Exposure to environmental contaminants or toxicants is one of many conditions or factors that compromise human quality of life. Evidence of illness associated with exposure to chemicals has been documented since the mid-19th century. A recent study has shown that toxic agents were responsible for 55,000 U.S. deaths in the year 2000, which is more than those caused bymotor vehicle crashes (1). In this study, toxic agents were associated with increased mortality from cancer, respiratory, and cardiovascular diseases. The characteristics and patterns of exposures from waste sites, unplanned releases, and other sources of pollution need to be understood clearly so as to prevent potential adverse human health effects and diminished quality of life. Often such exposures are to mixtures, but this is not well-documented in terms of the exact chemical composition and concentrations of the components of the mixture as a function of time and space. Amajority of people are exposed to chemical mixtures at low environmental levels through multiple routes of exposure and environmental media such as air, water, and soil. Exposures to mixtures are often estimated based on water and air consumption and the scarce data on composition. However, several

default assumptions inherent to such estimations are influenced by life style, personal habits, nutritional status, and genetic factors. Humans are exposed to a wide range of chemical mixtures due to the astounding number of chemicals used in commerce: more than seven million chemicals have been identified, and of these chemicals, 650,000 are in current use with more than 1,000 chemicals added per year (2). Even though humans are exposed to chemical mixtures on a daily basis, approximately 95% of resources allocated to health effects research are devoted to assessing the health effects of single chemicals (3).