 µg). The corresponding amounts of cadmium in these doses are 6.8 µg in Minnesota, 14.5 µg in Winnipeg, 24.4 µg in St. Louis, and 390 µg in Biltmore Beach. On the basis of these estimates, the subcommittee concludes that exposure to cadmium from the dispersion tests (except for Biltmore Beach, FL, an unpopulated remote area at the time of the Army's tests) did not exceed the background expo-

sures encountered in urban areas. It should be noted that cadmium intake directly from air (that is, via inhalation) contributes very little to total cadmium intake in urban and industrial areas. Although, on the days of the Army tests, the sites of the highest monitored ZnCdS concentrations had concentrations of airborne cadmium (in the form of ZnCdS) that were above the estimated urban average daily airborne cadmium in about half the test sites, these short-term high concentrations had minimal impact on indirect cadmium exposures (from water, food, and soil).