In the last 20 years, we’ve learned more and more about air pollution from Asia that blows across the Pacific Ocean and falls out of the atmosphere here in the Northwest.

Scientists have estimated that nearly one-fifth of the mercury in the Willamette River comes from foreign sources, much of it from China. Dust from storms in the Gobi desert can turn local skies milky white. When winds blow from Asia, pristine mountaintops like Mount Bachelor and Cheeka Peak on Washington’s Olympic Peninsula see increased levels of industrial chemicals that contribute to smog and can pose other health risks. (For a great overview, just read this story from Discover Magazine.)

But a recent study poses an interesting question, particularly in light of efforts to block Northwest coal export terminals: How much of the industrial pollution we get from fossil fuels burned in China is arguably because of us?

Specifically, the study published late last year in the Proceedings of the National Academy of Sciences (PNAS) estimates how much US pollution can be attributed to fossil fuels (largely coal) that are burned in China to produce the clothes, electronics, toys, furniture, and other manufactured goods that are consumed in other countries?

In Washington and Oregon, for instance, these published maps estimate that on the most polluted days in 2006, China’s export-related emissions contributed roughly between 12 and 20 percent of the region’s sulfate pollution, which can aggravate respiratory problems and acid rain. The contribution was between roughly 2 to 4 percent for ozone pollution in the Northwest, up to 10 percent for black carbon and between 4 and 5 percent for carbon monoxide.

Of all those export-related emissions, roughly 21 percent are attributable to goods that are bought and consumed in the United States, according to the study.

If one takes all of China’s air pollution into account—not just the trade-related emissions—the local contributions in 2006 would be roughly three times higher for SOx and NOx and four times higher for black carbon.

It’s important to keep in mind that these pollution estimates are based on models that carry high levels of uncertainty. So on some level, the actual numbers may matter less than the larger point, which is that a significant portion of the coal being burned in China (and the resulting emissions) is being driven by the consumption of goods in the US and elsewhere. While China can pursue cleaner alternatives, its appetite for energy—of all sorts—remains connected to the things that we buy.

How the pollution gets here

Coal combustion produces carbon dioxide, which generally joins a global pool of climate-warming gases that blanket and circle the planet. It also releases a number of shorter-lived chemicals that can fall back to earth as local pollution, including nitrogen oxide (NOx), sulfur oxide (SOx), carbon monoxide, particulate matter (including black carbon), and trace metals such as mercury. Those pollutants can make soils and lakes more acidic, contribute to smog, harm human tissues and hearts, increase risks of respiratory and neurological problems, and damage crops.

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    • In China, pollution problems are compounded because the combustion efficiency of coal burned there is relatively low and emission controls are relatively weak. The growing volume of Chinese exports, which according to the PNAS study rose by 390% from 2000 to 2007, has led to increases in those pollutants.

    But whether or not those pollutants get transported from China all the way to the US depends on a number of factors: They have to be injected high enough into the atmosphere to get picked up by the jet stream, they have to avoid getting rained out or chemically altered along the way, and they have to be propelled across the Pacific Ocean by vigorous winds. That’s why pollution transport tends to be sporadic, though it typically peaks in spring when under favorable conditions the journey from Asia to the US west coast takes 5 to 7 days.

    In fact, because shorter-lived pollutants are more likely to get deposited near a Chinese smokestack than get carried along the jet stream across the Pacific, China’s residents bear a much greater pollution burden from our consumption-based emissions, as these maps from the PNAS results show. (Note: these ranges are based on average annual concentrations rather than worst-case pollution days.)

    Embodied emissions

    The PNAS study, which was authored by researchers in China and the US, only counted emissions that are related to the energy required to manufacture export goods for and excludes pollution from traffic, coal dust, home heating, etc. Because coal burning provides nearly 70 percent of China’s energy, a large fraction of those emissions must be from coal.

    The emissions ’embodied’ in the goods we buy from China are emissions we would be producing in the US if those goods had been manufactured here. Outsourcing that manufacturing makes our own atmosphere cleaner than it would otherwise be. In fact, these maps estimate that some areas of the East Coast have seen reduced pollution as a result of importing Chinese goods.

    Here in the Northwest, where we’re more likely to see local fallout from China’s emissions, the opposite is more likely to be true, according to the study maps. So how much of a difference does that additional pollution make?

    The modeling study, at least, suggests that right now, the export-related pollution from China is probably contributing a relatively small share of the Northwest’s local pollution. If you consider the local impacts from China’s total emissions—not just the trade-related ones—that overseas pollution starts to become more significant. And as China’s emissions continue to grow, it could become harder for some West Coast cities to meet federal air quality standards over time.

    But the larger point embedded in the study is perhaps even more relevant, given the multiple proposals to turn Northwest ports and shorelines into loading docks for China’s fossil fuels. We shouldn’t ignore that part of the demand for that coal (overstated though it may be) is fueled by the iPads, bicycles, sneakers, and countless other things we buy and the the dynamics that make it so much cheaper to manufacture them overseas.