Tehran Air Quality Guide 2024: The Alborz Mountain Trap, School Closures & What the Sanctions Don’t Tell You
Tehran, population 16 million metro area, is one of the world’s most chronically polluted capitals. The city is geographically imprisoned — squeezed between 5,610m peaks and central desert. Schools close 15–22 days per winter. Here’s the full picture.
Monthly AQI — Tehran 2024
Winter inversions dominate Dec–Feb. Summer heat + desert dust create a secondary peak in Jul–Aug.
Tehran Pollutant Profile 2024
| Pollutant | Tehran | WHO |
|---|---|---|
| PM2.5 | 38 μg/m³ | 5 μg/m³ |
| PM10 | 95 μg/m³ | 15 μg/m³ |
| NO₂ | ~55 μg/m³ | 10 μg/m³ |
| O₃ | High summer | 60 μg/m³ |
| CO | Elevated winter | — |
The Alborz Mountain Inversion: Tehran’s Geographic Prison
The single most important fact about Tehran’s air quality is not traffic or population — it’s the Alborz Mountains. The city sits at ~1,200m altitude in a narrow strip between the Alborz (highest peak: Damavand at 5,610m, only 70km away) and the central Iranian desert to the south. This geometry creates one of the world’s most efficient pollution traps.
How the Inversion Forms (Winter)
- Cold dense air drains down from Alborz peaks overnight
- Pools in the Tehran basin (1,100–1,500m altitude)
- High-pressure anticyclone above creates a warm cap
- Warm air above, cold air below = stable stratification
- Vehicle emissions + heating fill the cold surface layer
- Mountains block northerly flushing winds
- Episodes last days to weeks without rain
Scale of Impact
- • AQI can reach 300–400 on worst inversion days
- • Schools close 15–22 days per winter season
- • Government offices periodically close
- • Odd-even vehicle restrictions imposed automatically at AQI 150+
- • Emergency hospital admissions for respiratory disease spike 40–60% during inversions
- • Visibility can drop below 2 km in central Tehran
Tehran School Closure Days by Year
| Winter Season | Closure Days | Note |
|---|---|---|
| 2019–20 | 14 days | Average winter |
| 2020–21 | 8 days | COVID lockdowns reduced traffic |
| 2021–22 | 18 days | Post-COVID rebound |
| 2022–23 | 22 days | Severe inversion episodes |
| 2023–24 | 19 days | Multiple December–January events |
Source: Tehran Municipality, Iranian Education Ministry. Data compiled from published alerts. Estimates for composite years.
How International Sanctions Worsen Tehran’s Air
The sanctions connection to air quality is direct and quantifiable. Iran’s oil sanctions have affected the country in two ways that specifically damage air quality:
Fuel Quality
Iran produces its own gasoline but cannot import refinery technology to reduce sulfur content. Iranian regular unleaded contains ~500–1,500 ppm sulfur, vs Euro 6 standard of 10 ppm. High-sulfur fuel means catalytic converters are rapidly poisoned and ineffective. A Euro 6 vehicle running on Iranian fuel effectively performs like a Euro 2 vehicle within 2–3 years.
Vehicle Technology
Iran’s domestic car industry (SAIPA, Iran Khodro) produces vehicles to Iranian National Standard (INS), which lags Euro 3–4 equivalents. Import bans on foreign vehicles prevent fleet modernization. An estimated 20–30% of Tehran’s fleet is over 20 years old. Iran Khodro Peugeot 405 clones from the 1990s remain common — these emit 8–12× more PM2.5 than a modern Euro 6 equivalent.
The CNG Partial Solution
Iran has the world’s second-largest CNG vehicle fleet (5+ million vehicles), partly as a domestic response to fuel quality limitations. CNG reduces PM2.5 by ~25–35% vs petrol in the same vehicle. But CNG vehicles still emit significant NOₓ (leading to secondary PM2.5 formation) and CO₂. It’s a meaningful improvement, not a solution.
Protection Guide: Living or Visiting Tehran
April–May are the only reliably safe months. If you must be in Tehran November–February, plan around inversion forecasts. IRIMO (Iran Meteorological Organization) provides 3-day AQI forecasts. IQAir has a Tehran station.
N95/FFP2 minimum for outdoor activity on AQI >150 days. KN95 is widely available in Tehran pharmacies. Standard surgical masks filter PM2.5 at <15% efficiency — inadequate for serious pollution events.
A HEPA air purifier in bedroom is essential for long-term residents. Look for CADR >150 m³/h for a typical Iranian apartment bedroom. Keep windows closed on inversion days. Iranian apartments typically use central heating (gas), which keeps residents indoors — counterintuitively, indoor air can be cleaner than outdoor during inversions.
Tehran Metro covers most tourist and business destinations. The 7 operational lines (with Line 8 and 9 under construction) serve Azadi, Vali Asr, Imam Khomeini Square, and University areas. Metro air is filtered and cleaner than street-level walking during peak pollution episodes.
Asthma patients: discuss increasing preventer inhaler frequency with your doctor for December–February stays. Carry reliever inhaler. Prescription salbutamol is available in Tehran pharmacies. Cardiovascular patients: avoid strenuous outdoor exercise entirely during AQI >150 periods.
Frequently Asked Questions
Why is Tehran one of the world's most polluted capitals despite being a 'modern' city?
Tehran faces three mutually reinforcing problems that are extremely difficult to solve simultaneously. First, geography: the city is compressed between the Alborz Mountains (5,610m peaks) to the north and central Iranian desert to the south. Cold dense air pools in the basin in winter, capped by warm air above — a temperature inversion that can last weeks. Second, fleet quality: international sanctions have prevented Iran from importing modern Euro 5/6 vehicles and low-sulfur fuel. An estimated 4 million vehicles circulate in Tehran, many with 1980s-era technology burning fuel with 3–5× the sulfur content of EU standards. Third, energy mix: Iran's cheap domestic natural gas has created overconsumption of energy across the economy — vehicle CNG is widespread, but the older fleet still dominates emissions.
How often do Tehran's schools close due to air pollution?
School closures are a routine winter event in Tehran. The city uses a three-tier alert system: Yellow Alert (AQI 150–200) — outdoor PE cancelled; Orange Alert (AQI 200–250) — all outdoor activities banned; Red Alert (AQI 250+) — full school closure. In a typical winter, schools close 15–22 days due to air pollution. In severe years, the number exceeds 30. Schools are typically in primary closure mode December through February, with March sometimes affected. Government offices also periodically close or shift to remote work on worst days. The economic cost is estimated at $1–2 billion annually from productivity loss alone.
What is the odd-even vehicle restriction in Tehran?
Tehran operates a mandatory odd-even license plate restriction during air quality emergencies. On designated 'even days', only vehicles with even last-digits in license plates can enter the central 'Traffic and Air Pollution Zone' (TAPO zone, covering central Tehran). On 'odd days', only odd plates. The restriction typically applies from 6:30 AM to 9:30 PM and covers about 30% of the city's core. When AQI exceeds 150 for sustained periods, the restriction is extended citywide and applied even during non-peak hours. Enforcement is by cameras and police at 40+ control points. The measure reduces traffic by approximately 25–30% on restricted days, providing measurable but temporary AQI improvement.
What time of year is best to visit Tehran for air quality?
April and May are the best months: winter inversions have broken, spring rains clean the air, and summer heat and dust haven't started. AQI typically runs 95–118 — uncomfortable by clean-city standards but manageable. The Nowruz (Iranian New Year, around March 21) holiday is also notable — millions of Tehranis leave the city, reducing traffic by 30–40% for about 2 weeks, and AQI drops noticeably despite still being spring. Avoid December–February entirely if you have respiratory conditions. The summer (July–August) is dual-problematic: extreme heat (40°C+) combines with Kavir (central desert) dust storms and high ozone to create its own pollution profile.
Is Tehran's air quality getting better or worse?
The honest answer is: worse on a structural basis, with intermittent policy interventions providing partial offsets. Iran's car fleet has grown from 3 million to 4 million vehicles in Tehran over the past decade. CNG conversion (Iran has the world's second-largest CNG fleet) is a genuine partial solution — CNG vehicles emit ~30% less PM2.5 than equivalent petrol — but fuel quality for remaining petrol/diesel vehicles has not improved due to sanctions. Tehran Metro is expanding (currently 7 operational lines, Line 8 and 9 under construction), which is meaningful. But the city's population also grew — from 13.8M to 16M metro area — absorbing transit gains. Climate change is increasing the frequency of blocking anticyclones that cause inversions. The trajectory is slowly negative.