• Carbon monoxide - produced by the incomplete burning of carbon-containing fuels, such as petrol, coal and wood.
• Sulphur dioxide - produced by burning fossil fuels (e.g. fuel oil and coil). A large proportion is produced by power stations and metal smelters which burn sulphur-containing coal, and also by the manufacturing industries which burn fuel oil.
• Nitrogen oxides - produced by petrol- or diesel-burning engines and coal/oil furnaces. In Hong Kong, the levels of nitrogen oxides are noticeably high because of the great proportion of diesel-driven vehicles operating in the territory. Diesel engines produce much less carbon monoxide but a lot more nitrogen oxides than engines.
• Hydrocarbons - formed from the evaporation of materials such as petrol, diesel and solvents when exposed to air. They can also be found in car exhaust as unburned hydrocarbons.
• Ozone - produced by the reaction of oxygen gas with free atoms of oxygen which are formed from the reactions between nitrogen oxides and hydrocarbons in sunlight.
• Particulates - produced by refuse incineration, factories, diesel vehicles, construction sites, and coal/charcoal burners. Particulates are solid or liquid particles which are so small that they remain suspended in the air for a long period of time. We see these particles in mass as smoke or haze.

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