 In 2003, the American Conference of Governmental Industrial Hygienists (ACGIH) set the time-weighted average threshold limit value for an 8-hour exposure at 10 parts per million (ppm).In 2014, the ACGIH adopted a lower threshold limit value of 0.1 ppm as an 8-hour time-weighted average.The California Occupational Safety and Health Administration set the permissible exposure limit at 5 ppm in 2010.