Understanding Pm2.5: Tiny Particles, Big Impact

Fine inhalable particles, or particulate matter 2.5 (PM2.5), are tiny particles or droplets in the air that are 2.5 micrometres or less in width. They are a form of particle pollution, which is a mixture of solid particles and liquid droplets found in the air. PM2.5 particles are emitted directly from sources such as construction sites, unpaved roads, fields, smokestacks, and fires, and they can also form in the atmosphere through chemical reactions. These particles can be harmful to human health, as they can travel deeply into the respiratory tract and reach the lungs, and may even enter the bloodstream.

PM2.5 sources: outdoor and indoor

PM2.5 refers to fine liquid or solid particles that are generally 2.5 micrometres and smaller in diameter. To put this in context, the average human hair is about 70 micrometres in diameter – making it 30 times larger than the largest fine particle. These particles come in many sizes and shapes and can be made up of hundreds of different chemicals.

Outdoor Sources

Emissions from the combustion of gasoline, oil, diesel fuel, or wood are the main sources of outdoor PM2.5 pollution. Outdoor PM2.5 sources, such as vehicle exhaust, are also a major contributor. Other sources include:

• Construction sites
• Unpaved roads
• Fields
• Smokestacks
• Fires
• Industrial sources
• Wind-blown dust from open lands
• Pollen
• Fragments of bacteria

Indoor Sources

Some of the particulate matter found indoors originates from outdoors, entering through doors, windows, and leakiness in building structures. However, indoor activities also generate particles, including:

• Smoking tobacco
• Cooking
• Burning wood, candles, or incense
• Pet dander
• Bacteria
• Mould
• Chemicals from cleaning products
• Building materials
• Fuel-burning equipment
• Hobbies

Health effects of PM2.5 exposure

PM2.5 refers to fine particles or particulate matter that are less than 2.5 micrometers in diameter. Due to their small size, they can be inhaled and penetrate deep into the lungs, depositing on the surface of the deeper parts of the lung. This can cause a range of adverse health effects, particularly in infants, children, pregnant women, and older adults with pre-existing heart or lung diseases.

Short-term exposure to PM2.5 has been linked to increased hospital admissions for heart or lung-related issues, acute and chronic bronchitis, asthma attacks, respiratory symptoms, and restricted activity days. It can also cause eye, nose, throat, and lung irritation, coughing, sneezing, a runny nose, and shortness of breath.

Long-term exposure to PM2.5 has been associated with an increased risk of mortality, including from heart disease and lung cancer. It has also been linked to an increased risk of ischemic heart disease, chronic obstructive pulmonary disease (COPD), lower-respiratory infections, stroke, type 2 diabetes, adverse birth outcomes, and reduced lung function growth in children.

The health effects of PM2.5 exposure are a global concern, with countries in Asia and Africa experiencing the highest rates of death attributable to PM2.5. China and India, for example, accounted for 58% of the total global mortality burden in 2019, with more than 4 million deaths worldwide attributed to long-term exposure to PM2.5 pollution.

The impact of PM2.5 on human health is influenced by various factors, including population size and age structure, access to healthcare, and socioeconomic development. Reducing exposure to PM2.5 is crucial, and guidelines recommend staying indoors, closing doors and windows, wearing qualified masks when outdoors, and minimizing the duration or intensity of outdoor activities.

PM2.5 and climate change

PM2.5 refers to particulate matter that is generally 2.5 micrometres or less in diameter. These particles are so small that they can be inhaled and enter the lungs, causing serious health problems. They are formed through the combustion of gasoline, oil, diesel fuel, or wood, as well as from industrial processes and motor vehicle exhaust.

PM2.5 is a significant concern in the context of climate change. Firstly, climate change can directly influence the concentration of PM2.5. For example, Westervelt et al. found that climate change leads to increased PM2.5 in most scenarios, which is linked to local temperature increases. Similarly, Allen et al. demonstrated that GHG-induced warming results in higher concentrations of both anthropogenic and natural aerosols, with a particular increase in black carbon, primary organic matter, and sulfate.

Secondly, the chemical composition of PM2.5 is also affected by weather patterns, which are influenced by climate change. A study analyzing the impact of weather on PM2.5 composition in the US between 1988 and 2017 found significant variations over space and time. For instance, wind speed decreased during both seasons, while relative humidity increased in the warm season and decreased in the cold season. These changes impacted the magnitude and direction of the "weather penalty" for different PM2.5 components.

The health risks associated with PM2.5 exposure are significant and include respiratory and cardiovascular issues, such as asthma, heart disease, and increased mortality. As climate change influences the concentration and composition of PM2.5, it can indirectly exacerbate these health risks. This is particularly concerning given that recent studies have projected an increase in premature mortality due to climate change-induced rises in air pollutant concentrations.

To address these challenges, future emission reductions are crucial. Understanding how aerosol burdens change under different emission scenarios can help improve air pollution policy decision-making and mitigate the health impacts of PM2.5 in a changing climate.

PM2.5 air quality standards

PM2.5 refers to particulate matter that is generally 2.5 micrometres and smaller in diameter. These fine particles are a form of air pollution, which can be solid or liquid and are often the result of complex reactions of chemicals such as sulfur dioxide and nitrogen oxides. PM2.5 is of particular concern as it can be inhaled and deposited in the deeper parts of the lung, causing serious health issues.

Air quality standards are set to protect public health, with the US Environmental Protection Agency (EPA) regulating PM2.5 levels in the US. The EPA establishes an Air Quality Index (AQI) for five major air pollutants, including PM2.5, which is regulated by the Clean Air Act. An AQI value of 100 or below is generally considered satisfactory, while values above 100 indicate unhealthy air quality. The EPA's rules to reduce emissions of pollutants that form PM2.5 help state and local governments meet the Agency's national air quality standards.

The World Health Organization (WHO) advises that no level of PM2.5 exposure has been shown to be completely free of health impacts. The WHO uses a more stringent threshold for PM2.5 levels than the US EPA, recommending levels below 10 μg/m3.

In 2002, the California Air Resources Board (CARB) adopted a new annual average standard for PM2.5 after an extensive review of the scientific literature. The national annual average PM2.5 standard was again revised in 2024, following new evidence of continued risk to human health at lower PM2.5 concentrations. The latest review resulted in the retention of the existing 24-hour average PM2.5 standards.

The New York State Departments of Health and Environmental Conservation (DEC) alert the public when particle pollution levels in outdoor air are expected to be unhealthy. An Air Quality Alert is issued when concentrations at monitoring stations in a specific region are predicted to be elevated.

Reducing PM2.5 emissions

PM2.5 refers to particulate matter that is generally 2.5 micrometres and smaller in diameter. These particles are so small that they can be inhaled and settle on the surface of the deeper parts of the lung, causing serious health issues. Exposure to PM2.5 has been linked to premature mortality, respiratory issues, and heart disease.

To reduce PM2.5 emissions, governments and organizations can implement the following strategies:

Outdoor Air Pollution

• Regulate emissions of pollutants that form PM2.5: This includes reducing the combustion of gasoline, oil, diesel fuel, and wood, which are significant sources of PM2.5 pollution.
• Improve industrial processes: Industries should adopt cleaner technologies and best practices to minimize the release of particulate matter and gaseous pollutants that contribute to PM2.5 formation.
• Control vehicle emissions: Implement stricter vehicle emission standards, promote the use of electric vehicles, and encourage the development of public transportation systems to reduce emissions from combustion engines.
• Manage construction sites: Implement dust control measures at construction sites to minimize the generation of dust particles, which contribute to both PM2.5 and PM10 pollution.
• Wildfire prevention and management: Wildfires are a significant source of PM2.5. Implementing controlled burns, creating fire breaks, and improving land management practices can help reduce the impact of wildfires on air quality.

Indoor Air Pollution

• Public awareness campaigns: Educate the public about the sources of indoor air pollution, such as tobacco smoking, cooking with gas or wood, burning candles, and using fireplaces. Encourage the use of exhaust fans and air purifiers to reduce indoor PM2.5 levels.
• Improve indoor air quality in public spaces: Implement measures to improve ventilation and air filtration in schools, offices, and other public buildings, especially in regions with consistently high PM2.5 levels.
• Encourage the use of air conditioners and air purifiers: In areas with high outdoor PM2.5 levels, spending more time indoors and using air conditioners or air purifiers can help reduce exposure to harmful particles.

Regional and International Cooperation

• Transboundary pollution management: Collaborate with neighbouring regions or countries to address transboundary pollution issues, as pollution from one area can affect the air quality in another.
• Share best practices: Encourage the sharing of successful emission control strategies and technologies between countries to accelerate global efforts in reducing PM2.5 emissions.

Scientific Research and Monitoring

• Invest in research: Fund scientific studies to better understand the sources, transport, and health impacts of PM2.5 pollution, which can inform the development of more effective emission control strategies.
• Establish monitoring networks: Expand air quality monitoring stations to accurately measure PM2.5 levels and identify areas requiring targeted emission reduction measures.

Frequently asked questions