FACTBOX - Key facts about Beijing's hard-to-clear smog

(Reuters) - Haze-shrouded Beijing may take more cars off the roads and shut more factories if air quality remains a problem during the Olympic Games, state media said on Monday.

The pollution has proved hard to shift days before athletes and visitors arrive, despite the implementation of two month even-odd car ban and temporary factory closures.

Here is some background about clean air and smog in Beijing, host of the August 8-24 Olympics.

* CLEAN AIR VS POLLUTED AIR:

-- Clean air is a balance of nitrogen (78 percent), oxygen (21 percent), water vapor, and small amounts of inert gases such as neon, carbon dioxide and helium.

-- Air pollutants -- substances not naturally occurring in the atmosphere -- affect this balance.

-- Main air pollutants include particulate matter (PM10), sulfur dioxide (SO2), nitrogen dioxide (NO2), ozone (O3), volatile organic compounds (VOCs) and carbon monoxide (CO).

* BEIJING'S MAIN AIR POLLUTANTS:

-- Particulate matter, a mix of dust, dirt, soot, smoke and various droplets emitted from cars, factories, coal-burning boilers and construction sites, is Beijing's worst pollutant.