Air Quality Measurement: Monitoring Our Air Pollution Levels

Air pollution is a critical issue that poses a significant threat to human health and the environment. With poor air quality contributing to climate change, biodiversity loss, pollution, and waste, it is essential to address this crisis through effective monitoring and regulation. Measuring air pollution is a complex task that involves various tools and methods to quantify and assess the level of pollutants in the atmosphere. This data is crucial for governments, organizations, and communities to make informed decisions and take targeted actions to improve air quality and protect public health.

Air Quality Index (AQI)

The Air Quality Index (AQI) is a tool developed by the US Environmental Protection Agency (EPA) to communicate information about outdoor air quality and health. The AQI is a numerical system that measures the level of air pollution in a given region. The higher the number, the worse the air quality. The index is split into six different categories, each colour-coded and corresponding to a range of index values. An AQI value of 50 or below represents good air quality, while a value over 300 indicates hazardous air quality. Values at or below 100 are generally considered satisfactory, with higher values indicating an increased level of unhealthiness, initially for certain sensitive groups of people, and then for everyone as the values continue to rise.

The AQI is used to report on the five major air pollutants regulated by the Clean Air Act: PM2.5, PM10, ground-level ozone, nitrogen dioxide, and sulfur dioxide. These pollutants are tied to human and environmental health impacts. For instance, PM2.5, which refers to particulate matter with a diameter of 2.5 micrometres or less, is linked to illnesses such as stroke, heart disease, lung disease, and cancer when inhaled.

Air quality monitors are equipped with sensors designed to detect these specific pollutants. Some monitors use lasers to scan particulate matter density in a cubic metre of air, while others rely on satellite imaging to measure energy reflected or emitted by the Earth. This data is crucial for governments to identify air pollution hotspots and take targeted action to protect and improve the well-being of their citizens and the environment.

AirNow.gov is a website that provides air quality data for local areas, as well as at the state, national, and global levels. It offers interactive maps, calculators, and activity guides to help users understand and respond to air quality levels.

Passive and active measurement

Air pollution is broadly measured in two ways: passively and actively. Passive devices are relatively simple and inexpensive. They collect ambient air samples, which are then analysed in a laboratory. Diffusion tubes, fastened to lamp posts to absorb specific pollutant gases, are one of the most common forms of passive measurement. Deposit gauges, which are large funnels that collect and drain particulates into sampling bottles, are another example of passive devices. Passive samplers can detect long-term averages to very low levels, depending on the environment. They are also the most cost-effective approach for estimating spatial gradients, surveying additional pollutants, identifying hotspots, and estimating individual exposure.

Active measurement devices are automated or semi-automated and tend to be more complex and sophisticated than passive devices. They use fans to suck in the air, filter it, and either analyse it automatically or collect and store it for later analysis in a laboratory. Active sensors use either physical or chemical methods. Physical methods measure an air sample without changing it, for example, by seeing how much of a certain wavelength of light it absorbs. Chemical methods change the sample through a chemical reaction and then measure that. Most automated air-quality sensors are examples of active measurement. Active sensors range from small handheld devices to large-scale static monitoring stations in urban areas.

Personal pollution sensors

Personal air pollution sensors are a niche class of devices designed to be carried on a keychain or necklace to measure the air quality wherever you go. They are also known as "wearables" and can be used to monitor indoor and outdoor air quality.

The market for personal air pollution sensors is still emerging, with start-up companies rushing to produce cheap air-monitoring sensors. These devices can cost a few hundred pounds, in contrast to the tens of thousands that larger-scale air quality monitors can cost. The data from these personal sensors can be used to inform personal decisions, such as whether to take medication for asthma, or to inform wider decisions, such as whether to close roads or ban cars in certain areas.

However, it is important to note that not all sensors are accurate or durable. Some cheaper devices are advertised as being for raising awareness of pollution, rather than giving accurate readings. It is recommended that sensors that claim to measure ambient pollution levels should undergo independent testing to validate their accuracy.

Despite this, personal air pollution sensors are becoming a valuable tool in health research, as they can show people's exposure to environmental factors such as noise and particulate matter. They can also be used to promote citizen awareness and behavioural changes regarding indoor and outdoor air pollution. For example, a study by Park, Chavez, Sousan, and others in 2023 used GPS-enabled portable air pollution sensors to provide citizens with insights about air pollutants during their daily commutes. This information can then be used to inform decisions about travel behaviour and route choices.

Satellite technology

Satellite images can identify large areas of pollution caused by fires, dust or sand storms, volcanic eruptions, large industrial sources, or the transport of man-made pollution from other regions. For example, the MODIS instrument records the location of fires on the ground by observing unusual hot spots. It can also be used to identify potential sources of pollution in the satellite image, although it is not always possible to identify the type of pollution, as smoke haze and dust can look alike.

Satellite data can be used to indicate criteria air pollutants (e.g. PM2.5 and NO2) and greenhouse gases (e.g. CH4 and CO2). Remote sensing instruments can be deployed on satellites, at ground level, and in aircraft. Remote sensing measurements are consistent across time and space, whereas ground monitoring protocols and instruments change over time and are not harmonized across countries.

The Tropospheric Emissions: Monitoring of Pollution (TEMPO) satellite instrument will hover 36,000 kilometers above North America, orbiting in sync with Earth’s rotation. It will measure air pollutants in the atmosphere, including ozone, nitrogen dioxide, formaldehyde, and fine particulate matter (PM2.5) every hour across greater North America down to a neighbourhood scale. It will not, however, directly measure PM2.5, one of the most harmful air pollutants.

Ground measurements are still needed for ground truthing, and sustained collaboration over time remains a challenge.

Health impacts

Air pollution is a mix of hazardous substances from both human-made and natural sources. It is a major threat to global health and prosperity, causing more than 6.5 million deaths each year worldwide. According to the World Health Organization, 99% of the global population breathes unclean air, contributing to 7 million premature deaths annually.

The main pathway of exposure from air pollution is through the respiratory tract. Pollutants enter the body through inhalation, leading to inflammation, oxidative stress, immunosuppression, and mutagenicity in cells throughout the body, impacting the lungs, heart, and brain, among other organs. Fine particulate matter, such as PM2.5, is of particular concern as it can penetrate deep into the lungs, enter the bloodstream, and travel to various organs, causing systemic damage to tissues and cells. This can result in reduced lung function, respiratory infections, aggravated asthma, and an increased risk of chronic diseases, including stroke, heart disease, chronic obstructive pulmonary disease, and cancer.

Maternal exposure to air pollution is associated with adverse birth outcomes, such as low birth weight, pre-term birth, and small gestational age births. Research also suggests a link between air pollution and diabetes, neurological development in children, and reproductive, neurological, and immune system disorders.

Ozone, a powerful lung irritant, is another significant pollutant. When inhaled, it reacts with the delicate lining of the small airways, causing inflammation and damage that can impact multiple body systems. High ozone levels can lead to breathing problems, including chest tightness, coughing, and shortness of breath, even in healthy young adults.

Additionally, psychosocial stress, such as poverty, racial/ethnic discrimination, and residency status, can amplify the harmful effects of air pollution. Disadvantaged communities of color often face disproportionate exposure to air pollution due to the placement of power plants, industrial facilities, and highways in their neighborhoods.

Frequently asked questions