Tehran Air Quality Index (AQI)
Iran's capital of 15 million is trapped between the 5,000m Alborz Mountains and central desert. Winter temperature inversions, an aging vehicle fleet, and sanctions-constrained fuel quality make Tehran one of the world's most chronically polluted capitals.
Monthly AQI Pattern — Tehran
Key Pollutants — Tehran 2024
| Pollutant | Level | WHO Guideline | Primary Source |
|---|---|---|---|
| PM2.5exceeds WHO | 38 μg/m³ | 5 μg/m³ | Vehicle exhaust (old fleet), industry, heating |
| PM10exceeds WHO | 95 μg/m³ | 15 μg/m³ | Desert dust from central Iran + construction |
| NO₂exceeds WHO | ~55 μg/m³ | 10 μg/m³ | Vehicle exhaust — ~4M vehicles, many pre-Euro 3 |
| O₃ (ozone) | High summer | — | Traffic NOx + intense summer UV radiation |
| CO | High | — | Incomplete combustion from old vehicles + heating |
The Alborz Mountain Inversion Trap
Tehran's most distinctive feature is its geographic prison. The Alborz Mountains rise from 1,200m (Tehran) to 5,610m (Damavand) within 50km — an impenetrable northern wall.
How Winter Inversions Form
- Cold air drains down from Alborz peaks into Tehran basin
- Anticyclone overhead creates stable, warm cap above
- Cold dense air trapped — vehicles + heating fill the layer
- Mountains block flushing northerly winds
- Episode lasts until rain or strong southerly wind breaks it
Impact
- • AQI can reach 250–350 (Very Unhealthy)
- • Schools close 10–20 days per winter season
- • Government offices occasionally close
- • Odd-even vehicle license plate restrictions imposed
- • Helicopter-measured upper-level ozone shows multi-layer pollution
Frequently Asked Questions
Why is Tehran's air quality so bad despite being a modern capital?
Tehran faces a perfect storm of geographic and structural challenges. Geographically, the city is compressed into a narrow strip between the Alborz Mountains (5,000m peaks) to the north and the central Iranian desert to the south. Cold winter air settles into the basin and is trapped by thermal inversions — warm air above acts as a lid, preventing pollutants from dispersing. Structurally, Iran's international sanctions have starved the country of modern vehicle technology and clean fuel imports. An estimated 4 million vehicles circulate in Tehran, many with 1980s-era technology running on substandard fuel.
What are Tehran's school closure days?
When AQI in Tehran exceeds 150 (Unhealthy), city authorities issue 'Air Pollution Alerts.' At AQI above 200, outdoor school activities are cancelled. At AQI above 250 (Very Unhealthy), schools close entirely — this happens multiple times per year, primarily December through February. The education ministry maintains a color-coded alert system. During these events, the elderly, children, and those with respiratory disease are advised to remain indoors.
How does the Alborz Mountain geography trap pollution?
Tehran sits at ~1,200m altitude in a basin with the Alborz Mountains (Damavand peak: 5,610m) immediately to the north. In winter, cold dense air flows down from the mountains and pools in the basin — a classic cold-air drainage inversion. Simultaneously, a high-pressure anticyclone typically sits over the region, creating a stable atmospheric cap. Vehicle exhaust and heating emissions accumulate below this inversion lid. The mountains also block northerly wind that might flush the basin. Only strong southerly winds or rainfall can break the episode.
Is Tehran's pollution getting better or worse?
The trajectory is negative. Iran's car fleet has grown faster than emission controls can offset. Sanctions limited access to Euro 5/6 engine technology and low-sulfur fuel. Natural gas vehicles (CNG) have been promoted as a partial solution — Iran has the world's second-largest CNG vehicle fleet — but the older petrol/diesel fleet still dominates. Climate change is also making temperature inversions more frequent. Some positive signals: Tehran Metro is expanding aggressively, and there's domestic pressure for stronger air quality policy.
What should expats and visitors know about Tehran's air?
December through February are dangerous months for sensitive individuals. Check the AQI forecast before arriving (IQAir or AirNow Tehran station). Pack FFP2/N95 masks — they are essential on inversion days and widely used by locals. Hotels with filtered HVAC provide significant protection. The best time to visit for air quality is April–May (spring): lower traffic during school exam season, frequent rain, and no summer heat yet. Avoid outdoor sightseeing on red/purple AQI days.