Monosodium Methanearsonate (MSMA), an Organic Arsenical
Monosodium methanearsonate (MSMA) is an organic arsenical herbicide used to control grasses and broadleaf weeds. It is registered for use on cotton, sod farms, golf courses, and highway rights-of-way, and may not be used in Florida except for on cotton in Calhoun, Columbia, Escambia, Hamilton, Holmes, Jackson, Jefferson, Okaloosa, Santa Rosa, Suwannee, Walton and Washington counties.
Background - Organic Arsenical Pesticides
MSMA is an organic arsenical, a class of herbicides that also includes DSMA, CAMA, cacodylic acid and its sodium salt. As of September 2009, all remaining products containing DSMA, CAMA, cacodylic acid and its sodium salt were canceled. These chemicals were previously registered for use on cotton, bearing and non-bearing fruit and nut trees, commercial turf, golf courses, athletic fields, parks and residential lawns. In 2009, the MSMA uses on athletic fields, parks, residential lawns, forestry, non-bearing fruit and nut trees, and citrus orchards were also canceled.
Uses of MSMA
Turf Uses
In January 2009, EPA reached an Agreement in Principle with registrants of MSMA to end its use on sod farms, golf courses, and highway rights-of-way after December 31, 2013. Read the Agreement in Principle to Implement the Organic Arsenicals Reregistration Eligibility Decision (RED). In September 2012, EPA modified the January 2009 Agreement in Principle with registrants to include a peer review by the National Academy of Sciences (NAS) of the carcinogenic mode of action (MOA) of inorganic arsenic before a decision was made regarding the remaining MSMA turf uses.
As a result, MSMA is still approved for use on sod farms, golf courses, and highway rights-of-way. A 2013 amended cancellation order for organic arsenicals describes the status of these uses and existing stocks provisions in more detail.
Following application, organic arsenicals like MSMA convert over time to a more toxic form in soil (inorganic arsenic) and can contaminate drinking water through soil runoff. To prevent exposure to inorganic arsenic in drinking water EPA requires the following restrictions for turf uses of MSMA.
Golf courses:
- Spot treatments only (100 square feet per spot), not to exceed 25 percent of the total golf course acreage per year.
- One broadcast treatment for newly constructed courses only.
Sod farms:
- 1-2 broadcast applications per season.
- 25-foot buffer around permanent water bodies.
Highway rights-of-way:
- Two broadcast applications per year ONLY on highway rights-of-way.
- 100-foot buffer around permanent water bodies.
Cotton
As of May 26, 2009, the use of MSMA to clear weeds in fields prior to planting cotton was no longer permitted on labels. MSMA can still be used to control weeds in cotton fields. This use is limited to two applications per season of two pounds active ingredient per acre for each application, once plants are growing (also called post-emergent application). Cotton growers also must maintain a 50-foot buffer zone around permanent water bodies.
Scientific Peer Review
The 2009 Agreement in Principle provided for a scientific peer review of the carcinogenic mode of action (MOA) of inorganic arsenic before the final phase-out of the remaining MSMA turf uses. Because of a December 2011 Congressional directive to the National Academy of Sciences (NAS) to evaluate the EPA Integrated Risk Information System (IRIS) risk assessment of the carcinogenic mode of action of inorganic arsenic, EPA and the MSMA registrants modified the January 2009 Agreement in Principle to include the NAS as the peer review body.
Therefore, EPA will postpone any decision regarding the phase-out of the remaining turf uses of MSMA until the Agency has analyzed and responded to the NAS review. The IRIS risk assessment website for inorganic arsenic describes the risk assessment process and schedule in more detail.
In addition, the Organic Arsenical Products Task Force (OAPTF) and the Arsenical Wood Preservatives Task Force (AWPTF) submitted a cancer reassessment package for inorganic arsenic in October 2020 for EPA's review under the Pesticide Registration Improvement Act (PRIA). EPA has reviewed this submission and the EPA concluded that the available data do not adequately support the proposed change for assessing inorganic arsenic cancer risk. EPA's May 30, 2023 report is available at www.regulations.gov on the MSMA docket.
Registration Review
Currently, MSMA is undergoing registration review, a program that re-evaluates all pesticides on a 15-year cycle. Registration review ensures pesticides will not cause unreasonable adverse effects when used according to label directions and precautions and there is a reasonable certainty of no harm from dietary and residential exposure. All documents related to the registration review of MSMA are located in the registration review docket EPA-HQ-OPP-2013-0107 at www.regulations.gov. In particular:
- The Preliminary Work Plan (March 2013)
- The Final Work Plan (September 2013)
- Draft Ecological Risk Assessment (March 2021)
- Draft Human Health Risk Assessment (March 2021)
- Proposed Interim Registration Review Decision (March 2021)
- MSMA Registration Review Report (September 2022)
In March 2021, EPA announced the availability of the Proposed Interim Registration Review Decision (PID) for MSMA for a 60-day public comment period. In the PID, EPA determined that, although sod farm, golf course, and highway rights-of-way uses of MSMA do not meet the FIFRA registration standard at this time, that due to the ongoing review of the carcinogenic MOA of inorganic arsenic, it is appropriate to delay removing these uses from labels until EPA’s review has been completed.
In September 2022, after receiving and considering public comments on the MSMA PID, EPA published a Registration Review Report for MSMA. EPA completed the report to update the public on aspects of the registration review process, including reiterating EPA’s previous regulatory position from the 2009 RED on the turf uses, providing updates on EPA’s review of data received since the RED, sharing the schedule for the registrant submission of rotational crop data for certain crops, and explaining EPA’s plans to evaluate MSMA in the future. As part of this action, the MSMA registrants committed to conduct studies to address rotational crop data deficiencies, which would otherwise have required label revisions.