Meghan Hodges
Particulate matter (PM) is a serious environmental pollutant that can cause a variety of adverse health effects. PM is made up of a mixture of solid particles and liquid droplets found in the air. These particles can be large or dark enough to be seen, such as dust, dirt, soot, or smoke, or they can be so small that they can only be detected using an electron microscope. PM2.5, fine inhalable particles with diameters of 2.5 micrometers and smaller, pose the greatest risk to health. They can get deep into the lungs and even into the bloodstream, causing serious health problems. Measuring PM concentration and understanding its sources are key to formulating policies to control its presence in the air and protect human health. There are various methods for measuring PM, including federal reference methods (FRMs), federal equivalent methods (FEMs), and low-cost sensors. While FRMs are considered the gold standard, FEMs like Beta Attenuation Monitors (BAMs) offer a more cost-effective and less labor-intensive alternative. Low-cost sensors, such as PurpleAir sensors, use laser counters or satellite imaging to detect particles and extrapolate PM values.
| Characteristics | Values |
|---|---|
| Particulate Matter (PM) Definition | Mixture of solid particles and liquid droplets found in the air |
| PM Health Impact | Linked to a variety of adverse health effects. Particles can be inhaled and cause serious health problems. |
| PM Size | Particles less than 10 micrometers in diameter can enter the lungs and may enter the bloodstream. Particles less than 2.5 micrometers in diameter (PM2.5) pose the greatest risk to health. |
| PM2.5 Health Impact | Can cause air pollution-related health issues, including impacts on lung and heart diseases. |
| PM Measurement Methods | Federal Reference Methods (FRMs), Federal Equivalent Methods (FEMs), Low-Cost Sensors, Laser Scanning, Satellite Imaging |
| Air Quality Index (AQI) | Scale from 0 to 500, with 50 or below considered safe, and above 100 deemed unhealthy |
| Global Monitoring Efforts | UNEP has supported the deployment of low-cost sensors in multiple countries, and aims to provide technical support to over 50 countries |
| Local Monitoring Efforts | Rules in California, Washington, and Oregon require employers with outdoor workers to monitor air quality and protect employees from wildfire smoke exposure |
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What You'll Learn
Federal Reference Methods (FRMs)
FRMs are highly accurate and reliable methods for measuring PM pollution. They are used to measure the concentration and particle size distribution of particulate matter in the air. FRMs can detect particles that are so small that they can only be seen using an electron microscope. These particles can be solid or liquid droplets and are often formed in the atmosphere due to complex reactions of chemicals, such as sulfur dioxide and nitrogen oxides, emitted from power plants, industries, and automobiles.
FRMs are also used to measure PM10 and PM2.5, which refer to particulate matter smaller than 10 micrometers and 2.5 micrometers, respectively. PM2.5 is especially harmful to human health and is linked to numerous adverse health effects. These fine particles can get deep into the lungs and even enter the bloodstream.
The EPA develops and evaluates FRMs for their air monitoring network, fostering innovation and advancing new technologies. The agency also reviews, tests, and approves other methods, known as Federal Equivalent Methods (FEMs), which are based on different sampling approaches. FEMs are similar to FRMs but are not standardised, allowing for various methods and hardware to be utilised. While FEMs may be less expensive and labour-intensive than FRMs, they still require significant resources and are not easily accessible to individuals.
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Federal Equivalent Methods (FEMs)
FEMs are not standardised, and many different methods and pieces of hardware can be used. One commonly used FEM is a Beta Attenuation Monitor (BAM), which uses beta ray attenuation to measure the mass concentration of PM in ambient air. This involves sending beta particles through a piece of filter tape that collects particulate matter, and measuring the difference in the degree of attenuation between the pre-and-post-sampled tape, which is directly proportional to the mass of PM in the sampled air.
The US Environmental Protection Agency (EPA) evaluates various methods for monitoring the concentrations of ambient air pollutants and designates them as either reference or equivalent methods. FEMs are used to determine whether alternative measurement techniques not accepted by the EPA meet the requirements in the Code of Federal Regulations (CFR). This helps to determine the legitimacy and accuracy of a given air sampling method.
FEMs may be less expensive and less labour-intensive than FRMs, but they are still not easily attainable by individuals.
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Low-cost sensors
There are a variety of low-cost sensors available, and they use different methods to collect data. Some common types of low-cost sensors include optical particle counters (OPCs), which use laser counters to detect particles and extrapolate PM values, and Beta Attenuation Monitors (BAMs), which use beta ray attenuation to measure the mass concentration of PM in the air. OPCs measure the amount of light scattered by particles to estimate their number and size, while BAMs involve sending beta particles through a piece of filter tape that collects particulate matter, with the difference in attenuation before and after sampling indicating the mass of PM in the air.
However, it is important to note that the accuracy and reliability of low-cost sensors have been questioned. Factors such as humidity and temperature can influence the recorded particle mass, and low-cost sensors may over-report above a certain Relative Humidity (RH) threshold. Additionally, there are currently no widely accepted indoor performance criteria for most low-cost air pollutant monitors. As a result, it is recommended to use low-cost sensors in conjunction with other approaches to improve indoor air quality.
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Optical particle counting (OPC)
OPCs are commonly used for counting large particles and have a lower size limitation of around 100 nm. They are often used to characterise cleanrooms and other contamination-controlled areas. Commercially available OPCs can size particles as small as 50 nm (0.05 μm) and particles as large as several hundred micrometres in diameter, although not in the same instrument. The dynamic sizing range of the light-scattering OPC is often a function of the smallest particle that can be sized.
The basic components of a light-scattering OPC include a detector chamber, a light source, and a time circuit. A sample is drawn into the detector chamber, which is illuminated by an intense light source. When a particle enters the chamber, the amount of light scattered towards the detector increases in proportion to the size of the particle. This increase in scattered-light intensity triggers the time circuit.
OPCs are one of the most popular methods of measuring PM pollution, along with federal reference methods (FRMs) and federal equivalent methods (FEMs). FRMs are considered the gold standard for measuring PM, but FEMs are more flexible and can be less expensive and less labour-intensive. Condensation particle counters (CPCs) are another type of particle counter that can be used in conjunction with OPCs to detect ultra-small particles.
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Air Quality Index (AQI)
The Air Quality Index (AQI) is a tool developed by the EPA (Environmental Protection Agency) to communicate information about outdoor air quality and health. The AQI is an index for reporting air quality that is updated daily, telling individuals how clean or polluted the outdoor air in their location is, along with any associated health effects that may be of concern.
The AQI translates air quality data into numbers and colours that help people understand when to take action to protect their health. For example, an AQI value of 50 or below represents good air quality, while an AQI value over 300 represents hazardous air quality. AQI values at or below 100 are generally thought of as satisfactory, while values above 100 indicate unhealthy air quality, first for certain sensitive groups of people, and then for everyone as values increase.
The EPA establishes an AQI for five major air pollutants regulated by the Clean Air Act, including ground-level ozone and airborne particles, which are the two pollutants that pose the greatest threat to human health. These pollutants can cause serious health problems for millions of people, and the EPA has developed national and regional rules to reduce emissions of pollutants that form PM, helping state and local governments meet the Agency's national air quality standards.
There are a number of ways to measure PM, including federal reference methods (FRMs), federal equivalent methods (FEMs), and low-cost sensors. FRMs are the gold standard for measuring PM, while FEMs are similar but are not standardised, with many different methods and pieces of hardware used. Low-cost sensors, such as PurpleAir sensors, use laser counters to detect particles, which is called optical particle counting (OPC).
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Frequently asked questions
PM stands for particulate matter, which refers to a mixture of solid particles and liquid droplets in the air.
PM2.5 refers to fine inhalable particles, with diameters that are generally 2.5 micrometers and smaller. These particles are often responsible for causing air pollution-related health issues.
There are several ways to measure PM pollution, including Federal Reference Methods (FRMs), Federal Equivalent Methods (FEMs), and low-cost sensors. FRMs are considered the gold standard for measuring particulate matter. FEMs are similar but are not standardized, and low-cost sensors use methods such as laser counters or satellite imaging to detect particles and extrapolate PM values.
Measuring PM pollution is crucial as it directly impacts human health and the environment. By understanding the concentration and sources of PM pollutants, governments and communities can take targeted action to protect and improve public health and environmental well-being.
The Air Quality Index (AQI) is a commonly used scale to determine air quality. It translates air quality data into numbers and colours, helping people understand when to take action to protect their health. An AQI of 50 or below is generally considered safe, while readings above 100 are deemed unhealthy.
