SHENZHEN, China  In this sprawling metropolis in southeastern China stand two hulking brown buildings erected by a private company, the Longgang trash incinerators. They can be smelled a mile away and pour out so much dark smoke and hazardous chemicals that hundreds of local residents recently staged an all-day sit-in, demanding that the incinerators be cleaner and that a planned third incinerator not be built nearby.

After surpassing the United States as the world’s largest producer of household garbage, China has embarked on a vast program to build incinerators as landfills run out of space. But these incinerators have become a growing source of toxic emissions, from dioxin to mercury, that can damage the body’s nervous system.

And these pollutants, particularly long-lasting substances like dioxin and mercury, are dangerous not only in China, a growing body of atmospheric research based on satellite observations suggests. They float on air currents across the Pacific to American shores.

Chinese incinerators can be better. At the other end of Shenzhen from Longgang, no smoke is visible from the towering smokestack of the Baoan incinerator, built by a company owned by the municipal government. Government tests show that it emits virtually no dioxin and other pollutants.