Produced by coal-fired power plants, vehicle exhausts, factories and construction, air pollution is made up of several chemicals that are hazardous to people.
Here's a quick summary of the major components of concern in air pollution.
Particulate matter (PM) are tiny particles of solid matter suspended in gas or liquid. They can be composed of a variety of substances, such as soot, acids, chemicals, metals, soil and dust. It is often made up of partially combusted fuels, and comes from coal combustion and diesel engines.
When inhaled, PM can cause asthma, chronic bronchitis, breathing difficulties, lung cancer, cardiovascular issues, birth defects and premature death. People with heart or lung diseases, children and older adults are more vulnerable to PM pollution.
The size of the particle is directly related to how dangerous they are. The smaller the particle, the further they can penetrate into the human body and the lungs.
Larger particles of pollution can be filtered out by mucus and cilia in the nose and throat. But PM10 – particles smaller than 10 micrometers – can enter deep into the lungs and into the bronchioles. PM2.5 – those smaller than 2.5 – are even more dangerous, as they can enter into the areas where gas is exchanged between the lungs and the bloodstream, and thus into other organs. PM2.5 has been linked to increased incidence of heart disease, heart attacks and other cardiovascular issues.
Head to our handy infographic to learn more about PM2.5 and how to protect yourself from it.
Sulphur dioxide (SO2)
Sulphur dioxide is a major air pollutant. This colorless gas is produced naturally by volcanoes, but in China and Hong Kong, its main sources are coal and gasoline. This means that coal-fired power plants, industry and vehicles are the biggest sources of sulphur dioxide pollution.
Exposure to sulphur dioxide has been linked to asthma, breathing difficulties, impaired lung function, respiratory illness and cardiovascular disease. It can also lower infant birth weight and increase the rate of birth mortality. Children, the elderly, and those with asthma or long-term lung and cardiovascular disease are the most vulnerable to its effects.
In addition, when sulphur dioxide combines with water, it can form acid rain. Acid rain can cause deforestation, acidified lakes and streams, and loss of biodiversity.
Nitrogen dioxide (NO2)
Nitrogen dioxide is a reddish-brown gas produced by power plants, heating furnaces and the engines of ships and vehicles. It can react with chemicals in the air to form fine particles, and thus contribute to particulate matter pollution.
Low levels of nitrogen dioxide exposure can cause irritation of the eyes, nose, throat and lungs, as well as shortness of breath, tiredness, nausea and impaired lung functions. Nitrogen dioxide can also lead to premature birth, low infant birth weight and birth mortality. Children may face greater risk of respiratory disease.
High levels of exposure can cause rapid burning, swelling and spasms of tissues in the throat and upper respiratory tract, reduced oxygen supply, build-up of fluid in the lungs and death.
Ozone is beneficial when it's high in our atmosphere, where it shields us from the sun's ultraviolet rays. But down at street-level, ozone has various health hazards and is a main component of smog.
Ozone levels are higher on sunny days, when the sunshine catalyses a chemical reaction between air pollutants (such as nitrogen dioxide) and volatile organic compounds, which are produced from vehicles, industry and solvents.
Ozone causes irritation of the eyes and nasal passage, asthma attacks, breathing difficulties, impaired lung functions and other respiratory diseases.
Carbon monoxide (CO)
Carbon monoxide is a colorless, odorless and tasteless gas. In higher doses, it can be highly toxic.
Carbon monoxide is produced as exhaust from internal combustion engines, as well as by the combustion of fuels, including coal, natural gas, oil and propane.
Carbon monoxide poisoning can result in headache, nausea, vomiting, dizziness and fatigue. It may also lead to confusion, disorientation and visual disturbances. Higher levels of poisoning can result in coma, seizures and death.
Polycyclic aromatic hydrocarbons (PAHs)
PAHs are produced by burning coal, and are especially dangerous for those who live near coal-fired power plants. They are also found in oil and tar.
Long-term exposure can increase the rate of lung cancer. PAH exposure during pregnancy has been linked to low birth weight, reduced IQ and impaired nervous system development.
According to a case study in Tongliang township, Chongqing, the concentration of PAHs in the air rose by 3.5 times whenever the coal-fired power plant was operating.