* EPA chief says no change on course particle standards
* EPA wants to end "myth" it plans farm dust regulation
* Critics use issue as symbol of runaway regulators
* Microscopic particles can cause health problems (Adds environmentalist reaction, bylines, background)
By Timothy Gardner and Charles Abbott
WASHINGTON, Oct 17 (Reuters) - The U.S. Environmental Protection Agency said on Monday it will not tighten controls on dust particles on farms when it sends the rule to the White House for its regular five-year review.
Some Republicans, including presidential hopeful Herman Cain, have said the EPA would expand the rule, which mainly affects heavy industry and vehicles, and regulate farms more broadly.
The agency sent a letter late last week to Senate Agriculture Committee Chairwoman Debbie Stabenow and fellow farm-state Democrat Amy Klobuchar saying it will keep soot standards in place, but not tighten the rules to regulate larger particulates.
Critics have used the potential of farm-dust regulation as a headline issue in complaining of runaway regulation. Nebraska Sen Mike Johanns, a Republican, said he has 26 co-sponsors for a bill to bar regulation of dust from farm fields and dirt roads.
An EPA spokeswoman said the letter should end the "myth" of broader farm-dust regulation. Concern arose from proposals during the review process for stricter rules.
Microscopic particles can be inhaled and can cause serious health issues such as difficult breathing and irregular heartbeat. Power plants, factories, motor vehicles, fires and construction are among the sources of particles, soot, haze and dust.
In standards in place since the 1980s, EPA regulates particulate matter up to 10 microns in diameter. By comparison, a human hair is about 70 microns in diameter. Sand and particles larger than 10 microns are not regulated.
"Based on my consideration of the scientific record, analysis provided by EPA scientists and advice from the Clean Air Science Advisory Council, I am prepared to propose the retention -- with no revision -- of the current PM10 standard and form when it is sent to the (White House) for interagency review," wrote EPA administrator Lisa Jackson in the letter to the senators.
The National Farmers Union applauded "final clarification" by EPA that it does not plan to regulate farm dust.
"We hope this action finally puts to rest the misinformation regarding dust regulation and eases the minds of farmers and ranchers across the country," said NFU president Roger Johnson.
An environmental activist was not pleased. "I am sorry to say it appears to be another triumph of political pressure over science," said Frank O'Donnell, president of Clean Air Watch.
Last month the White House delayed new EPA rules on smog- forming gases. President Barack Obama said the decision was part of an effort to ease regulatory burdens on business. [ID:nN1E7810W0] The EPA is under pressure from Republicans and businesses to delay air pollution standards, but says it is moving forward on rules to restrict mercury and other emissions from power plants.
The EPA's Clean Air Science Advisory Council issued a paper this year recommending revision of the dust rules to protect public health.
Livestock and farm groups have said it would be impossible to comply with stricter rules on exposure to dust, which they say is a natural part of farming.
(Reporting by Timothy Gardner and Charles Abbott; editing by Sofina Mirza-Reid and David Gregorio)