Air Quality Science (Part I): AQI and Particulate Matter – The Basics You Need to Know
Air Quality Science (Part I): AQI and Particulate Matter – The Basics You Need to Know
Air quality is directly linked to every breath we take. Terms like “AQI,” “PM2.5,” or “haze” often appear in weather reports or mobile apps—but what do they really mean? This article breaks down the Air Quality Index (AQI) and particulate matter (PM), helping you understand the fundamentals behind air pollution.
What Is AQI? Your Air’s “Health Report”
1. Definition
The Air Quality Index (AQI) is a standardized tool that translates complex pollution data into a single number and color-coded category. It allows the public to quickly understand the health risks associated with current air quality.
2. AQI Numbers and Colors
The AQI ranges from 0–500: the higher the number, the worse the pollution. Each range corresponds to a specific color and health meaning:
3. How AQI Is Calculated
AQI is based on six major pollutants: PM2.5, PM10, CO, O₃, NO₂, and SO₂. Each pollutant is converted into a sub-index, and the highest value determines the final AQI. In most cities, PM2.5 and PM10 are the dominant drivers of AQI levels.
Particulate Matter (PM): The Key Player in Air Pollution
1. What Is PM?
Particulate matter is a mixture of solid particles and liquid droplets suspended in the air. Sizes vary: some are visible like dust or smoke, while others are so small they require electron microscopes to detect.
- PM10: particles ≤10 micrometers, small enough to reach the respiratory tract.
- PM2.5: particles ≤2.5 micrometers, which penetrate deep into the lungs and may enter the bloodstream.
For comparison, a human hair is about 70 micrometers wide—roughly 30 times larger than PM2.5.
2. Sources of PM
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Primary emissions: vehicle exhaust, power plants, industrial processes, construction dust, agricultural burning, wildfires.
- Secondary formation: chemical reactions in the atmosphere involving SO₂, NOx, and VOCs that generate new fine particles.
3. Health Impacts of PM
Scientific evidence consistently shows that PM, especially PM2.5, has severe health impacts:
- Respiratory system: aggravates asthma, bronchitis, and COPD; reduces lung function.
- Cardiovascular system: increases risk of heart attacks, stroke, and arrhythmias.
- Long-term effects: linked to lung cancer, reduced life expectancy, and impaired lung development in children.
- Other impacts: reduces visibility (haze) and harms ecosystems when deposited on plants, soil, or water.
4. WHO Guidelines
- The World Health Organization (2021) significantly tightened its Air Quality Guidelines: Annual PM2.5: ≤ 5 μg/m³
- 24-hour PM2.5: ≤ 15 μg/m³
Even low concentrations of PM are now recognized as harmful, emphasizing the urgent need for mitigation and protection.
Conclusion
- AQI provides a quick snapshot of daily air quality, with particulate matter usually being the main determinant.
- PM2.5 and PM10 have diverse sources and pose serious health threats.
- Understanding AQI and PM empowers us to take informed actions—like limiting outdoor activity during high-pollution days or using air purifiers indoors.
In the next article of this series, we will focus on carbon dioxide (CO₂) and indoor temperature & humidity — often-overlooked factors that significantly affect our health and performance.
References
1. AirNow. Technical Assistance Document for the Reporting of Daily Air Quality
2. U.S. EPA. Particulate Matter (PM) Basics
3. WHO. Global Air Quality Guidelines, 2021
4. California Air Resources Board. Inhalable Particulate Matter and Health
