Air quality is represented by annual mean concentration of fine particulate matter (PM10 and PM2.5, i.e. particles smaller than 10 or 2.5 microns).

空气质量用年平均细颗粒物的浓度表示(PM 10至PM 2.5，即粒径小于等于10或2.5微米的细颗粒)。

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Fine particulate matter - particles smaller than 2.5 micrometers - can evade hairlike projections in the nose and trachea that trap larger particles.

细颗粒物——小于2.5微米的颗粒——能躲过鼻子和气管中绒毛的保护作用。

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The southwest Iranian city of Ahvaz walked away with the unfortunate distinction of having the highest measured level of airborne particles smaller than 10 micrometers.

伊朗西南部城市阿瓦士不幸成为空气最糟城市，该市空气中小于10微米的颗粒物含量是最高的。

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But HEPA filters cannot remove disease-causing particles smaller than zero point three microns.

VOA: special.2009.07.14

Efforts to make the particles smaller have worked, but have raised safety concerns because they can penetrate the skin and potentially interact with cells.

WSJ: European Sunscreen Roadblock on U.S. Beaches

Moving around unseen are an estimated 500, 000 particles ranging in size between 1-10cm across, and perhaps tens of millions of other particles smaller than 1cm.

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Respiratory health researches have for years suspected a relationship between raised levels of particulate matter (defined as airborne particles smaller than 2.5 micrograms) and cardiac arrest.

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