Manganese is a naturally occurring metal utilized in numerous processes including the manufacture of steel alloys and dry-cell batteries, for wastewater treatment, and as a paint pigment. Manganese emissions have also been associated with incineration of sewage sludge and medical waste, welding, and the wear of certain parts (e.g., brakes, tires and rail car wheels) on mobile sources. While trace amounts of manganese are required for normal human function, “manganese compounds” – which include any unique chemical substance with manganese as part of its infrastructure – are listed as toxic air pollutants in the Clean Air Act. As such, US EPA is required to regulate manganese emissions. US EPA’s current manganese air toxics regulation is based on its calculated inhalation reference concentration (RfC), which essentially represents the acceptable chronic human respirable exposure limit.

Recently, federal and state environmental and public health agencies have been increasingly focused on potential human health risks from manganese exposure. For example, in 2009, US EPA began monitoring outdoor air near schools for air toxics including manganese. Similarly, the Agency for Toxic Substances and Disease Registry (ATSDR) has been investigating health concerns from manganese emission inhalation in several communities over the past few years. These efforts have spawned an increase in private toxic tort litigation (primarily based on nuisance theories).

Yet, US EPA’s current inhalation manganese RfC of 0.05 µg/m3 has not been updated since its original 1993 publication. Since then, a number of key epidemiological studies have been published or are underway justifying a significantly less stringent RfC. Manganese was originally chosen for reassessment as part of US EPA’s Integrated Risk Information System (IRIS) Agenda for 2008. However, in May 2009, US EPA implemented a new IRIS process and manganese reassessment was not completed. This year, with additional compelling science in its back pocket, US EPA renewed its intention to reassess manganese by including it in the “high priority” list for 2012.

A less stringent acceptable chronic human respirable exposure limit for manganese is important. Issuance of a revised RfC by US EPA will likely lead to other regulatory revisions affecting manganese emitters in the US. Indeed, Minnesota is already reviewing its manganese standard, while Michigan has been actively advocating for US EPA reassessment of the manganese RfC. Further, in its IRIS Progress Report issued last month, US EPA noted the development of a Memorandum of Understanding with ATSDR and California EPA’s Office of Environmental Health Hazard Assessment designed to ensure cooperation in developing health assessments. This is significant since California recently adopted a final manganese chronic Reference Exposure Level (REL) of 0.09 µg/m3, a three-fold increase from its 2009 proposed REL of 0.03 µg/m3. Given these developments, manganese emitters should continue to stay informed about the status of US EPA’s reassessment so that they can support development of a less stringent RfC. Such a revision would have impacts in both the regulation and enforcement realm, but also in private party litigation.