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Air Quality Monitoring

Credit: iStock.

A number of pollutants found in the air we breathe are known to have harmful effects on human health and the environment. The health implications are serious, with outdoor air pollution in both cities and rural areas causing an estimated 4.2 million premature deaths worldwide in 2016. For these reasons, monitoring air quality is vital for human health and the environment, and this importance is reflected by the regulation and legislation surrounding air quality. In this list, we look at the different pollutants that affect air quality, how they are produced and how they can be monitored and measured.

What is air pollution?

Air pollution generally refers to any airborne substance that can cause harm to human health. Human health implications of air pollution tend to affect the cardiovascular and respiratory systems. Short-term health implications include inflammation of the airways and lungs, worsening of heart and lung diseases and reduction in oxygen uptake by red blood cells. Long-term effects include cardiovascular and respiratory diseases, cancers and mortality. More recently, the term air pollution may also refer to airborne substances that can cause harm to the environment.

Historically, air pollution causes have predominantly been a result of rapid industrialization, where the combustion of fossil fuels has resulted in high levels of smoke and sulfur dioxide entering the atmosphere. Presently, the main cause of air pollution is traffic emissions, where petrol and diesel combustion engine vehicles emit carbon monoxides, nitrogen oxides, particulate matter and volatile organic compounds into the atmosphere. These pollutants do not necessarily cause air quality issues in the vicinity of the pollution source but can impact air quality over long distances. In 2016, 91% of the world population was living in places where the World Health Organization (WHO) air quality guideline levels were not met.

Types of air pollution

The air pollutants that must be monitored are summarized in the table below:

Nitrogen Oxides
NOx (NO/NO2)
Sulfur Dioxide
Carbon Monoxide
Particulate Matter
PM10 and PM2.5

Nitrous oxides (NOx ):

Nitrous oxides are produced by combustion processes. Road transport is the main source, followed by electricity generation. Nitric oxide (NO) is the primary product, which quickly oxidizes in the atmosphere to produce nitrogen dioxide (NO2). Nitrogen dioxide has a variety of health impacts, including causing inflammation of airways and an increased response to allergens in sensitive individuals. High levels of nitrogen oxides also damage the environment by reducing vegetation growth and causing acidification and/or eutrophication leading to biodiversity loss. Nitrogen dioxide can also generate further air pollutants – it reacts with hydrocarbons in the presence of sunlight to produce photochemical pollutants such as ozone and can be further oxidized in air to acidic gases, which contribute to acid rain.

Sulphur dioxide (SO2 ):

Sulphur dioxide is produced by combustion of sulfur-containing fuels, including coal and heavy oils, mainly by power stations and refineries. Sulphur dioxide causes constriction of the airways in the lungs, particularly in those suffering from asthma and chronic lung disease. Environmental effects include degradation of chlorophyll in plants leading to vegetation loss and acidification of soils and waters leading to biodiversity loss.

Ozone (O3 ):

Ozone arises from the chemical reaction between other air pollutants, notably nitrogen dioxide and volatile organic compounds in the presence of strong sunlight. Exposure to high concentrations causes irritation to the eyes and nose and damage to airways. Ozone at ground level can damage plants and lead to biodiversity and vegetation loss.

Carbon monoxide (CO):

This is formed by the incomplete combustion of carbon fuels, most notably from road transport, industry and homes. Carbon monoxide reduces oxygen uptake by red blood cells, reducing the delivery of oxygen to organs around the body. This is particularly harmful to those with existing diseases which affect the heart or brain.

Particulate matter:

Particulate matter is comprised of a wide range of materials from various sources, both homogenic and natural. The biggest sources of particulate matter pollution include road transport, quarrying and construction. PM10 relates to particles of less than 10 micrometers in diameter, while PM2.5 relates to matter of less than 2.5 micrometers in diameter. Short-term and long-term exposure to particulate matter is associated with respiratory and cardiovascular illness and diseases.


Other air pollutants include: benzene, from combustion of carbon fuels; polycyclic aromatic hydrocarbons (PAHs), from domestic coal and wood-burning fires and industrial processes; 1,3-butadiene, from petrol combustion; lead, from the combustion of coal, iron and steel; ammonia, from agriculture; hydrogen sulfide, from power plants and oil refineries; and toxic air contaminants (such as heavy metals, asbestos and formaldehyde), from burning coal and metals.

Measuring air pollution

Concern over the effects of modern-day pollution on air quality has led to the introduction of recommended limits to air pollutants by the World Health Organisation (WHO). Many regions and countries also have their own set of guidelines for pollutant limits and recommended testing procedures, including techniques such as UV-visible spectroscopy. In the UK, levels are governed by the Department for Environment, Food and Rural Affairs (DEFRA) and in the US, levels are governed by the United States Environmental Protection Agency (EPA).

Air quality monitoring and sampling depends on the purpose of the testing and the pollutants to be monitored. The table below lists major pollutants and their methods of monitoring:

Monitoring Method
 Nitrogen Oxides
• Chemiluminescence 

• Diffusion Tubes
 Sulphur Dioxide
• Ultra-violet Fluorescence
• UV Absorption
 Carbon Monoxide
• Infrared Absorption
Particulate Matter
• TEOM (Tapered Element Oscillating Microbalance)