Moscow's air is polluted at the best of times, but this smog, heavy with carbon monoxide, makes breathing difficult.

莫斯科的空气，在最好的时候是受到了污染，而这种烟雾里含有大量的一氧化碳，让人呼吸困难。

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This makes breathing difficult and painful, and can also limit the amount of oxygen a person takes into their body with each breath.

这使呼吸变得困难且痛苦，并且会限制每次呼吸时进入人体的氧气量。

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COPD is a serious lung disease that makes breathing difficult. Symptoms can include breathlessness, chronic cough and excessive phlegm.

COPD是一种严重的肺病可造成呼吸困难，症状包括呼吸困难、久咳和痰多。

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It makes breathing difficult. The leading cause is cigarette smoking.

VOA: special.2009.06.23

Much of the fighting in the operation is at elevations above 11, 000 feet, where thin air makes breathing more difficult.

CNN: Al Qaeda, Taliban deaths mount in Operation Anaconda

Along with images of the puffy-faced Chavez came a government explanation for why no one has heard from the longtime president since his surgery: He's breathing through a tracheal tube that makes speech difficult.

NPR: Photos Of Hugo Chavez Shown After 2-Month Absence