Particulate matter (PM) in the air is a priority pollutant internationally due to its adverse effects on public health as well as its negative effects on sensitive ecosystems, materials and on climate. Additionally, it is a key contributor to low visibility incidents.Particulate pollution in the atmosphere derives from a wide range of sources including transportation of natural materials like sand; dust from construction activities; traffic and industry. Secondary particles are formed from atmospheric reactions between gases, such as the photochemical oxidation of sulphur dioxide.

Airborne particles cover a wide range of sizes. Large particles generally settle out onto surfaces and cause nuisance effects through soiling of buildings, monuments and fabrics. Human health concerns are generally centered around fine particulate matter with an aerodynamic diameter of 10 micrometres (μm) or less, known as PM₁₀. These particles can penetrate deep into the human respiratory system and are responsible for a variety of health effects including effects on the respiratory and cardiovascular systems, asthma and premature death. Health effects of particulate matter are greater in the presence of high concentrations of SO₂, as the two pollutants act synergistically.

All combustion processes produce oxides of nitrogen. Nitrogen dioxide (NO₂) is associated with adverse human health effects and is produced mainly through the atmospheric oxidation of nitric oxide, which is produced during combustion of hydrocarbon fuels. Sources of nitrogen oxide emissions include road traffic, power stations and industry.

Nitrogen dioxide can affect the respiratory system causing inflammation of the airways. People with asthma or respiratory diseases are particularly sensitive and can show effects at low concentrations.

Sulphur dioxide (SO₂) is a gas produced from burning fuel and processing materials which contain sulphur contaminants. Typical sources include oil-fired power stations, oil and gas processing, industry and road traffic.

Inhalation of sulphur dioxide causes constriction of the airways leading to breathing difficulties, particularly in those suffering from asthma and chronic lung disease.

SO₂ can also lead to the formation of acid rain which can lead to widespread ecosystem damage in rivers and lakes, sometimes thousands of kilometers from the original emission source. Direct and indirect impacts on vegetation are also known to occur at lower concentrations than those affecting human health.

Carbon monoxide (CO) is a colourless, odourless gas which is produced by the incomplete combustion of hydrocarbon fuels. The main source of CO is road traffic, in particular petrol-engine vehicles. The main threat to human health from carbon monoxide is from the formation of carboxyhaemoglobin in the blood, which substantially reduces the uptake and transport of oxygen in the body. People who suffer from existing diseases which affect delivery of oxygen to the heart and brain (e.g. coronary artery disease) are at particular risk.

Ozone (O₃) is a secondary pollutant which is formed in the atmosphere during chemical reactions involving nitrogen oxides, volatile organic compounds (VOCs) and sunlight. These chemical reactions take place over several hours or even days, depending on the VOCs present. As a consequence, ozone measured at a particular location could have been formed from precursor compounds produced hundreds or even thousands of kilometers away. Once formed, ozone then drifts in the atmosphere under the influence of meteorological conditions. Maximum concentrations are therefore generally found some distance downwind from the source emissions. Concentrations in urban areas with high levels of traffic are often reduced due to the reaction between ozone and nitric oxide, resulting in the formation of nitrogen dioxide.

Ozone is also toxic to plants and can cause leaf necrosis and affect photosynthesis, and therefore reducing production of crops and trees.

Volatile organic compounds (VOCs) is a term given to a wide class of chemical compounds including aliphatic, aromatic and halogenated hydrocarbons, aldehydes, ketones esters and amines. They are important in the formation of ozone and can have a variety of health effects. Some species are directly toxic to humans and some produce offensive odours at very low concentrations.

Lead is a naturally occurring metallic element which is found in the atmosphere as small particles. Industrial sources of lead include battery manufacture and secondary metal smelting. Road traffic used to be the dominant source of lead in urban areas, as it was used as an additive in petrol. In recent years, its use in petrol has been phased out and this is no longer a major source.

Exposure to lead can have a variety of human health effects, including effects on the synthesis of hemoglobin, effects on the kidneys, gastrointestinal system and effects on the nervous system. It has also been shown to have a negative effect on childhood neurological development.