💨 Air Quality Index (AQI) and PM2.5 Explained: A Complete Health Guide

What is the Air Quality Index?

The Air Quality Index (AQI) is a standardised scale developed by the US Environmental Protection Agency (EPA) to communicate how clean or polluted the outdoor air is at any given moment. It converts complex measurements of multiple pollutants into a single number that tells you how concerned you should be about your breathing environment today.

Rather than presenting raw concentration data (micrograms per cubic metre, parts per billion), which most people cannot intuitively interpret, the AQI translates measurements into a 0–500 scale with colour-coded categories ranging from "Good" to "Hazardous." This approach — since adopted in various forms by governments worldwide — makes air quality information immediately actionable for ordinary people.

The US AQI considers six major pollutants regulated under the Clean Air Act: ground-level ozone (O₃), particle pollution (PM2.5 and PM10), carbon monoxide (CO), sulfur dioxide (SO₂), and nitrogen dioxide (NO₂). On most days and in most locations, PM2.5 is the pollutant that determines the overall AQI reading — which is why PM2.5 and AQI are so closely linked in public health guidance.

What is PM2.5?

PM2.5 refers to "particulate matter 2.5 micrometres or smaller in diameter" — airborne particles so tiny they are roughly 30 times smaller than the width of a human hair. These particles are the product of combustion (vehicle exhaust, wildfires, power plants, cooking), industrial processes, and secondary formation from gas-to-particle chemical reactions in the atmosphere.

Their microscopic size is what makes PM2.5 medically significant. Larger particles (10–100 µm) are filtered by nasal hairs and the upper respiratory tract. PM2.5 particles bypass these defences and travel deep into the lungs, reaching the alveoli where gas exchange occurs. From there, the smallest particles can cross into the bloodstream and reach the heart, brain, and other organs.

Concentrations are measured in micrograms per cubic metre (µg/m³). The WHO's 2021 air quality guideline sets the safe annual average PM2.5 level at 5 µg/m³ — a threshold that much of the world's population exceeds even in ordinary conditions. The 24-hour guideline is 15 µg/m³.

The AQI Scale: 0 to 500

Health Effects at Each AQI Level

AQI 0–100: Low to moderate risk

At these levels, the air is acceptable for most people under most circumstances. People without underlying health conditions can exercise, work, and spend extended time outdoors without air quality concern. For very sensitive individuals — those with severe asthma or certain heart conditions — moderate AQI (51–100) may still trigger minor symptoms.

AQI 101–150: Sensitive group caution

At "Unhealthy for Sensitive Groups" levels, the affected populations — children under 12, adults over 65, people with asthma, COPD, heart disease, or diabetes — should limit prolonged outdoor exertion and monitor for symptoms. Healthy adults generally notice little effect at this range. If you're in a sensitive group, this is the level at which an indoor alternative to outdoor exercise makes sense.

AQI 151–200: General public affected

At "Unhealthy" levels, essentially everyone faces some health risk with prolonged outdoor exposure. Common effects include eye, nose, and throat irritation; reduced lung function; coughing; and aggravation of cardiovascular conditions. Healthy adults should avoid jogging or cycling for extended periods. Sensitive groups should remain indoors when possible.

AQI 201–300: Very unhealthy — act now

These levels occur during significant pollution events: wildfire smoke enveloping a city, dense industrial smog, or severe atmospheric inversions. At 200+, short-term exposures can trigger cardiovascular events in at-risk individuals and cause measurable lung function reduction in healthy adults. Wearing a well-fitting N95 respirator (not a cloth mask) significantly reduces inhaled particle load. Air purifiers with HEPA filters reduce indoor PM2.5 concentration substantially.

AQI 301–500: Hazardous — emergency conditions

AQI values above 300 represent serious public health emergencies. They occur during major wildfire events, extreme industrial accidents, or severe regional smog episodes. These conditions trigger school closures and public health advisories. Remaining indoors in a well-sealed space with HEPA filtration is the primary protective measure. N95 respirators are essential if outdoor exposure is unavoidable.

Who is Most at Risk?

Air pollution affects everyone, but certain groups face disproportionately higher health risks:

• Children under 12: Lungs are still developing; faster breathing rates mean higher particle intake per body weight. Air pollution exposure in childhood is associated with reduced lifelong lung function capacity.
• Older adults (65+): Natural immune and cardiovascular reserves decline with age. PM2.5 exposure is a leading trigger for cardiac events in this group.
• People with asthma or COPD: Particle exposure directly irritates inflamed airways, triggering bronchospasm, exacerbations, and emergency hospitalisations even at moderate AQI levels.
• People with heart disease: PM2.5 particles in the bloodstream contribute to arterial inflammation, blood clotting, and arrhythmias. Even short-term spikes above AQI 100 increase cardiovascular event rates in cardiac patients.
• Diabetics: Diabetes amplifies the cardiovascular impact of particulate exposure. Studies link high air pollution with increased insulin resistance.
• Outdoor workers: Construction, farming, and landscaping workers face chronic elevated exposure, often without the option to stay indoors.

Where Does Air Pollution Come From?

PM2.5 and AQI are shaped by many emission sources, which vary by location, season, and prevailing winds:

• Vehicle exhaust: The dominant urban source in most cities. Diesel engines emit fine particles directly; petrol engines emit precursors that form secondary PM2.5.
• Wildfires: The fastest-growing source of extreme air quality events in the western United States, Canada, and Australia. A single large wildfire can push AQI across thousands of kilometres to hazardous levels.
• Industrial facilities: Power plants, smelters, and manufacturing emit sulfur dioxide and nitrogen oxides that react in the atmosphere to form fine particulate sulfate and nitrate.
• Agricultural burning: A major source in Southeast Asia, Sub-Saharan Africa, and parts of South America, producing seasonal air quality crises.
• Household burning: Wood-burning stoves, fireplaces, and open cooking fires are dominant sources in many lower-income countries and contribute seasonally even in wealthy nations.
• Atmospheric inversions: Temperature inversions trap pollution near the surface, dramatically amplifying local AQI even when emission rates are normal. Common in valley cities (Los Angeles, Beijing, Delhi) during winter.

How to Protect Yourself

The most effective actions differ by AQI level:

• Check the AQI before outdoor activity: Do not rely on how the air looks. High humidity can make the air appear hazy even at moderate AQI; crystal-clear days can carry elevated ozone levels.
• Time your outdoor activity: On days with elevated AQI, early morning (before rush hour) and evening often have lower particle concentrations than midday or afternoon.
• Wear the right mask: Cloth masks and surgical masks have minimal effect on PM2.5. N95 and N99 respirators, fitted properly, remove 95–99% of particles. P100 elastomeric respirators provide the highest protection for extended high-AQI exposure.
• Use air purifiers indoors: HEPA-rated air purifiers can reduce indoor PM2.5 by 65–85% when running continuously. Keep windows closed during high-AQI events.
• Stay hydrated: Proper hydration supports mucus membranes that help filter particles in the upper airway.
• Monitor with real-time data: AQI can change significantly hour-to-hour during pollution events. Consulting a live air quality monitor before going outside gives you the most current picture.

Monitoring Live Air Quality

EarthIntelPro monitors real-time PM2.5 and AQI for 30+ global cities using the Open-Meteo Air Quality API, which integrates data from the Copernicus Atmosphere Monitoring Service (CAMS) — the EU's flagship atmospheric monitoring programme — and produces hourly gridded air quality forecasts covering the entire globe.

The PM2.5 and Haze cards on the EarthIntelPro dashboard show current readings with a severity badge and plain-English guidance on what to do right now. The global map plots readings across all monitored cities simultaneously so you can see regional patterns at a glance — particularly useful during wildfire events when smoke plumes can affect dozens of cities at once.

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