In this criteria document, the National Institute for Occupational Safety and Health (NIOSH) reviews the critical health effects studies of hexavalent chromium (Cr[VI]) compounds in order to update its assessment of the potential health effects of occupational exposure to Cr(VI) compounds and its recommendations to prevent and control these workplace exposures. NIOSH reviews the following aspects of workplace exposure to Cr(VI) compounds: the potential for exposures (Chapter 2), analytical methods and considerations (Chapter 3), human health effects (Chapter 4), experimental studies (Chapter 5), and quantitative risk assessments (Chapter 6). Based on evaluation of this information, NIOSH provides recommendations for a revised recommended exposure limit (REL) for Cr(VI) compounds (Chapter 7) and other recommendations for risk management (Chapter 8). This criteria document supersedes previous NIOSH Cr(VI) policy statements, including the 1975 NIOSH Criteria for a Recommended Standard: Occupational Exposure to Chromium(VI) and NIOSH Testimony to OSHA on the Proposed Rule on Occupational Exposure to Hexavalent Chromium [NIOSH 1975a, 2005a]. Key information in this document, including the NIOSH site visits and the NIOSH quantitative risk assessment, were previously submitted to the Occupational Safety and Health Administration (OSHA) and were publicly available during the OSHA Cr(VI) rule-making process. OSHA published its final standard for Cr(VI) compounds in 2006 [71 Fed. Reg. 10099 (2006)]. Cr(VI) compounds include a large group of chemicals with varying chemical properties, uses, and workplace exposures. Their properties include corrosion-resistance, durability, and hardness. Workers may be exposed to airborne Cr(VI) when these compounds are manufactured from other forms of Cr (e.g., the production of chromates from chromite ore); when products containing Cr(VI) are used to manufacture other products (e.g., chromate-containing paints, electroplating); or when products containing other forms of Cr are used in processes that result in the formation of Cr(VI) as a by-product (e.g., welding). In the marketplace, the most prevalent materials that contain chromium are chromite ore, chromium chemicals, ferroalloys, and metal. Sodium dichromate is the most common chromium chemical from which other Cr(VI) compounds may be produced. Cr(VI) compounds commonly manufactured include sodium dichromate, sodium chromate, potassium dichromate, potassium chromate, ammonium dichromate, and Cr(VI) oxide. Other manufactured materials containing Cr(VI) include various paint and primer pigments, graphic arts supplies, fungicides, and corrosion inhibitors. An estimated 558,000 U.S. workers are exposed to airborne Cr(VI) compounds in the workplace. Some of the industries in which the largest numbers of workers are exposed to high concentrations of airborne Cr(VI) compounds include electroplating, welding, and painting. An estimated 1,045,500 U.S. workers have dermal exposure to Cr(VI) in cement, primarily in the construction industry. Cr(VI) is a well-established occupational carcinogen associated with lung cancer and nasal and sinus cancer. NIOSH considers all Cr(VI) compounds to be occupational carcinogens [NIOSH 1988b, 2002, 2005a]. In 1989, the International Agency for Research on Cancer (IARC) critically evaluated the published epidemiologic studies of chromium compounds. IARC concluded that "there is sufficient evidence in humans for the carcinogenicity of chromium[VI] compounds as encountered in the chromate production, chromate pigment production and chromium plating industries" (i.e., IARC category "Group 1" carcinogen) [IARC 1990]. Cr(VI) compounds were reaffirmed as an IARC Group 1 carcinogen (lung) in 2009 [Straif et al. 2009; IARC 2012]. The National Toxicology Program (NTP) identified Cr(VI) compounds as carcinogens in its first annual report on carcinogens in 1980 [NTP 2011]. Nonmalignant respiratory effects of Cr(VI) compounds include irritated, ulcerated, or perforated nasal septa. Other adverse health effects, including reproductive and developmental effects, have been reviewed by other government agencies [71 Fed. Reg. 10099 (2006); ATSDR 2012; EPA 1998; Health Council of the Netherlands 2001; OEHHA 2009]. Studies of the Baltimore and Painesville cohorts of chromate production workers [Gibb et al. 2000b; Luippold et al. 2003] provide the best information for predicting Cr(VI) cancer risks because of the quality of the exposure estimation, large amount of worker data available for analysis, extent of exposure, and years of follow-up [NIOSH 2005a]. NIOSH selected the Baltimore cohort [Gibb et al. 2000b] for analysis because it has a greater number of lung cancer deaths, better smoking histories, and a more comprehensive retrospective exposure archive. The NIOSH risk assessment estimates an excess lifetime risk of lung cancer death of 6 per 1,000 workers at 1 microg Cr(VI)/m3 (the previous REL) and approximately 1 per 1,000 workers at 0.2 microg Cr(VI)/m3 (the revised REL) [Park et al. 2004]. The basis for the previous REL for carcinogenic Cr(VI) compounds was the quantitative limitation of the analytical method available in 1975. Based on the results of the NIOSH quantitative risk assessment [Park et al. 2004], NIOSH recommends that airborne exposure to all Cr(VI) compounds be limited to a concentration of 0.2 microg Cr(VI)/m3 for an 8-hr TWA exposure, during a 40-hr workweek. The REL is intended to reduce workers' risk of lung cancer associated with occupational exposure to Cr(VI) compounds over a 45-year working lifetime. It is expected that reducing airborne workplace exposures to Cr(VI) will also reduce the nonmalignant respiratory effects of Cr(VI) compounds, including irritated, ulcerated, or perforated nasal septa and other potential adverse health effects. Because of the residual risk of lung cancer at the REL, NIOSH recommends that continued efforts be made to reduce exposures to Cr(VI) compounds below the REL. The available scientific evidence supports the inclusion of all Cr(VI) compounds into this recommendation. Cr(VI) compounds studied have demonstrated their carcinogenic potential in animal, in vitro, or human studies [NIOSH 1988b; 2002; 2005a,b]. Molecular toxicology studies provide additional support for classifying Cr(VI) compounds as occupational carcinogens. The NIOSH REL is a health-based recommendation derived from the results of the NIOSH quantitative risk assessment conducted on human health effects data. Additional considerations include analytical feasibility and the achievability of engineering controls. NIOSH Method 7605, OSHA Method ID-215, and international consensus standard analytical methods can quantitatively assess worker exposure to Cr(VI) at the REL. Based on a qualitative assessment of workplace exposure data, NIOSH acknowledges that Cr(VI) exposures below the REL can be achieved in some workplaces using existing technologies but are more difficult to control in others [Blade et al. 2007]. Some operations, including hard chromium electroplating, chromate-paint spray application, atomized- alloy spray-coating, and welding may have difficulty in consistently achieving exposures at or below the REL by means of engineering controls and work practices [Blade et al. 2007]. The extensive analysis of workplace exposures conducted for the OSHA rule-making process supports the NIOSH assessment that the REL is achievable in some workplaces but difficult to achieve in others [71 Fed. Reg. 10099 (2006)]. A hierarchy of controls, including elimination, substitution, engineering controls, administrative controls, and the use of personal protective equipment, should be followed to control workplace exposures. The REL is intended to promote the proper use of existing control technologies and to encourage the research and development of new control technologies where needed, in order to control workplace Cr(VI) exposures. At this time, there are insufficient data to conduct a quantitative risk assessment for workers exposed to Cr(VI), other than chromate production workers or specific Cr(VI) compounds other than sodium dichromate. However, epidemiologic studies demonstrate that the health effects of airborne exposure to Cr(VI) are similar across workplaces and industries (see Chapter 4). Therefore, the results of the NIOSH quantitative risk assessment conducted on chromate production workers [Park et al. 2004] are used as the basis of the REL for all workplace exposures to Cr(VI) compounds. The primary focus of this document is preventing workplace airborne exposure to Cr(VI) compounds to reduce workers' risk of lung cancer. However, NIOSH also recommends that dermal exposure to Cr(VI) compounds be prevented in the workplace to reduce adverse dermal effects including skin irritation, skin ulcers, skin sensitization, and allergic contact dermatitis. NIOSH recommends that employers implement measures to protect the health of workers exposed to Cr(VI) compounds under a comprehensive safety and health program, including hazard communication, respiratory protection programs, smoking cessation, and medical monitoring. These elements, in combination with efforts to maintain airborne Cr(VI) concentrations below the REL and prevent dermal contact with Cr(VI) compounds, will further protect the health of workers.