"Today, air pollution presents significant environmental risk in some border communities," according to the air quality report by Mexico's Environment Ministry and the U.S. Environmental Protection Agency (EPA).

"Many border residents are frequently exposed to elevated concentrations of carbon monoxide, sulfur dioxide, ozone and particulate matter," the report obtained by Reuters said.

The findings raise alarms about potential harm to the ecosystem and human health in the 2,000-mile (3,200 kms) border area - home to more than 4.8 million Mexicans and 5.8 million U.S. citizens. The border population is expected to more than double in the next 20 years.

The study found that virtually every major community from Ciudad Juarez and its U.S. neighbor El Paso, Texas to the Pacific Ocean, has toxins that exceeded air quality standards in at least one of six categories of contaminants: particulate matter - soot mixed with gaseous vapors - sulfur dioxide, carbon monoxide, nitrogen dioxide, lead and ozone.

Particulate matter has been linked to respiratory disease, and can aggravate heart problems. Carbon monoxide poisoning can cause visual impairments and even death.

Only San Diego County, Calif., escaped the worst-air list. That state has some of the strictest emissions laws in the United States.

Mexican environmental expert Gildaro Acosta said Mexican industry - unfettered by strict emission control laws - and U.S. businesses that exceed legal standards are the worst culprits.

Acosta said visitors to Agua Prieta and Nogales, Sonora, are surprised to see smog in the sparsely-populated western frontier. He put much of the blame on the region's two copper smelting plants and a lime plant.

Blasting and stone crushing at the lime plant produces tons of dust per day, he said.

"There are times you can feel the dust in the air," he said, adding that the visibility in Agua Prieta is less than 75 yards (meters). Community groups in Agua Prieta are monitoring cases of lupus, a disease of the autoimmune system, he said.

DUST AND INDUSTRY BLAMED

The study is a U.S.-Mexican effort to monitor spiraling pollution fueled by a rising population, industry and traffic since Mexico joined NAFTA in 1994.

Results of soil and water pollution studies will also be released when the full report is made public later this month.

Conducted from 1996-2000, the air quality study blames the region's pollution on dust from unpaved streets and industry, vehicle exhaust, power plants and outdated methods of brick making - all exacerbated by the desert topography and climate that encircles most of the border.

The Nogales community has raised concerns about possible clusters of myeloma, a cancer of the bone marrow, the report concedes.

While the air quality report does not draw conclusions about pollution-linked disease, the EPA announced last month that it will help fund an environmental health clinic for children located at a hospital near Mexico City.

The report also tells how communities have reduced toxic emissions.

In 1997, the Mexican government stopped using lead as an additive in gasoline, and since October 1999 Ciudad Juarez has joined El Paso in distributing oxygenated gasoline in winter.

That effort reduces the carbon monoxide levels during the cold months, when weather patterns keep pollutants close to the ground for much of the day.

The report concludes that both countries have a stake in reducing toxins along the frontier.