Air Pollution Metrics: Understanding The Units

Air pollution is measured using an air quality index (AQI), which is an indicator developed by government agencies to communicate to the public how polluted the air currently is or how polluted it is forecast to become. The AQI is calculated based on the concentrations of various pollutants, such as ozone, nitrogen dioxide, sulfur dioxide, PM2.5, and PM10. PM2.5 refers to particulate matter measuring 2.5 microns or less in diameter, while PM10 includes particles less than 10 microns in size. These particles can be solid or liquid and are composed of a complex mixture of acids, ammonium, water, carbon, metals, and other substances. High levels of PM2.5 can have significant health impacts, and understanding its sources is crucial for formulating control policies. Different countries have their own AQI standards, and the specific calculations and bands may vary, but the overall principle is that as air pollution levels rise, the AQI increases, indicating a higher associated public health risk.

Air Quality Index (AQI)

The Air Quality Index (AQI) is a tool used by government agencies to communicate to the public about the current level of air pollution and how it is forecast to change. As air pollution levels rise, so does the AQI, along with the associated public health risks. The AQI is particularly important for children, the elderly, and individuals with respiratory or cardiovascular problems, who are typically the first groups affected by poor air quality.

The AQI is calculated from the concentrations of certain pollutants, including ozone, nitrogen dioxide, sulfur dioxide, PM2.5, and PM10. PM2.5 refers to fine particulate matter, which is widely regarded as one of the most harmful pollutants to human health. It is a microscopic particle, 2.5 microns in width, and almost 30 times smaller than the diameter of a human hair. When levels of PM2.5 are high, these particles form a haze in the sky, making their way into people's respiratory tracts and reaching the lungs.

The United States Environmental Protection Agency (EPA) has developed an AQI that is divided into six color-coded categories, each corresponding to a range of index values. An AQI value of 50 or below represents good air quality, while a value over 300 represents hazardous air quality. Each category corresponds to a different level of health concern, with colors making it easy for people to quickly determine whether the air quality is reaching unhealthy levels in their communities.

The Daily Air Quality Index, recommended by the Committee on the Medical Effects of Air Pollutants (COMEAP), is the most commonly used AQI in the UK. This index has ten points, which are further grouped into four bands: low, moderate, high, and very high. Each band comes with advice for at-risk groups and the general population.

Particulate matter (PM)

PM is classified based on the diameter of the particles. PM10 refers to particles with a diameter of 10 micrometers or less, which can be inhaled into the lungs and induce adverse health effects. PM2.5 refers to fine particulate matter, which are particles with diameters of 2.5 micrometers or less. These particles are so small that about 30 of them would equal the width of a human hair. They can get deep into the lungs and may even enter the bloodstream. PM2.5 is the main cause of reduced visibility (haze) in parts of the United States, including national parks and wilderness areas.

PM2.5 is often responsible for causing air pollution-related health issues. Long-term exposure has been linked to premature death, particularly in people with chronic heart or lung diseases, and reduced lung function growth in children. It can come from various sources, including construction sites, unpaved roads, fields, smokestacks, and fires. Most particles form in the atmosphere as a result of complex reactions of chemicals such as sulfur dioxide and nitrogen oxides, which are pollutants emitted from power plants, industries, and cars.

Measuring the concentration of PM2.5 and understanding its sources is crucial for formulating policies to control its presence in the air we breathe. Global satellite data and ground-level monitoring are used to measure PM2.5 levels, but more research and data are needed. The United States Environmental Protection Agency (EPA) has developed an Air Quality Index (AQI) that is used to report air quality and protect people's health when PM reaches harmful levels.

Primary and secondary PM2.5

Air pollution is typically measured using the Air Quality Index (AQI), which is an indicator developed by government agencies to communicate the level of air pollution to the public. The AQI is calculated based on the concentrations of various pollutants, including PM2.5, and as air pollution levels rise, the AQI increases along with the associated public health risks.

PM2.5 refers to particulate matter with a diameter of 2.5 microns or less, making them approximately 30 times smaller than the width of a human hair. These particles are a mixture of solids, liquid droplets, and aerosols, and they can contain various compounds, including inorganic ions, metallic compounds, and organic matter. Due to their small size, PM2.5 particles can be inhaled and cause adverse health effects, especially in individuals with pre-existing heart or lung conditions.

Primary PM2.5 particles are released directly from sources such as dust storms, forest fires, volcanoes, transportation, fuel burning, and industrial processes. These particles enter the atmosphere and can have direct health impacts, as mentioned earlier.

Secondary PM2.5 particles, on the other hand, are formed through chemical reactions between primary pollutants. For example, ozone (O3) is a secondary pollutant created when hydrocarbons (HC) and nitrogen oxides (NOx) react in the presence of sunlight. Another secondary pollutant is secondary organic carbon (SOC), which is produced through the photochemical reactions of gaseous precursors from primary organic carbon (POC).

The distinction between primary and secondary PM2.5 particles is important because it helps us understand the sources and mechanisms of air pollution. By identifying the primary sources and the chemical reactions that form secondary particles, scientists and policymakers can develop strategies to reduce air pollution and mitigate its health impacts.

Sources of PM2.5

The air quality index (AQI) is an indicator developed by government agencies to communicate the level of air pollution to the public. As air pollution levels rise, so does the AQI, along with the associated public health risks. The AQI is calculated based on the concentrations of pollutants like ozone, nitrogen dioxide, sulfur dioxide, PM2.5, and PM10. PM2.5, or fine particulate matter, is a microscopic particle that is often responsible for causing air pollution-related health issues. It is 2.5 microns in width, almost 30 times smaller than the diameter of a human hair. When levels of PM2.5 are high, these particles form a haze in the sky and can make their way into people's respiratory tracts, reaching the lungs.

PM2.5 is primarily composed of inorganic ions, carbonaceous compounds (including black carbon, organic carbon, and secondary organic aerosol), and mineral dust. Sources of PM2.5 include direct emissions such as:

• Forest fires and agricultural waste burning
• Windblown mineral dust from arid regions
• Inefficient fuel combustion
• Residential energy use
• On- and off-road vehicles
• Energy generation
• Solvent use
• Industrial processes
• Agricultural fertiliser application

Most particles form in the atmosphere as a result of complex reactions of chemicals such as sulfur dioxide and nitrogen oxides, which are pollutants emitted from power plants, industries, and automobiles. Coal-burning power plants, vehicle emissions, and industrial sources are major contributors to PM2.5 pollution.

It is important to measure the concentration and understand the sources of PM2.5 to formulate policies to control its presence in the air. While satellite data has advanced the measurement of PM2.5 levels, ground-level monitoring is still considered essential, requiring sophisticated equipment and sustained funding.

Health risks of air pollution

Air pollution is measured using the Air Quality Index (AQI), an indicator developed by government agencies to communicate the current and forecasted levels of air pollution to the public. As air pollution levels rise, so does the AQI, along with the associated public health risks. The AQI is calculated based on the concentrations of pollutants such as ozone, nitrogen dioxide, sulfur dioxide, PM2.5, and PM10. PM2.5 refers to particulate matter measuring 2.5 microns or less in diameter, which can cause air pollution-related health issues.

Air pollution exposure is associated with a range of health risks and adverse outcomes, affecting almost every organ in the body. The main pathway of exposure is through the respiratory tract, where pollutants can cause inflammation, oxidative stress, immunosuppression, and mutagenicity in cells. These effects can impact the lungs, heart, and brain, among other organs, leading to various diseases and increased mortality risks.

Short-term exposure to higher levels of outdoor air pollution is linked to reduced lung function, respiratory infections, aggravated asthma, cardiac problems, and increased hospital admissions. Long-term exposure to fine particulate matter, such as PM2.5, increases the risk of non-communicable diseases with a longer onset, including stroke, heart disease, lung cancer, chronic obstructive pulmonary disease (COPD), and other types of cancer.

Children are particularly vulnerable to the health effects of air pollution. Higher pollution levels are associated with an increased risk of short-term respiratory infections, which can lead to more school absences. Children living in urban areas, near busy roads, or in communities with high ozone levels are more likely to develop asthma and bronchitis symptoms in adulthood. Air pollution exposure during pregnancy can also impact fetal health, increasing the risk of pre-term birth, low birth weight, and maternal and fetal illness.

Other health risks associated with air pollution include adverse pregnancy outcomes, diabetes, cognitive impairment, and neurological diseases. Additionally, psychosocial stress, such as poverty and racial/ethnic discrimination, can amplify the harmful effects of air pollution. People of color are more likely to be exposed to air pollution and suffer more significant health consequences due to systemic racism and the higher prevalence of chronic conditions in these communities.

Frequently asked questions