sent in lemon, orange, other citrus fruits, and to a lesser extent in vegetables and plants.

d-Limonene is the major constituent of lemon and orange oils; it is also present in other essential oils. Besides being naturally present in fruits, vegetables, and their products (such as orange juice, which contains 100 ppm of d-limonene), it is used as a flavoring agent and is found in common food items, such as ice cream (68 ppm), baked goods (120 ppm), gelatins and puddings (48 to 400 ppm), and nonalcoholic beverages (31 ppm) (NTP 1990, NICNAS 2002). d-Limonene is a food additive on the Food and Drug Administration’s Generally Recognized as Safe List (Opdyke 1978). Consumption of d-limonene has been estimated to be 0.2 to 2 mg/kg body weight per day (or 14 to 140 mg/70 kg/d) (IARC 1999). Because d-limonene can be isolated from a large number of natural sources and has a desirable odor and taste, it is the isomer that is commercially produced and is mainly used in soap, personal hygiene products, medicinal cosmetics, and perfume, in addition to its wide use in foods and beverages. In 1976, 68,000 kg of d-limonene was produced and used in the United States mainly as a fragrance and flavoring agent; by 1984, the consumption increased to 254,000 kg. The use of limonene has continued to increase because consumers prefer natural and organic food additives to synthetic products. d-Limonene seems to have some medicinal properties. It has been shown experimentally to have protective effects against certain types of cancer and was evaluated in phase I clinical trials with advanced cancer patients (Gould 1997).

Because it has good solvent properties, relatively low toxicity, and a pleasant odor, d-limonene is used increasingly as an industrial solvent to replace chlorinated hydrocarbons as a remover/stripper for wax, paints, ink, and adhesives and in degreasing operations and other applications (NICNAS 2002). It has been substituted for xylene in slide preparation in many histopathology laboratories. Because of its widespread presence in botanical and commercial products and its increasing industrial uses, d-limonene is released into the environment from biogenic and anthropogenic sources (NICNAS 2002).

Occurrence and Use in Spacecraft

d-Limonene has been considered for use on the International Space Station (ISS) as a cleansing solvent. On the ISS, low-toxicity water-soluble solvents (especially alcohols such as ethanol and isopropanol) are used in medical applications and for hardware cleansing. These volatile, highly soluble, and low-molecular-weight compounds, which are released into the ISS air after their use, are readily removed together with water vapor by the humidity removal system as water condensate. In the ISS water purification system, the contaminants in the condensate are removed by charcoal filtration and catalytic oxidation. This water recycling system has a limited capacity for removing these water-borne organics. Thus, the ISS program has placed a restriction on the use of water-soluble volatile organic compounds (VOCs).

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