Air Pollution Measurement: Understanding The Units

Air pollution is a critical issue that poses a significant threat to human health and the environment. With rising emissions contributing to climate change, biodiversity loss, and pollution, it is essential to measure and understand air pollution levels to implement effective solutions. Various methods and technologies are employed to quantify air pollution, providing data that drives policy decisions and protective actions. This data is often collected by governments, local councils, research bodies, and environmental groups, who use it to assess the impact of air pollution on specific areas and populations. One of the most widely used tools for measuring air pollution is the Air Quality Index (AQI), which assigns numerical values to indicate the level of air pollution and the associated health risks. Other methods include the use of personal pollution sensors, stationary measuring devices, and satellite-driven technologies, each offering unique advantages and limitations in the ongoing battle against air pollution.

Air Quality Index (AQI)

The Air Quality Index (AQI) is a tool used to communicate information about outdoor air quality and health. It was developed by the Environmental Protection Agency (EPA) to provide information about the health effects of the five most common air pollutants and how to avoid them. The AQI includes six colour-coded categories, each corresponding to a range of index values. The higher the AQI value, the greater the level of air pollution and the associated health concerns. For example, an AQI value of 50 or below represents good air quality, while a value over 300 indicates hazardous air quality. Each category is assigned a specific colour to enable people to quickly determine whether the air quality is reaching unhealthy levels in their communities.

The AQI for an individual location is the highest of the air quality index values for each pollutant being monitored at that location. The calculation of the AQI requires an air pollutant concentration over a specified averaging period, obtained from an air monitor or model. Concentration and time together represent the dose of the air pollutant. The health effects associated with a given dose are determined by epidemiological research. Air pollutants vary in potency, and the function used to convert from air pollutant concentration to AQI varies by pollutant.

The AQI was designed to be a simple and accessible way for the public to judge the air quality in their vicinity. The six categories of air quality are: Good, Satisfactory, Moderately Polluted, Poor, Very Poor, and Severe. Each category corresponds to a different level of health concern. When AQI values exceed 100, air quality is considered unhealthy, initially for certain sensitive groups of people, and then for everyone as values increase. The Air Quality Health Index (AQHI) is a scale designed to help people understand the impact of air quality on their health and protect themselves from air pollution. It provides a number from 1 to 10+ to indicate the level of health risk associated with local air quality.

The definition of the AQI in a particular nation reflects the discourse surrounding the development of national air quality standards in that country. Each state and territory in Australia, for example, is responsible for monitoring air quality and publishing data according to the National Environment Protection (Ambient Air Quality) Measure (NEPM) standards. AQI values are influenced by various factors, including human-caused emissions, such as fossil fuel use, and natural sources, like dust storms and smoke from wildfires.

Particulate matter (PM2.5, PM10, PM1)

Particulate matter, or PM, refers to a mixture of solid particles and liquid droplets found in the air. Some particulate matter is large or dark enough to be seen with the naked eye, like dust, dirt, soot, or smoke. Other particulate matter is so small that it can only be detected using an electron microscope.

PM2.5 refers to particulate matter that is 2.5 micrometers or smaller in diameter. To put this in context, a human hair is about 70 micrometers in diameter, making it 30 times larger than the largest fine particle. PM2.5 is emitted directly from sources such as construction sites, unpaved roads, fields, smokestacks, and fires. It also forms in the atmosphere as a result of complex reactions between chemicals such as sulfur dioxide and nitrogen oxides, which are pollutants emitted from power plants, industries, and automobiles.

PM2.5 is the particulate matter that poses the greatest risk to health. It is small enough to be inhaled deeply into the lungs and may even enter the bloodstream. Short-term exposure to PM2.5 has been linked to premature mortality, increased hospital admissions for heart or lung issues, acute and chronic bronchitis, asthma attacks, emergency room visits, respiratory symptoms, and restricted activity days. Long-term exposure to PM2.5 has been associated with reduced lung function growth in children and premature death, particularly in those with chronic heart or lung diseases.

PM10 refers to particulate matter with a diameter between 2.5 and 10 micrometers. PM10 is more likely to deposit on the surfaces of the larger airways of the upper region of the lung. Short-term exposure to PM10 has been associated with the worsening of respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD), leading to hospitalisation and emergency department visits. The effects of long-term exposure to PM10 are less clear, although studies suggest a link between long-term exposure and respiratory mortality.

PM1 is particulate matter with a diameter of 1 micrometer or less.

Nitrogen dioxide

NO₂ is a criteria air pollutant tracked by the World Health Organization (WHO) and most regional air quality management agencies due to its effects on human and environmental health. The WHO sets recommended maximum levels for NO₂, at 25 μg/m3 for each 24-hour mean and 10 μg/m3 for the annual mean level. However, these levels are only guidelines rather than regulated standards. Short-term exposure to concentrations of NO₂ higher than 200 μg/m3 can cause inflammation of the airways and may increase susceptibility to respiratory infections. NO₂ can also exacerbate symptoms in individuals with pre-existing lung or heart conditions.

In the United States, NO₂ is regulated as a criteria pollutant under the National Ambient Air Quality Standards (NAAQS) set by the Clean Air Act. The NAAQS establishes both primary and secondary standards, with 1-hour averages of 100 ppb as the primary standard and an annual mean of 53 ppb as the primary and secondary standard for NO₂. These standards aim to protect human health and the environment, using NO₂ as an indicator of general nitrogen oxide pollution.

Since NO₂ is a product of fuel combustion, it can be measured to evaluate the polluting effects of energy production at power plants quantitatively. Measuring nitrogen dioxide emissions and the associated public health benefits of their reduction can help build a case for actions that improve environmental health and mitigate climate change, such as transitioning from fossil fuels to clean energy sources.

NO₂ concentration varies significantly worldwide, with some regions experiencing steep declines in recent years while others continue to struggle with extremely high levels, significantly impacting air quality and public health. Air quality monitoring is particularly sparse in Africa, Central Asia, and Latin America, despite their dense populations, which may result in a disproportionate impact from air pollution. Governments must implement legislation mandating monitoring and invest in infrastructure to enhance data reliability.

Ozone

Ground-level ozone is formed when volatile organic compounds (VOCs) react with nitrogen oxide emissions in the presence of sunlight. This typically occurs when pollutants from cars, power plants, industrial boilers, refineries, and other sources react with one another. Ground-level ozone can be transported by wind, affecting both urban and rural areas. It is particularly likely to reach unhealthy levels on hot, sunny days, but it can also remain high during colder months.

To protect public health, organisations like the Environmental Protection Agency (EPA) and local clean air agencies monitor ozone levels. Ozone is measured using ozone analyzers that utilise ultraviolet (UV) light to determine the concentration of ozone in the air. The EPA has established National Ambient Air Quality Standards (NAAQS) for ozone and other criteria air pollutants, aiming to limit their levels in outdoor air based on health criteria. These standards help guide efforts to improve air quality in areas that do not meet the national standards, known as "nonattainment areas".

Carbon monoxide

The presence of carbon monoxide is typically measured in parts per million (ppm) or milligrams per cubic meter (mg/m3). For example, a study in Chile found that for a 1.3-mg/m3 increase in carbon monoxide concentration, the relative risk for migraine associated with interquartile-range increases was 1.11. Additionally, the emission rates of 23 different types of incense were measured, ranging from 144 to 531 mg/hour.

Standards and regulations are in place to control CO pollution and maintain safe levels. For instance, in the United States, the Environmental Protection Agency (EPA) sets and reviews standards for CO in outdoor air under the Clean Air Act. These standards assist state, tribal, and local agencies in ensuring that CO levels remain within acceptable ranges. Similar guidelines for indoor air quality have been established by the World Health Organization (WHO), providing recommendations to reduce exposure to CO and other pollutants.

Frequently asked questions