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Microbial activity plays a pivotal role in the biogeochemical cycling of iron and manganese, especially at the redox gradients characteristic of stratified aquatic environments. These cycles are driven by microbial transformations between oxidized and reduced forms of the metals, allowing organisms to exploit them for metabolic energy and structural purposes.Iron Cycling Across Redox GradientsIn neutral, oxygen-rich surface waters, iron is predominantly found in its oxidized, insoluble ferric...

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Proposal for a revised Reference Concentration (RfC) for manganese based on recent epidemiological studies.

Lisa A Bailey1, Julie E Goodman, Barbara D Beck

  • 1Gradient Corporation, Cambridge, MA 02138, USA.

Regulatory Toxicology and Pharmacology : RTP
|August 19, 2009
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New research proposes updated manganese (Mn) Reference Concentrations (RfCs) for air quality, suggesting lower safe exposure levels than the 1993 US EPA guideline. These updated RfCs aim to better protect public health from manganese neurotoxicity.

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Area of Science:

  • Environmental Health Sciences
  • Toxicology
  • Occupational Health

Background:

  • The United States Environmental Protection Agency (US EPA) established a Reference Concentration (RfC) for manganese (Mn) in 1993 based on subclinical neurological effects in workers.
  • The 1993 RfC of 0.05 microg/m(3) was derived using a lowest observable adverse effect level (LOAEL) of 150 microg/m(3) and applied uncertainty factors (UFs).
  • Concerns exist regarding the adequacy of the 1993 RfC given new scientific literature on manganese neurotoxicity.

Purpose of the Study:

  • To propose updated, scientifically-supported manganese (Mn) Reference Concentrations (RfCs) based on recent occupational exposure studies.
  • To re-evaluate the safe inhalation exposure levels for manganese, considering chronic exposure durations and subclinical neurobehavioral effects.
  • To provide more accurate risk assessment values for airborne manganese.

Main Methods:

  • Reviewed 12 recent occupational studies of eight cohorts with chronic manganese exposure, focusing on subclinical neurobehavioral and motor system effects.
  • Identified three studies suitable for RfC derivation, utilizing no observable adverse effect levels (NOAELs).
  • Calculated human equivalent concentrations (HECs) from occupational NOAELs and applied uncertainty factors (UF) for intraspecies variability; also employed benchmark dose (BMD) modeling.

Main Results:

  • Three recent occupational studies identified a NOAEL of approximately 60 microg/m(3) for respirable manganese.
  • Conversion and UF application yielded a proposed RfC of 2 microg/m(3) for manganese.
  • Benchmark dose modeling resulted in a similar proposed RfC of 7 microg/m(3) for manganese.

Conclusions:

  • Recent scientific literature supports lower Reference Concentrations for airborne manganese than the current 1993 US EPA value.
  • The proposed RfCs of 2 microg/m(3) and 7 microg/m(3) offer updated estimates for safe manganese inhalation exposure.
  • Overall confidence in the proposed RfCs is medium, necessitating further research and consideration for regulatory updates.