Monosodium Methanearsonate (MSMA), an Organic Arsenical
Monosodium methanearsonate (MSMA) is an organic arsenical pesticide currently registered for use in the United States. MSMA is a broad spectrum herbicide used to control grasses and broadleaf weeds. It can only be used on cotton, sod farms, golf courses, and highway rights-of-way, and cannot be used in Florida except for on cotton in certain counties.
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 application to 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.
In January 2009, EPA reached an agreement 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 2009, EPA modified the January 2009 agreement with registrants to include a peer review by the National Academy of Sciences
As a result, MSMA is still allowed for use on sod farms, golf courses, and highway rights-of-way. The 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.
- Spot treatments only (100 sq feet per spot), not to exceed 25 percent of the total golf course acreage per year.
- One broadcast treatment for newly constructed courses only.
- 1-2 broadcast applications.
- 25-foot buffer around permanent water bodies.
- Two broadcast applications ONLY on highway rights-of-way.
- 100-foot buffer around permanent water bodies.
The use of MSMA before planting cotton is no longer permitted. MSMA can still be used to control weeds in cotton fields. This use is limited to two applications per year of two pounds active ingredient per acre each, once plants are growing (also called post-emergent application). Cotton growers also must maintain a 50-foot buffer zone around permanent water bodies.
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 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 2009 Agreement 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 on October 29, 2020 for EPA’s review under the Pesticide Registration Improvement Act (PRIA). EPA is currently evaluating this submission.
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 (March2013)
- 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)
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, due to the ongoing review of the cancer mode of action of inorganic arsenic, it is appropriate to delay removing these uses from labels until EPA’s review of the MOA for inorganic arsenic has been completed. EPA is seeking public comment on the proposed decision and will consider the comments prior to issuing a registration review decision.
It remains legal to purchase and use organic arsenical herbicides according to label directions and precautions. When used according to the label, organic arsenical herbicides do not pose an imminent hazard. Consumers who choose to use these products should always take special care to read and follow the label precautions and directions. The Organic Arsenicals; Amendments to Terminate Uses; Amendment to Existing Stocks Provisions – FR Notice, March 27, 2013 clarifies the existing stocks provisions for the turf uses, and for previously canceled uses.
If you choose to discontinue use, contact your state or local hazardous waste disposal programs or your local solid waste collection service for information on proper disposal. See also Safe Disposal of Pesticides.