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.

颗粒物是指空气中的灰尘、散土、烟灰、烟雾及液态小滴等颗粒。

PM10是指大气中直径小于10微米的颗粒物，称为可吸入颗粒物，由于其可进入呼吸系统并聚集，对人体健康有一定隐患。

PM2.5是指大气中直径小于2.5微米的颗粒物，称为可入肺颗粒物，尽管粒径小，但对人体的危害最大。

国家环保部从2011年11月16日开始就新的《环境空气质量标准》征求公众意见。

新标准首次将有害健康的灰霾“元凶”——PM2.5纳入常规空气质量评价体系。