空氣的污染物可分為污染氣體和懸浮物。懸浮物通常以顆粒的大小來區分，而當中的可吸入懸浮粒子則可影響人體。PM2.5是指空氣中直徑小於或等於2.5微米的可吸入懸浮粒子：
PM2.5微細懸浮粒子直徑小， 還不到人的頭髮絲粗細的1/20，可以穿透肺泡到達血液。 由於富含大量的有毒及有害物質， 而且在空氣中的停留時間長、輸送距離遠，因而對人體健康和環境的影響非常大。
Particulate matter (PM) is the term for particles found in the air. PM2.5 particles are air pollutants with diameter of 2.5 micrometers or less that are suspended in the air. Particles less than 10 micrometers in diam
eter can be inhaled into and accumulate in the respiratory system. Particles less than 2.5 micrometers in diameter (PM2.5), because of their small size, can lodge deeply into the lungs and cause serious damage to health.
健康空氣行動從2012年1月11日開始，逢星期一至五，每天監測微細懸浮粒子(PM2.5)濃度，並通過Twitter () 、微博(@健康空氣行動)和facebook實時發表數據。
免責聲明：我們的PM2.5數據是通過型號為SidePak AM510的便攜式檢測儀所錄得，此檢測儀由美國TSI公司製造，廣為高校研究者使用。 此監測數據有誤差和不確定性，只可作參考使用，且不能以此推算香港其他地點的PM2.5濃度。我們監測和報導PM2.5的目的僅為協助關心PM2.5的市民大致了解PM2.5的濃度範圍，以及提高公眾意識。
Starting from January 11, 2012, Clean Air Network (CAN) monitors PM2.5 everyday, Mon - Fri, and publish the data through Twitter (), Weibo (@健康空氣行動)和facebook. Disclaimer: the PM2.5 data we release is measured by a portable PM2.5 monitor (TSI SidePak AM510) which is widely used by university researchers. The data is subjected to variation and uncertainty, and is for REFERENCE ONLY, NOT to be extracted to other locations in HK. The goal of our PM2.5 reporting is solely to help our concerned citizens to get a rough idea of what the PM2.5 level is like and to raise awareness.
