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The Health Effects of Air Pollution

The Health Effects of Air Pollution

3 June 2020


The impact of air pollution on human health has been widely recognized. According to the World Health Organization (WHO)'s publication on Global Health Risk (2009), urban outdoor air pollution is ranked as the 14th global risk factor for mortality in Year 2004 (Figure 1).

The effects of air pollution on health are dependent on several factors. Apart from the concentrations and chemical properties of the pollutants, the person's age and general state of health, the duration of exposure, factors such as the weather condition and the distance from the emission sources also affect the nature and extent of the health effects observed. Those who exercise outdoor also increase their exposure to air pollutants. However, under general circumstances where exposure is brief, the health risk is usually low.

Sources of Air Pollution

Urban air pollution in Hong Kong is partly caused by emissions from motor vehicles, especially diesel vehicles. Emissions from power plants are also an important local air pollution source.

Hong Kong also experiences regional air pollution which is the result of emissions from power plants, industry and motor vehicles in both Hong Kong and the Pearl River Delta region.

Potential Health Effects

The potential health effects of air pollution range from subtle physiological changes inside the body to florid symptoms such as nose and throat irritation, shortness of breath, coughing and chest tightness. People suffering from asthma or chronic respiratory diseases will experience an increase in symptoms when exposed to air pollutants. Although individual's reaction to air pollutant depends on various factors, people of all age groups are affected by poor air quality. Vulnerable groups such as children and the elderly are especially susceptible to the effects of air pollution.

Studies based on local data have revealed that there was a strong association between high pollution incidents and both hospital admissions and premature deaths for respiratory and cardiovascular diseases.

Outdoor air pollutants can increase the risk of cancer. In June 2012, the International Agency for Research on Cancer (IARC) under the WHO has reclassified diesel engine exhaust from ‘probably carcinogenic to humans (Group 2A)’ to ‘carcinogenic to humans (Group 1)’. Based on latest scientific evidence, the IARC has further classified outdoor air pollution as ‘carcinogenic to humans (Group 1)’ in October 2013.

The Common Air Pollutants

The common air pollutants include carbon monoxide, nitrogen dioxide, ozone, particulate matter, sulphur dioxide and lead.

Air Pollutant Sources Health Effects
Carbon Monoxide
Carbon monoxide is produced primarily by incomplete combustion of fossil fuels. In Hong Kong, the majority of ambient carbon monoxide comes from vehicular emissions.
Carbon monoxide reduces the oxygen carrying capacity of red blood cells. The health effects depend on the duration of exposure and the concentration of carbon monoxide inhaled. Typical symptoms of exposure to low levels of carbon monoxide include headache, dizziness and tiredness. Higher concentration of carbon monoxide can lead to impaired vision, disturbed coordination and eventually death.

Nitrogen Dioxide

High temperature combustion process results in nitrogen oxides emissions (including nitrogen dioxide), which may come from stationary sources such as power stations, and mobile sources such as motor vehicles.

Nitrogen dioxide irritates the mucosa of the eyes, nose, throat and the lower respiratory tract. Exposure to low level of nitrogen dioxide may cause increased bronchial reactivity and in those with asthma increased response to allergens. Nitrogen dioxide also aggravates existing chronic respiratory diseases. Long-term exposure to nitrogen dioxide can lower a person's lung function and resistance to respiratory infections.


Ozone is formed from photochemical reaction between nitrogen oxides and volatile organic compounds, which are predominantly pollutants from motor vehicles and industries.

In the upper atmosphere, ozone shields the earth from the harmful ultraviolet radiation. In ground level, ozone is a highly reactive gas which can irritate the eyes and bring upper and lower respiratory symptoms to healthy people. It may also provoke asthmatic attacks in people having asthma. Ozone can also increase a person's susceptibility to respiratory infection and aggravate pre-existing respiratory illnesses.

Particulate Matter (PM)

There are numerous sources of PM related to human activities as well as natural sources. Combustion of fossil fuels (such as from power plants and vehicles) and biomass burning (such as wildfires) are the key source categories. In general, PM consists of a complex mixture of solid and liquid particles of organic and inorganic substances suspended in the air.

PM is further identified according to their aerodynamic diameter, as either PM10 (particles with an aerodynamic diameter smaller than 10 µm) or PM2.5 (aerodynamic diameter smaller than 2.5 µm). The latter are more dangerous since, when inhaled, they may get deeper into the lungs. A number of research studies have shown association between increase in PM concentration and increase in daily hospital admissions and premature deaths from respiratory and cardiovascular diseases. Persons with pre-existing cardiovascular and respiratory diseases were found to be most susceptible. PM has been classified as carcinogenic to humans (Group 1) by IARC.

Sulphur Dioxide

Sulphur dioxide is formed primarily from combustion of sulphur containing fossil fuels. In Hong Kong, the main source of sulphur dioxide is the power stations. Vehicular and marine vessels as well as industrial fuel combustion also contribute to the atmospheric sulphur dioxide.

Sulphur dioxide irritates eyes and nose. Inhalation of sulphur dioxide causes narrowing of the airways (bronchoconstriction), which people suffering from asthma and chronic respiratory diseases are more sensitive to than other people.


Lead occurs naturally in the earth's crust but the natural sources contribute only a small fraction of the amount of lead found in air, food, water and dust. The majority of lead in these media arises from automobile and industrial emissions and from the use of lead-containing solder and paints. The lead content in vehicular exhaust is the result of using organolead compound as fuel additives. However, a ban on the use of leaded petrol in Hong Kong in 1999 has completely eliminated the emissions of lead from motor vehicles.

Lead is a highly toxic and is known to damage the nervous system and kidney, and interfere with the synthesis of haemoglobin. Children are more vulnerable to the effects of lead, which can result in learning disabilities and impaired neurobehavioural functioning.


Information related to Local Air Quality and Air Quality Health Index

In Hong Kong, the Air Pollution Control Ordinance, Cap. 311, is the main legislation on air pollution control. It is enforced by the Environmental Protection Department (EPD). EPD also monitors the local air quality and communicates the short-term health risk posed by air pollution to the general public through the Air Quality Health Index (AQHI) currently.

AQHIs are reported on a scale of 1 to 10 and 10+ and are grouped into five health risk categories of “Low”, “Moderate”, “High”, “Very High” and “Serious”. The suggested precautionary actions under each AQHI category can be found in the following EPD's website for AQHI -

To help improve understanding on the health risk of air quality and the actions you can take to improve air quality, the EPD produced a video series entitled “Clean Air and You”. The series can be found in the following EPD's website -

Consideration for the Use of Masks

In general, people should reduce outdoor physical exertion and the time of their stay outdoors on high pollution days. If outdoor physical exertion is unavoidable, some people may indicate a wish to use masks. It should be noted that common masks (such as surgical masks and N95 masks) are not useful in preventing the inhalation of gaseous air pollutants such as carbon monoxide, nitrogen dioxide, ozone and sulphur dioxide.

Although surgical masks may filter larger particles in the air, studies have shown that they are not effective in the filtration of PM of less than 10 microns, let alone PM of less than 2.5 microns.

The efficiency of N95 masks to filter PM depends on whether they are fitted properly. Moreover, they are uncomfortable to use and increase the effort of breathing especially if worn over an extended period of time.

For information on assessment of medical fitness to use respirators in conditions of high AQHI, medical practitioners may refer to the relevant “Guidance for Physicians”. Medical practitioners can also refer to the Letter to Doctors (3 Jan 2014) for more health advice to be provided to their patients and the public when AQHI reached 10+.


Figure 1: Deaths attributed to 19 leading risk factors, by country income level (2004)

Source: Global Health Risk - Mortality and burden of disease attributable to selected major risks, the World Health Organization (2009)