NIOSH Manual of Analytical Methods, Fourth edition. Third supplement. Cincinnati, OH: National Institute for Occupational Safety and Health, 2003 Mar; :167-178
Workplace exposure standards have been established for several soluble metals and metalloids to take into account the increased bioavailability of some metal compounds. Exposure standards for soluble compounds can be up to 500 times lower than the exposure standards for less soluble compounds for the same metal. However, there is often confusion among chemists, industrial hygienists, and laboratories over what is meant by "soluble" when the metal species, extraction fluid, or solubility conditions are not specified in the exposure standard nor in the supporting exposure standard documentation . In addition, the metals and metal compounds may interact chemically or physically with the sampling media or with each other. Such complications can affect the stability and speciation of the metals and their compounds, and must be addressed in order to obtain meaningful results. These issues are becoming more important in workplace airborne metals exposure monitoring. The solubility of a metal will depend on the chemical form of the metal, the fluid used to extract the metal, and the conditions under which the extraction occurs (e.g., temperature, volume, time). Unfortunately, the degree of method specificity needed to obtain measurements that are reproducible among laboratories is generally either missing or is subject to a variety of interpretations from exposure standards and supporting documentation. The need for a better definition of what is meant by the term "soluble" in relation to exposure standards was first raised in 1994, but to date no significant improvement has occurred within United States exposure standard setting organizations. Therefore, to meet the needs of analysts, laboratories, and laboratory clients for better definition of the analyte of interest, and to improve measurement reproducibility among laboratories, various organizations are attempting to achieve international consensus on extraction of soluble metal compounds. New guidelines have been promulgated in an International Standard,  and this will serve to fill the void and improve the situation.