灰霾是一种由固体颗粒形成的空气污染，其核心物质是空气中悬浮的灰尘颗粒，气象学上称为气胶颗粒。霾中含有数百种大气化学颗粒物质，它们在人们毫无防范的时候侵入人体呼吸道和肺叶中，从而引起呼吸系统疾病、心血管系统疾病、血液系统、生殖系统等疾病，诸如咽喉炎、肺气肿、哮喘、鼻炎、支气管炎等炎症，长期处于这种环境还会诱发肺癌、心肌缺血及损伤；雾霾的危害如同吸烟 。而霾也常常引发交通事故。1952年伦敦的烟霞造成了超过4000人死亡。

烟霞的成因与逆温层的出现有关。在一般情况，地面气温较半空为暖，因此空气会上升并在半空散开。但若上升的暖空气遇到逆温层的出现时，空气不能上升而造成累积，形成烟霞。

Smog is a type of air pollutant. The word "smog" was coined in the early 20th century as a portmanteau of the words smoke and fog to refer to smoky fog. The word was then intended to refer to what was sometimes known as pea soup fog, a familiar and serious problem in London from the 19th century to the mid 20th century. This kind of visible air pollution is composed of nitrogen oxides, sulfur oxides, ozone, smoke or particulates among others (less visible pollutants include carbon monoxide, CFCs and radioactive sources). Man-made smog is derived from coal emissions, vehicular emissions, industrial emissions, forest and agricultural fires and photochemical reactions of these emissions.