dust mineralogy from spectroscopy, measured soil composition, and plausible alteration mechanisms (Morris et al., 1995; Bell et al., 2000).

The committee also concluded that Martian airborne dust could present the same chemical hazards as Martian soil, so soil and dust should be characterized in the same way. In addition, certain soils could contain harmful organic compounds. However, the oxidizing environment at the surface would most likely have destroyed any organic compounds contained in the surface layers of the soil. Notwithstanding these conclusions, in the absence of further data, astronauts should avoid direct skin contact with soil.

Cancer versus Noncancer Risks

Generally, EPA considers that there is no safe threshold (no safe level) for human exposure to certain genotoxic, cancer-inducing compounds. “Genotoxic” refers to the process by which cancer-causing compounds directly interact with DNA. In contrast, there is a defined threshold level above which noncancer effects are induced. An estimate of the safe inhalation concentration is referred to as the reference concentration (RfC). If hazardous compounds are below the RfC threshold, noncancer effects are not expected to occur. If the RfC of a toxic compound is exceeded, there could be harmful health effects. It should be noted that many compounds do not have RfCs established.

An examination of the Integrated Risk Information System (IRIS) provided by the EPA reveals that the allowed safe concentrations for cancer are much more stringent than the safe concentrations that give comparable risk estimates for noncancer effects (Table 4.1 is a representative sample). Therefore, if NASA protects astronauts against the risk of developing cancer in the long term as a result of having been exposed to particulate matter on Mars, NASA will also be protecting astronauts from acute and short-term noncancer effects that could potentially interfere with mission success.

Toxic Metals and Other Inorganic Elements

Airborne dust and soil could contain trace amounts of hazardous chemicals, including compounds of toxic metals that are known to cause cancer over the long term if inhaled in sufficient quantities. Soil analyses conducted by the Viking missions established maximum possible concentration limits for a few toxic elements based on the detection capabilities of the instruments on the landers (Table 4.2), and Mars Pathfinder measurements established that chromium is present in Mars soil. Although analogous measurements have not been made on airborne dust, soil and dust are commonly assumed to have similar chemical compositions (McSween and Keil, 2000).

TABLE 4.1 Representative Listing of Reference Concentrations for Cancer-Causing Compounds and for the Noncancerous Effects of Those Compounds from EPA's IRIS Database (milligrams per cubic meter)

Compound

Reference Concentration (RfC) Safe Dose for Noncancer Effects

Concentration That Gives a Cancer Risk of 1 in 1,000,000a

Beryllium

2 × 10-5

4 × 10-7

Acrylonitrile

2 × 10-3

1 × 10-5

Acrolein

2 × 10-5

No information given

Acrylic acid

1 × 10-3

No information given

Aniline

1 × 10-3

No information given

Antimony trioxide

2 × 10-4

No information given

Carbon disulfide

7 × 10-1

No information given

Dichlorobenzene

8 × 10-1

No information given

Styrene

1 × 100

No information given

Chromium VI

No information given

8 × 10-8

Arsenic

No information given

2 × 10-7

Cadmium

No information given

6 × 10-7

a Higher dose poses a greater risk.

Based on a survey of Environmental Protection Agency (EPA) exposure risk estimates, the elements that are toxic at the lowest concentrations are hexavalent chromium (Cr VI), arsenic (As), cadmium (Cd), and beryllium (Be) (see Box 2.2).

Hexavalent Chromium

Chromium contained in naturally occurring geologic materials is primarily in the trivalent state (a +3 ion), which is a stable form of chromium and minimally toxic to humans. Hexavalent chromium (Cr VI, a +6 ion), the highly toxic form of chromium, is rarely encountered in natural geologic materials. In fact, hexavalent chromium is only found naturally on Earth in the rare mineral crocoite (PbCrO4) and is more often produced by humans for industrial purposes (ATSDR, 2000).



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