As people in Beijing and northern China struggle with severe air pollution this winter, the toxic air is also making life hard for plants and even food crops of China, say researchers who have been looking at how China's plants are affected by air pollution.

Beijing's extreme smog event this week has made headlines, with the American Embassy calling the pollution levels “hazardous” and Beijing writer Zheng Yuanjie blogging that "the air smells like sulfur perfume, as the capital city currently looks like a poisonous huge gas can," according to a report on Al Jazeera.

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“In the last 50 years there has been a 16-fold increase in ozone pollution” in the Beijing area, said Hanqin Tian of Auburn University in Alabama, who studies the effects of China's pollution and climate change on plants. He said the soup of pollutants, including harmful sulfur and nitrogen compounds “is definitely expanding into new areas; into the countryside.”

Ozone is particularly harmful to plants because it damages the pores on leaves, called stomata, which plants use to regulate how much water transpires from the leaves. That, in turn, affects how much water a plant must take up through its roots. Changes in water uptake by plants have been documented in other parts of the world, including the United States, as having major impacts on regional groundwater and surface water supplies.

“So you could affect the water cycle,” said Hanqin Tian. That's probably not such a good thing in a changing climate and in northern China, where droughts have become a chronic problem, he explained.

The ozone pollution in this case is not the naturally occurring ozone layer up in the stratosphere, which protects Earth's surface from ultraviolet radiation. Rather, it's an entirely man-made byproduct of internal combustion engines in cars.

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In studies of the long-term productivity of plants, Tian Hanqin and some of his colleagues show that ozone pollution, along with climate change, has been lowering plant productivity in China, which reduces the amount of carbon and other pollutants that the plants can absorb to combat all the emissions from the burning of fossil fuels.

The worst effects on plants are likely to be in areas where the growing pollution problem is just fairly new, said Arthur Chappelka, also a plant researcher at Auburn University. Some plants are more resistant to pollutants than others, he said, and the plants that are living today in long-polluted urban areas are likely to be only those that are very pollution tolerant.

Away from the cities, however, where crops are needed to feed China's vast population, the effects of the growing pollution on crops is a significant concern.

“In some ways it affects the crop production and food security of China,” said Hanqin Tian. “Air quality is really important for human health, plants and ecosystem and sustainability."

The problem is bound to get worse as China continues to develop economically, he said, and so he and other researchers continue to urge the Chinese government to take action to reduce emissions from cars and industries.