Health Impacts

The four major components of air pollution are:

• Particulate Matter, PM
• Nitrogen Dioxide, NO₂
• Sulfur Dioxide, SO₂
• Ozone, O₃

Three other pollutants cited by the Hong Kong Government, and included in their Air Pollution Index are:

• Volatile Organic Compounds, VOC
• Carbon Monoxide, CO
• Lead, Pb

Particulate Matter

Direct Health Impacts: Heart & Lung
Sensitive populations: Young & unborn children

Particulate matter is tiny solid and liquid particles suspended in the air. It is made up of various organic and inorganic substances, including sulfate, nitrates, ammonia, salt, carbon, mineral dust, and water. Particles are identified according to their size, and PM₁₀ refers to particles that are 10 micrometres or smaller in aerodynamic diameter. The smaller the particles, the more dangerous they are, since when inhaled they may reach the deepest regions inside the lungs and interfere with your ability to absorb oxygen into your bloodstream.

The Hong Kong Government currently tracks PM₁₀ only, even though the greatest risk to health comes from even finer particles.
In 2003, the WHO revisited the second version of the Air Quality Guidelines^[2] it had announced in 1996 in order to take into account scientific studies conducted in the intervening period. One of its key recommendations was the recommendation to use fine particulate matter, PM_2.5, to supplement the commonly used PM₁₀ (which includes fine and coarse particles), as an indicator for health effects. This recommendation was based on several epidemiological studies showing that PM_2.5 has adverse effects on mortality, lung function, cardio-pulmonary disease and children’s lung growth and function. Moreover, and importantly, the 2003 WHO study found that fine particles are more dangerous to human health than coarse ones. The WHO study provides an exhaustive treatment of the health impacts of particulates, especially PM_2.5.

Fine particulates (PM_2.5) are largely responsible for our loss of visibility and make up about 70% of the particles measured as PM₁₀ here in Hong Kong. Our annual average level of PM_2.5 is about 45 micrograms per cubic metre. In Beijing, average levels of over 100 were reported in 2005. In the USA in 2002 the average PM_2.5 levels were reported as 12.5 and 90% of monitoring sites recorded levels below 16 micrograms. The new World Health Organization Annual Guideline level is only 10 micrograms.

Chronic long-term exposure to particles increases the risk of developing cardiovascular (heart) and respiratory (lung) diseases, as well as lung cancer. Acute exposure to particles (i.e. short term exposure to very high quantities) is also associated with a higher risk of death due to cardio-respiratory diseases. ^[1]

Nitrogen dioxide, NO₂

Direct Health Impacts: Lung
Sensitive populations: Children

Nitrogen dioxide (NO₂) is a gas formed mainly by combustion processes in power plants, heating furnaces, and in the engines of ships and vehicles. It has a reddish-brown appearance, which you can often see in car exhaust fumes.

In most urban locations, the nitrogen oxides that yield NO₂ are emitted primarily by motor vehicles, making it a strong indicator of vehicle emissions (including other unmeasured pollutants emitted by these sources). Nitrogen dioxide emissions contribute to fine particulates, as NO₂reacts with other chemicals in the air to form nitrate particles. It also reacts with volatile organic compounds under exposure to sunlight to form Ozone (O₃), which is a major component of photochemical smog.

The photochemical reaction sequence initiated by solar-radiation-induced activation of NO₂ can contribute significantly to total PM₁₀ or PM_2.5 mass. For these reasons, NO₂ is a key precursor for a range of secondary pollutants whose effects on human health are well documented. In other words, the measurement of NO₂ continues to be critical because it a reliable surrogate for other harmful emissions, whose effects have been more thoroughly documented, specifically, particulates.

Exposure to low levels of NO₂ can cause irritation of the eyes, nose, throat, lungs, possibly leading to cough and shortness of breath, tiredness, and nausea. It can result in fluid build-up in the lungs 1 or 2 days after exposure. Exposure to high levels of NO₂ can cause rapid burning, spasms, and swelling of tissues in the throat and upper respiratory tract, reduced oxygen supply to body tissues, build-up of fluid in the lungs, and death. At short-term concentrations of more than 200 micrograms per cubic metre, it is a toxic gas which causes significant inflammation of the airways. Recent studies of adolescents in the United States found a strong association with reduced lung growth and function in those exposed to NO₂at levels much lower than those experienced in Hong Kong. ^[1]

Sulphur dioxide, SO₂

Direct Health Impacts: Heart & Lung
Sensitive populations: Children, the elderly

SO₂ is a colourless gas with a sharp odour. It is produced from the burning of fossil fuels (coal and oil) and the smelting of mineral ores that contain sulphur. The main man-made source of SO₂ is the burning of sulphur-containing fossil fuels for domestic heating, power generation and motor vehicles.

SO₂ can inflame the respiratory system, affect the functioning of the lungs, and causes irritation of the eyes. Inflammation of the respiratory tract causes coughing, mucus secretion, aggravation of asthma and chronic bronchitis and makes people more prone to respiratory tract infections. Hospital admissions for cardiac disease and mortality from all causes increase on days with higher SO₂ levels.

In July 1990 the Hong Kong government reduced the amount of sulphur allowed in fuel to 0.5% by weight. Although this was quite a modest restriction, it had a dramatic effect on both air quality and public health. Sulphur dioxide levels fell by up to 80% in the most polluted districts. There was a marked reduction in children’s respiratory illnesses (cough, phlegm and wheezing) and also a reduction in deaths from heart and lung disease in people aged 46 and older. Unfortunately SO₂ emissions generally have increased across the Pearl River Delta and neighbouring areas since then. Emissions from ships using heavy residual oil, and also power stations, have been a major problem. ^[1]

Ozone, O₃

High up in the atmosphere, the ozone layer helps protect the earth from harmful ultraviolet rays. However, at ground level, ozone is a harmful gas and is one of the main components of photochemical smog. It is formed by chemical reaction under exposure to sunlight between other pollutants such as nitrogen oxides (NOx) from vehicle and industry emissions, and volatile organic compounds (VOCs) from vehicles, solvents and industry. The highest levels of ozone pollution occur during periods of sunny weather.

Excessive ozone in the air can have a marked effect on human health. It can cause significant irritation of the eyes, nasal passages and lungs with breathing problems, asthmatic attacks, reduced lung function and other lung diseases ^[1]

Ozone has not been studied as much as PM. Nevertheless, the WHO 2003 report, Health Aspects of Air Pollution with Particulate Matter, Ozone and Nitrogen Dioxide^[2], acknowledged the evidence that ozone produces short-term effects on mortality and respiratory morbidity, even at the low ozone concentrations experienced in many cities in Europe. As a result, the WHO revised its Air Quality Guideline for ozone.

Most recently, the New England Journal of Medicine published an 18-year study of respiratory morbidity resulting from low-level ozone exposure. The study “demonstrate[d] a significant increase in the risk of death from respiratory causes in association with an increase in ozone concentration.”^[3]

Volatile Organic Compounds & Smog

Volatile organic compounds (VOCs) are found in a wide variety of products such as solvent-based paints, printing inks, many consumer products, organic solvents and petroleum products. In addition to motor vehicles, the use of these VOC-containing products releases VOCs that eventually cause air pollution and smog.

VOCs play a significant role in the formation of ozone and fine particulates in the atmosphere. Under sunlight, VOCs react with nitrogen oxides emitted mainly from vehicles, power plants and industrial activities to form ozone, which in turn helps the forming of fine particulates. The accumulation of ozone, fine particulates and other gaseous pollutants results in smog that reduces visibility. The economic growth in the Pearl River Delta comes with increased formation of these pollutants, thus aggravating the regional smog phenomenon. Smog is particularly severe under stagnant weather conditions, e.g. with typhoon approaching, or with prevailing northerly winds that are common in autumn.

Smog can irritate our eyes, nose and throat, or can worsen existing heart and respiratory problems such as asthma. People with heart and lung problems, the elderly and children whose respiratory systems are still developing are most at risk. Healthy adults of all ages who exercise or work vigorously outdoor are more sensitive to the pollution because of their higher level of exposure than people who are less active outdoor. Prolonged exposure to severe smog condition may cause permanent damage to lung tissue and affect our immune system. ^[4]

Carbon Monoxide, CO

Inhaled O₃ enters the blood stream and binds preferentially to haemoglobin (the substance that carries oxygen to the cells), thereby replacing oxygen. It results in reducing the amount of oxygen which is delivered to body organs and tissues. The percentage of haemoglobin inactivated by CO depends on the amount of air breathed in, the concentration of CO in air and the duration of exposure. In a healthy person, elevated CO concentration exposure is associated with visual impairment, reduced work capacity, reduced mental function and poor learning ability. Persons suffering from heart and circulatory problems, fetuses, young infants, pregnant women and elderly people are likely to be more susceptible to the effects of CO. ^[5]

Lead, Pb

Lead is a toxic heavy metal which can be found in total or respirable suspended particulates. Lead particles from vehicular exhausts which are of a size smaller than 10 micrometres can affect both adults and children, but small children (and foetuses) are at greatest risk because of their smaller size, breathing patterns and the metabolism of lead in their bodies. The effects of lead exposure include damage to the nervous system, red blood cells, kidneys and potential increases in high blood pressure. Other health effects may result in decreased co-ordination and mental abilities. The effects of lead exposure can be treated and reversed, providing treatment is timely and lasts the entire course of therapy. If no treatment is given, permanent brain damage can result.^[6]

Proportion of sources

Notes

1. ^ Hedley Environmental Index FAQ (April 2009)
2. ^ Health Aspects of Air Pollution with Particulate Matter, Ozone and Nitrogen Dioxide, World Health Organisation, 2003.
3. ^ Long-Term Ozone Exposure and Mortality, Michael Jerrett, Ph.D., Richard T. Burnett, Ph.D., C. Arden Pope, III, Ph.D., Kazuhiko Ito, Ph.D., George Thurston, Sc.D., Daniel Krewski, Ph.D., Yuanli Shi, M.D., Eugenia Calle, Ph.D., and Michael Thun, M.D., The New England Journal of Medicine, 2009.
4. ^ Volatile Organic Compounds and Smog, HKSAR Government Environment Protection Department website (April 2009)
5. ^ Health Effects of Carbon Monoxide, HKSAR Government Environment Protection Department website (April 2009)
6. ^ Health Effects of Lead, HKSAR Government Environment Protection Department website (April 2009)