Air quality, for travellers

AQI scales compared

Different countries use different AQI scales. The same underlying PM2.5 concentration can produce wildly different AQI numbers across scales. The relevant conversions for travellers:

• U.S. EPA AQI: 0-500. PM2.5 35 µg/m³ = AQI 100; 55 = AQI 150; 150 = AQI 200; 250 = AQI 300.
• WHO 2021 guidelines: PM2.5 annual mean 5 µg/m³; 24-hour mean 15 µg/m³. The strictest published standard.
• China AQI: same 0-500 scale, different breakpoints. China AQI 100 = PM2.5 75 µg/m³ (compared with U.S. AQI 100 at 35 µg/m³). A Chinese “Moderate” corresponds to an EPA “Unhealthy for Sensitive Groups.”
• India CPCB AQI: 0-500; AQI 100 = PM2.5 60 µg/m³.
• European CAQI: 1-100 short index; less commonly referenced.
• Singapore PSI: 0-300+. PSI 100 = PM2.5 roughly 40 µg/m³ (24-hour).

The practical impact: a reading reported as “AQI 150” means different things in different countries. The travel-safe approach is to default to either the U.S. EPA scale (more conservative) or to the underlying PM2.5 µg/m³ number itself. The Field Manual’s smoke-days guide covers the wildfire-smoke-specific scaling.

What pollutes cities

Year-round urban air pollution has multiple sources; the mix varies by region:

• Vehicle emissions: NOx, CO, particulate from diesel exhaust. Dominant in dense old-fleet cities (Cairo, Mexico City, Lagos, Mumbai, Delhi, Jakarta).
• Coal-power generation: SO2, particulate. Major source in northern China, India, Vietnam.
• Industry: refining, steel, cement. Pearl River Delta, Indo-Gangetic plain, Tehran basin.
• Crop burning: seasonal stubble burning in Punjab India and Pakistan produces Delhi’s catastrophic winter pollution. Similarly in southern Africa, Mato Grosso, Sumatra (peat fires).
• Wildfire smoke: increasingly globally distributed. See the wildfire-smoke guide.
• Dust storms: Saharan dust affects Mediterranean; Gobi dust affects Beijing/Seoul; Middle Eastern dust affects Gulf cities.
• Domestic heating: wood and coal heating in winter in Eastern Europe, Northern China, Indo-Gangetic plain.

Year-round baselines

The cities with the highest year-round PM2.5 baselines per IQAir 2024 World Air Quality Report:

• Begusarai, India: annual average around 118 µg/m³ in 2023. The highest-polluted major city in the world.
• Delhi, India: annual average 92 µg/m³. Winter peaks exceed 500.
• Lahore, Pakistan: annual average 99 µg/m³. Indo-Gangetic plain pattern.
• Dhaka, Bangladesh: annual 81 µg/m³.
• Hotan, Tashkent, several Central Asian cities: 60-100 µg/m³ annual.
• Kabul, Afghanistan: 60-80 µg/m³.
• Beijing, China: now around 35 µg/m³ annual, down from 90+ in 2013. The most-improved major city in recent decades.
• Mexico City, Mexico: around 22 µg/m³ annual; significantly improved since 1990s peaks.
• Mumbai, India: 50-60 µg/m³ annual.
• Bangkok, Thailand: 25-35 µg/m³ annual.
• Jakarta, Indonesia: 35-45 µg/m³ annual.

For comparison, WHO guideline is 5 µg/m³ annual. Most major Asian capitals exceed this 5x to 20x year-round.

Seasonal peaks

• Delhi winter (November to February): the world’s worst predictable air-pollution event. Crop burning, temperature inversion, Diwali fireworks. Daily PM2.5 500+ for weeks. Asthma exacerbations, school closures, tourist hospital admissions documented.
• Singapore haze season (September-October): Indonesian peat-fire smoke. PSI 200+ for days in bad years.
• Beijing winter (December-February): coal-heating contribution; though improved, still produces peak events.
• Seoul yellow-dust spring (March-May): Mongolian and Chinese desert dust plus local emissions.
• Mexico City winter (November-February): temperature inversion in the basin; daily AQI 150-200.
• U.S. Pacific Northwest summer (July-October): wildfire smoke; recurring annually since 2017.
• Athens and southern European summer (July-September): wildfire smoke; recurring.
• Bangkok January-April: crop burning in northern Thailand and Laos plus dry-season urban emissions.
• Cairo winter (November-February): cool inversion plus year-round vehicle pollution.

Indoor strategy

When the AQI outside is bad for days, indoor strategy matters:

• Hotel room HVAC: ask if there’s a HEPA filter; some chains (Marriott, Hilton, IHG in South/Southeast Asia) routinely install. Set HVAC to recirculate.
• Portable HEPA air purifier: rent from local services in Delhi, Bangkok, Beijing; some hotels have them. CADR rating matters; pick by room size.
• DIY box-fan plus MERV-13 filter: USD 40 improvised; reasonable performance for one room.
• Sealed windows: gaffer-tape door and window gaps if necessary.
• Avoid indoor combustion: candles, incense, gas stoves without ventilation all add to indoor PM2.5.
• Plan indoor sightseeing: museums, shopping malls, indoor markets in winter Delhi or haze- season Singapore. Outdoor itineraries on peak-AQI days are degraded experiences.

Masks for chronic AQ

For chronic background air-quality exposure (as opposed to acute wildfire-smoke events):

• N95 / KN95 / KF94 / FFP2: filter 94-95 percent of PM2.5. Good fit essential. The standard traveller mask for AQI 150+.
• Cambridge Mask, Vogmask, MagicAir: branded commercial masks marketed for chronic-AQ use; performance similar to certified N95 if NIOSH-rated.
• Surgical and cloth masks: minimal protection. Useful only for very-short exposure or as comfort.
• Mask hygiene: replace N95 every 8-40 hours of wear depending on conditions; or daily for chronic-AQ use.
• Fit checks: blow out; if glasses fog or you feel air around the nose bridge, the seal is wrong. Most masks fit poorly straight from the package; adjustment required.

Vulnerable groups

• Children: smaller airways, higher ventilation rate per kg. Threshold for action one AQI category earlier than adults. Delhi paediatric ER admissions correlate strongly with peak-pollution days.
• Elderly: existing cardiopulmonary comorbidities; reduced reserve.
• Asthma and COPD: any AQI elevation may trigger; carry rescue inhaler; pre-medicate before high-AQ outdoor activity.
• Cardiovascular disease: PM2.5 acute exposure increases MI and stroke risk within hours.
• Pregnancy: emerging evidence of adverse-outcome association. WHO and CDC treat as sensitive group.
• Long-term travellers (digital nomads, expats): chronic-exposure risks accumulate over months. Health- screening before extended stays in high-AQ cities.

Country brief

• India: Delhi winter November-February PM2.5 500+; severe respiratory hazard. CPCB SAFAR readings; consider south India alternatives.
• China: Beijing winter improved but still elevated; MEE readings; air-quality apps work (with VPN).
• Singapore: NEA PSI; September-October haze.
• Malaysia: APIMS; same haze season.
• Thailand: Bangkok and Chiang Mai January-April peak; PCD readings.
• Indonesia: BMKG; Sumatra and Kalimantan peat fires September-October.
• South Korea: AirKorea; spring yellow-dust peaks; KF94 standard.
• Egypt: Cairo year-round elevated; winter inversion peaks.
• Mexico: Mexico City winter inversions; SIMAT readings; substantial improvement from 1990s.
• United States: EPA AirNow; West Coast wildfire summer; East Coast sporadic 2023+.

One more time

AQI scales differ by country; default to U.S. EPA scale or raw PM2.5 µg/m³ for traveller decisions. Mask at AQI 150+; indoor preference at AQI 200+; consider postponing or leaving at AQI 300+. HEPA purifier in the sleep room. N95-grade mask with good fit; surgical and cloth masks do not protect. The Field Manual’s smoke-days guide covers acute wildfire-smoke events specifically; the heat-dome guide covers urban heat as the parallel environment-and-health challenge.