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INTRODUCTION

KEY POINTS

  • The term solvent refers to a class of liquid organic chemicals of variable lipophilicity and volatility, small molecular size, and lack of charge.

  • Absorption of inhaled volatile organic compounds occurs in the alveoli, with almost instantaneous equilibration with blood in the pulmonary capillaries.

  • Solvents are readily absorbed from the gastrointestinal tract and across the skin.

  • Most solvents produce some degree of CNS depression.

The term solvent refers to a class of organic chemicals of variable lipophilicity and volatility with small molecular size and lack of charge. Solvents undergo ready absorption across membranes of the lung, gastrointestinal (GI) tract, and skin. Lipophilicity generally increases with increasing numbers of carbon and/or halogen atoms, while volatility decreases. Organic solvents are frequently used to dissolve, dilute, or disperse materials that are insoluble in water. Naphthas and gasoline are complex mixtures, often consisting of hundreds of compounds.

According to molecular structure or functional group, classes of solvents include aliphatic hydrocarbons, many of which are halogenated (i.e., halocarbons), aromatic hydrocarbons, alcohols, ethers, esters/acetates, amides/amines, aldehydes, ketones, and complex mixtures that defy classification. The main determinants of a solvent’s inherent toxicity are: (1) its number of carbon atoms, (2) whether it is saturated or has double or triple bonds between adjacent carbon atoms, (3) its configuration (i.e., straight chain, branched chain, or cyclic), (4) whether it is halogenated, and (5) the presence of functional groups. Subtle differences in chemical structure can translate into dramatic differences in toxicity.

Nearly everyone is exposed to solvents during normal activities. Environmental exposures to solvents in air and groundwater use multiple exposure pathways (Fig. 24–1). Even household use of solvent-contaminated water may result in solvent intake from inhalation and dermal absorption as well as ingestion. In many cases, risk assessment guidelines stipulate that risks be determined for physiologically diverse individuals who are exposed to several solvents by multiple exposure pathways. For many common/toxic solvents, toxicity factors, such as reference concentrations (RfCs), reference doses (RfDs), and cancer slope factors (CSFs) are available.

FIGURE 24–1

Solvent exposure pathways and media. (Adapted from EPA Risk Assessment Guidance for Superfund. Human Health Evaluation Manual Part A, Interim Final. Washington, DC: Office of Emergency and Remedial Response, 1989.)

The U.S. Occupational Safety and Health Administration (OSHA) has established legally enforceable permissible exposure limits (PELs) for over 100 solvents. The majority of existing PELs were adopted from a list of threshold limit values (TLVs) previously published by the American Conference of Governmental Industrial Hygienists (ACGIH). Whereas the ACGIH’s TLVs for an 8-hour workday, 40-hour workweek are designed to be protective for a working lifetime, its short-term exposure limits (STELs) and ceiling values are designed to protect against the acute effects of high-level, short-term solvent exposure. If warranted, ACGIH will assign a skin notation to a solvent, ...

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