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AQI Levels Explained

Understanding the Air Quality Index

The Air Quality Index (AQI) translates complex air pollution data into a simple, color-coded scale that anyone can understand. Think of it as a health thermometer for air—just as temperature tells you how hot or cold it is, AQI tells you how clean or polluted your air is.

The AQI runs from 0 to 500—the higher the value, the greater the level of air pollution and health concern.

AQI Level Breakdown

🟢 Good (0-50)

Air Quality: Satisfactory with little to no health risk

What It Means: Air pollution poses minimal concern. You can breathe easily and enjoy all outdoor activities without restrictions. This is ideal air quality that we should all aspire to experience daily.

Health Implications:

  • No health effects expected
  • Air quality is excellent
  • Perfect for all outdoor activities
  • Ideal for people with respiratory conditions

Who's Affected:

  • Nobody. Even the most sensitive individuals can breathe freely.

Recommended Actions:

  • ✅ Exercise outdoors without concern
  • ✅ Keep windows open for fresh air
  • ✅ Enjoy all outdoor activities
  • ✅ This is the air quality we all deserve

Visibility:

  • Clear skies
  • Excellent visibility
  • No haze or pollution visible

Real-World Example:

  • Mountain towns on clear days
  • Coastal areas with ocean breezes
  • Rural areas far from cities
  • Cities during rain (temporarily)

🟡 Moderate (51-100)

Air Quality: Acceptable, but sensitive individuals may experience minor effects

What It Means: Air quality is generally acceptable for most people. However, unusually sensitive individuals may experience minor respiratory symptoms. This is still considered "normal" air quality in many cities worldwide.

Health Implications:

  • Acceptable for most people
  • Very sensitive individuals may notice mild effects
  • Generally safe for outdoor activities
  • Slight respiratory discomfort possible for small subset of population

Who's Affected:

  • Very few people: Only those with unusual sensitivity to air pollution
  • Most people won't notice any difference from Good air

Recommended Actions:

  • ✅ General public: Continue normal outdoor activities
  • ⚠️ Unusually sensitive people: Consider reducing prolonged outdoor exertion if symptoms occur
  • ✅ Windows can stay open
  • ✅ Exercise outdoors is fine for most

Visibility:

  • Generally clear
  • Slight haze may be visible
  • No significant reduction in visibility

Real-World Example:

  • Most U.S. cities on average days
  • European cities typically
  • Urban areas with good air quality management

🟠 Unhealthy for Sensitive Groups (101-150)

Air Quality: Sensitive groups should reduce prolonged outdoor exertion

What It Means: This is where health concerns begin for vulnerable populations. Children, elderly, people with asthma or heart disease, and those with respiratory conditions should start paying attention and making adjustments.

Health Implications:

  • Sensitive groups: May experience respiratory symptoms
  • General public: Less likely to be affected
  • Increased risk of asthma attacks
  • Potential for breathing discomfort during exercise

Who's Affected: People in sensitive groups, including:

  • Children and teenagers (developing lungs)
  • Elderly (65+)
  • People with asthma or lung disease
  • People with heart disease
  • Pregnant women
  • Outdoor workers (due to prolonged exposure)

Recommended Actions:

Sensitive Groups:

  • ⚠️ Reduce prolonged or heavy outdoor exertion
  • ⚠️ Take more breaks during outdoor activities
  • ⚠️ Watch for symptoms: coughing, shortness of breath
  • ⚠️ Keep rescue inhaler accessible (if asthmatic)
  • ⚠️ Consider moving intense exercise indoors

General Public:

  • ✅ Continue normal activities
  • ✅ Outdoor exercise generally fine
  • Be aware if symptoms develop

Indoor Air:

  • Consider closing windows if sensitive
  • Air purifiers beneficial for sensitive individuals

Visibility:

  • Noticeable haze
  • Slightly reduced visibility
  • Sky may appear less blue

Real-World Example:

  • Major cities on typical days
  • Urban areas with traffic pollution
  • Areas affected by distant wildfire smoke

Use Our Calculator: At AQI 125 for 8 hours (work/school day), you're breathing the equivalent of approximately 2.5 cigarettes. Use our AQI to Cigarettes Calculator to see your personal exposure.

🔴 Unhealthy (151-200)

Air Quality: Everyone may begin to experience health effects

What It Means: Air quality is now unhealthy for everyone, not just sensitive groups. This is when you should modify your behavior—reduce outdoor activities, especially anything strenuous. The general public will start to notice impacts.

Health Implications:

  • Everyone: May experience reduced lung function, breathing discomfort
  • Sensitive groups: Increased risk of serious effects
  • Asthma attacks more likely
  • Heart disease patients at elevated risk
  • Respiratory irritation common

Who's Affected:

  • Sensitive groups: Significantly affected, should avoid outdoor exertion
  • General public: Will notice effects, especially during exercise
  • Everyone: Should reduce outdoor exposure

Recommended Actions:

Sensitive Groups:

  • 🚫 Avoid prolonged outdoor exertion
  • 🚫 Stay indoors as much as possible
  • 🚫 No outdoor sports or heavy exercise
  • ✅ If must go outside, keep it brief and light
  • ✅ Wear N95 mask if outdoor exposure unavoidable

General Public:

  • ⚠️ Reduce prolonged outdoor exertion
  • ⚠️ Limit outdoor exercise—move workouts inside
  • ⚠️ Take more breaks if working outside
  • ⚠️ Watch for symptoms: coughing, breathing difficulty

Indoor Air:

  • 🚫 Close windows
  • ✅ Run air purifiers
  • ✅ Use air conditioning on recirculate mode
  • ✅ Create clean air room for sleeping

Schools & Activities:

  • Indoor recess recommended
  • Cancel outdoor sports practice
  • Reschedule outdoor events if possible

Visibility:

  • Significant haze visible
  • Reduced visibility (often <5 miles)
  • Sky appears gray or brown
  • Sun may appear orange/red

Real-World Example:

  • Wildfire smoke events
  • Severe pollution days in major Asian cities
  • Winter inversion pollution in mountain valleys
  • Heavy industrial pollution episodes

Cigarette Equivalent: At AQI 175 for 8 hours = approximately 6 cigarettes. That's nearly a third of a pack per day from just breathing. Calculate your exposure.

🟣 Very Unhealthy (201-300)

Air Quality: Health alert—everyone may experience serious health effects

What It Means: This is a health emergency. Air quality is very unhealthy for everyone. Outdoor exposure should be minimized. This level triggers public health warnings in most jurisdictions.

Health Implications:

  • Everyone: Likely to experience health effects
  • Sensitive groups: Serious health effects expected
  • Emergency room visits increase significantly
  • Heart attacks and strokes more likely
  • Respiratory distress common
  • Schools often close

Who's Affected:

  • EVERYONE is at risk
  • Sensitive groups face severe health risks
  • Even healthy adults will feel effects
  • This is dangerous air

Recommended Actions:

Everyone:

  • 🚫 Avoid ALL outdoor physical exertion
  • 🚫 Stay indoors with windows/doors closed
  • 🚫 Cancel outdoor events, sports, activities
  • ✅ Run air purifiers continuously
  • ✅ Wear N95 mask if must go outside briefly
  • ✅ Limit even indoor physical activity

Sensitive Groups:

  • 🚫 Stay indoors entirely if possible
  • 🚫 Avoid all physical exertion, even light
  • ✅ Keep medications accessible
  • ✅ Monitor symptoms closely
  • ✅ Seek medical care if breathing difficulty
  • ✅ Consider temporary evacuation if prolonged

Indoor Air:

  • 🚫 Do NOT open windows or doors
  • ✅ Seal gaps around windows/doors
  • ✅ Run air purifiers on high continuously
  • ✅ Create clean air room
  • ✅ Use AC on recirculate mode (never fresh air intake)

Schools & Work:

  • Schools should close
  • Non-essential outdoor work cancelled
  • Essential workers need respiratory protection

Visibility:

  • Very poor visibility (<1-2 miles)
  • Thick, visible haze or smoke
  • Can smell/taste pollution in air
  • Sky appears brown, orange, or white
  • Sun completely obscured or deep red

Real-World Example:

  • Major wildfire smoke events (California, Australia)
  • Severe pollution crisis (Delhi winter)
  • Industrial accident pollution plumes
  • "Airpocalypse" conditions

Cigarette Equivalent: At AQI 250 for 8 hours = approximately 15-20 cigarettes—that's a full pack per day just from breathing. Emergency action required. See the numbers.

🟤 Hazardous (301-500)

Air Quality: Health emergency—entire population at severe risk

What It Means: This is a public health emergency. Air quality has reached hazardous levels dangerous to all humans. This triggers emergency response protocols. Avoid all outdoor exposure. This is rare but increasingly occurring during extreme wildfire events.

Health Implications:

  • Serious health effects for everyone
  • Emergency room capacity exceeded
  • Significant increase in deaths
  • Healthy people will experience serious effects
  • Sensitive groups at critical risk
  • Long-term health consequences from even brief exposure

Who's Affected:

  • EVERYONE faces serious health risk
  • No one is safe
  • This is life-threatening air

Recommended Actions:

Everyone:

  • 🚫 STAY INDOORS - This is not optional
  • 🚫 Do NOT go outside except absolute emergencies
  • 🚫 No outdoor exposure, even brief
  • 🚫 Cancel everything non-essential
  • ✅ Shelter in place with maximum air purification
  • ✅ N95 mask essential for any outdoor exposure
  • ⚠️ Consider evacuation if conditions persist

Sensitive Groups:

  • 🚫 Extremely high risk
  • ✅ Medical-grade air purification essential
  • ✅ Keep emergency contacts ready
  • ✅ Medications accessible and stocked
  • ⚠️ Strongly consider evacuation
  • ⚠️ Go to hospital if breathing difficulty

Indoor Air:

  • 🚫 Seal room completely
  • ✅ Multiple air purifiers running on high
  • ✅ Wet towels under doors
  • ✅ Plastic sheeting over windows
  • ✅ Create airtight clean air room
  • ✅ Monitor indoor PM2.5 continuously

Community Response:

  • Schools closed indefinitely
  • Non-essential businesses close
  • Public transportation may be suspended
  • Emergency services only
  • Evacuation orders possible
  • Government emergency protocols activated

Visibility:

  • Visibility near zero (<few hundred feet)
  • Cannot see across street
  • Thick, choking smoke or smog
  • Breathing is difficult even indoors without filtration
  • Apocalyptic appearance

Real-World Example:

  • 2023 Canadian wildfire smoke in NYC (AQI 484)
  • Delhi during peak winter pollution crisis (AQI 400-500)
  • California Camp Fire aftermath (AQI 500+)
  • Indonesia peat fire smoke (AQI 500+)

Cigarette Equivalent: At AQI 400 for 8 hours = approximately 40+ cigarettes—that's TWO PACKS per day from just breathing. This is a life-threatening emergency. Understand your exposure.

Special Considerations by AQI Level

For Exercise and Physical Activity

| AQI Range | Light Exercise (Walking) | Moderate Exercise (Jogging) | Intense Exercise (Running, Sports) | |-----------|-------------------------|----------------------------|-----------------------------------| | 0-50 | ✅ Unrestricted | ✅ Unrestricted | ✅ Unrestricted | | 51-100 | ✅ Normal | ✅ Normal | ✅ Normal | | 101-150 | ✅ Normal | ⚠️ Reduce duration | ⚠️ Sensitive groups move indoors | | 151-200 | ⚠️ Reduce duration | ⚠️ Move indoors | 🚫 Move indoors | | 201-300 | 🚫 Minimal outdoor time | 🚫 Indoor only | 🚫 Indoor only | | 301-500 | 🚫 Stay indoors | 🚫 Stay indoors | 🚫 Stay indoors |

For Children (More Vulnerable)

Children breathe faster and their lungs are still developing. Use more conservative thresholds:

  • AQI >100: Reduce prolonged outdoor play
  • AQI >150: Indoor recess, cancel outdoor sports
  • AQI >200: Stay indoors entirely
  • AQI >300: Emergency measures, consider school closure

For Outdoor Workers

If you must work outside:

  • AQI <100: Normal work
  • AQI 100-150: More breaks, watch for symptoms
  • AQI 150-200: Reduce exertion, N95 masks recommended
  • AQI 200+: Work should be suspended if possible, N95 essential

How AQI is Calculated

The AQI is based on the highest pollutant measurement, typically from:

  1. Ground-level Ozone (O₃)
  2. Particle Pollution (PM2.5 and PM10)
  3. Carbon Monoxide (CO)
  4. Sulfur Dioxide (SO₂)
  5. Nitrogen Dioxide (NO₂)

PM2.5 is most commonly the pollutant reaching unhealthy levels and driving the AQI.

Conversion: The EPA uses a formula to convert raw pollutant concentrations (µg/m³ for PM2.5) into the AQI scale (0-500).

Regional and Seasonal Variations

Urban vs. Rural

  • Urban areas: Traffic and industry mean higher baseline AQI
  • Rural areas: Generally lower, but vulnerable to distant wildfire smoke

Seasonal Patterns

  • Summer: Ozone formation on hot, sunny days (AQI spikes afternoon)
  • Winter: Heating emissions, inversions trap pollution (worse mornings)
  • Wildfire season: Smoke can send AQI to Hazardous levels hundreds of miles away
  • Monsoon/rainy season: Rain washes pollutants, temporarily improves AQI

Time of Day

  • Early morning (5-7 AM): Often cleanest air
  • Rush hours (7-9 AM, 5-7 PM): Worst air in urban areas
  • Midday (summer): Ozone peaks
  • Evening: Variable

Tools and Resources

Check Your Local AQI

  • AirNow.gov (United States) - Official EPA site
  • IQAir (Global) - Real-time worldwide data
  • Purple Air (Crowdsourced) - Dense network of sensors
  • Weather apps - Many now include AQI

Set Up Alerts

Most air quality apps allow you to:

  • Set threshold alerts (notify when AQI >100, etc.)
  • Daily morning notifications
  • Hourly updates during poor air

Understand Your Exposure

Use our AQI to Cigarettes Calculator to translate air quality into cigarette equivalents—a metric everyone understands. See exactly what breathing your local air for 8 hours equals in cigarettes smoked.

Taking Action Based on AQI

Your Personal Threshold

Different people have different sensitivity. Determine yours:

  • Start paying attention at AQI 100 if sensitive (asthma, elderly, children)
  • Start modifying behavior at AQI 150 if generally healthy
  • Everyone should respond at AQI 200

Create Your Action Plan

AQI < 100:

  • Live normally
  • Enjoy outdoor activities
  • Fresh air is beneficial

AQI 100-150:

  • Monitor symptoms
  • Reduce prolonged outdoor exertion if sensitive
  • Have indoor backup plans

AQI 150-200:

  • Move exercise indoors
  • Close windows
  • Run air purifiers
  • Wear N95 if must be outside >30 min

AQI 200+:

  • Stay indoors
  • Seal home
  • Continuous air purification
  • N95 mask for any outdoor exposure
  • Consider evacuation if AQI >300 persists

The Bottom Line

The AQI is your daily guide to protecting your respiratory health. Just as you wouldn't go outside in a blizzard without a coat, you shouldn't exercise outdoors when AQI is high without taking precautions.

Key takeaways:

  • Check AQI daily before planning outdoor activities
  • Green/Yellow (0-100): Breathe freely
  • Orange (101-150): Caution for sensitive groups
  • Red (151-200): Everyone should limit outdoor time
  • Purple/Maroon (201+): Stay indoors, emergency measures

Your lungs don't get a second chance. Protect them by respecting the AQI.

For more detailed protection strategies, see our guides on Air Purifiers, N95 Masks, and Creating a Clean Air Room.

Calculate Your Exposure

Now that you understand AQI levels, use our calculator to see what your current air quality means in cigarette equivalents.

Try the Calculator