WASHINGTON A leading research for the California Air Resources Board wrote a letter early this summer that said the selective catalytic reduction process that most diesel engine manufacturers will use to comply with 2010 federal emission standards could emit toxic byproducts.

The letter was filed in federal court here by Navistar Inc., which is suing the U.S. Environmental Protection Agency over its approval of selective catalytic reduction for use in 2010 diesel engines to meet federal emissions rules.

The letter was submitted to a federal appeals court to bolster Navistars claim that EPA could be endangering public health by skipping the federal rulemaking process in approving SCR.

Written to "alert" a Boston nonprofit organization planning tests on 2010 engines, the letter said SCR technology represents a "large departure from conventional emission controls by introducing a liquid additive containing an organic form of nitrogen."

The June letter, written by Bart Croes, chief of CARB research, to the Health Effects Institute, said a comprehensive literature search showed that the "primary concern is the release of organonitrogen compounds, many of which are carcinogenic or toxic in other ways."

"Some toxic air contaminants that have been identified with SCR technology include hydrogen cyanide, cyanic acid, nitromethane, hydrazine, acrylonitrile, acrylamide, acetonitrile, and acetamide," Croes wrote in his letter. "It is hoped that any exotic substances emitted from SCR technology will be at levels insignificant to exposure health effects."

"Although we are encouraged by findings to date, which suggest that the technology can deliver significant reductions of many species of toxicological relevance, this work has also documented the increase in some emissions such as some metals, nitrous oxide and nanoparticles," the CARB letter said.

SCR systems reduce NOx in the aftertreatment by mixing the exhaust output with diesel exhaust fluid, a blend of pure urea and demineralized water, through a catalyst that converts NOx into nitrogen and water vapor.

Navistar, which is based in Warrenville, Ill., is the only diesel engine maker planning to use exhaust gas recirculation to meet EPAs 2010 emission standards.

The manufacturers using SCR include independent truck engine manufacturer Cummins Inc., Detroit Diesel Corp. and Volvo Group America.

An EPA spokeswoman declined comment on the letter.

Hector Maldonado, a CARB air pollution specialist who worked on the CARB study, downplayed the significance of the letter.

"There is a possibility, at least in a theoretical sense, that some of these compounds could be formed. However, based on the experience we have to date, there are no alarm bells being rung, but at the same time, we are aware that there is a possibility."

Maldonado said CARB supports the idea of after-treatment control of NOx emissions.

"The irony is that I think, if anything, SCR is incredibly effective in terms of NOx reduction," Maldonado said. "Its some of the exotic compounds, unregulated emissions that might be formed, that give us some cause for concern."

However, he said using the letter to buttress a court argument against SCR is, "in my opinion, a blatant misrepresentation."