Particulate matter, or PM, is the term for particles found in the air, including dust, dirt, soot, smoke, and liquid droplets.

Particles less than 10 micrometers in diameter (PM10) pose a health concern because they can be inhaled into and accumulate in the respiratory system.

Particles less than 2.5 micrometers in diameter (PM2.5) are believed to pose the greatest health risks.

The Ministry of Environmental Protection began gauging public opinion on revised air quality standards on Nov. 16, 2011.

The new standards will, for the first time, include readings of concentrations less than PM2.5, a major cause of haze harmful to health, the ministry said in a statement.

顆粒物是指空氣中的灰塵、散土、煙灰、煙霧及液態小滴等顆粒。

PM10是指大氣中直徑小于10微米的顆粒物，稱為可吸入顆粒物，由于其可進入呼吸系統并聚集，對人體健康有一定隱患。

PM2.5是指大氣中直徑小于2.5微米的顆粒物，稱為可入肺顆粒物，盡管粒徑小，但對人體的危害最大。

國家環保部從2011年11月16日開始就新的《環境空氣質量標準》征求公眾意見。

新標準首次將有害健康的灰霾“元兇”——PM2.5納入常規空氣質量評價體系。