Backers of California's proposed high-speed rail system frequently tout the long-term air-quality benefits of getting people out of cars and planes and onto electric-powered trains.

But any reductions in air pollution won't start for at least a decade, when the trains would start carrying passengers between Merced and the Los Angeles Basin. Meanwhile, building the system in the San Joaquin Valley is expected to pump tons of dust, greenhouse gases and other pollutants into the air. International experts warn it could take years for the benefits of train ridership to make up for the harm caused during construction.

The California High-Speed Rail Authority expects to pay millions of dollars to make up for construction emissions in the Valley.

"Building in an emissions-free manner is not possible, of course," said Lisa Marie Burcar, a spokeswoman for the rail authority. "But offsetting those emissions to result in the same outcome is."

In its environmental impact report for the Merced-to-Fresno section -- one of the first portions of the statewide train system planned to be built -- the rail authority allows that "construction ... has the potential to cause temporary and significant localized air quality impacts" on the Valley's air between 2013 and 2022.

Work would include demolition, land grading, earthmoving, pouring concrete, building stations and laying tracks.

All that work, and the equipment used to do it, are expected to produce reactive organic compounds and nitrogen oxides -- two chemicals that mix in the atmosphere to create ozone -- as well as dust and carbon dioxide and other greenhouse gases.

The pollution anticipated from high-speed rail construction would be a small fraction of emissions already generated in the region. But in the Valley, already struggling to meet state and federal air-quality standards, any extra pollution is a major worry, said David Barber, of the San Joaquin Valley Air Pollution Control District.

Construction pollution not only has "dire consequences" for healthy air, but it threatens the San Joaquin Valley's ability to comply with federal mandates under the federal Clean Air Act, Barber told rail-authority board members this month in Fresno.

The Valley faces several deadlines over the next 11 years to meet standards for ozone and fine particles, called PM-2.5. PM-2.5 is made up of dust and other particles that are 2.5 microns in size or smaller. A human hair, by comparison, is between 50 and 70 microns in thickness.

Barber said failure to reach those standards will have "dramatic and potentially devastating consequences in the form of federal sanctions on the Valley." Penalties could include severe limits on industrial development and the loss of billions of dollars in federal highway funds.

The rail authority outlined steps it will take to limit air pollution. Contractors will have to use the cleanest possible machinery; and trucks, for example, will have to be newer models. Temporary concrete plants will have to be at least 1,000 feet from daycare centers, schools, hospitals, senior-care centers, homes or parks.

Even so, the authority admits emissions would be significant, so it promises to give the Valley air district money to reduce emissions from other sources to offset rail building pollution.

"The air district strongly believes this is the right approach, given the seriousness of air-quality concerns in the Central Valley," Barber told the authority's board.

Burcar said the authority will include similar requirements in the upcoming environmental report for the Fresno-Bakersfield section. In all, the agency estimates it will spend $10 million to $20 million to counter pollution.