The really big difference between air quality in Salt Lake City and Beijing -- indeed, any city of size that lies north of the Yangtze River -- stems from particulates and other emissions in the exhaust from many widely dispersed furnaces whose associated boilers feed into district heating systems, which are the primary means for heating apartment and office buildings ("China or Utah?" HCN, 2/18/13). Any city to the south of the Yangtze is prohibited by the central government from using such systems. Where they do exist, the furnace-boilers use the cheapest coal available, and most of the equipment is old, inefficient and not well maintained. Thanks to the much shorter exhaust stacks -- compared to power plants' tall stacks -- and complete lack of emissions control technology, the exhaust mix produced by these small furnaces is easily trapped close to the ground by air inversions that are common during cold winter months. There, it combines with motor vehicle exhaust to produce a much more toxic mix than any city in the U.S. ever experiences.