Smoke days: when to leave a city

What AQI actually means

The Air Quality Index is a unit-free 0 to 500 number derived from the underlying pollutant concentration, usually PM2.5 (particles smaller than 2.5 micrometres) during wildfire smoke. The translation between AQI and PM2.5 in micrograms per cubic metre (µg/m³) matters because not all countries use the same AQI scale. The U.S. EPA scale and the WHO guidelines diverge enough to mislead travellers.

• U.S. EPA AQI 50 ≈ PM2.5 12 µg/m³ (24-hour average). Good.
• AQI 100 ≈ PM2.5 35 µg/m³. Moderate; sensitive groups should reduce outdoor activity.
• AQI 150 ≈ PM2.5 55 µg/m³. Unhealthy for sensitive groups.
• AQI 200 ≈ PM2.5 150 µg/m³. Unhealthy for everyone.
• AQI 300 ≈ PM2.5 250 µg/m³. Very unhealthy. Healthy adults’ lung function measurably drops within hours.
• AQI 500 ≈ PM2.5 500+ µg/m³. Hazardous. Comparable to standing downwind of a small fire.

The 2021 WHO guideline for PM2.5 is 15 µg/m³ over 24 hours. Almost all wildfire-smoke events exceed this for days at a time across affected regions. The 2023 Canadian wildfire season produced AQI readings above 400 across the US Northeast (New York, Boston, Washington DC). The 2025 Greek wildfire season produced sustained AQI 200+ across Athens. The recurring Indonesian peat-fire haze produces AQI 300 to 500 across Singapore, Malaysia, southern Thailand for weeks.

Health-based decision thresholds

The U.S. EPA category labels reflect what is actually known about health impact at each level:

• AQI 0 to 50 (Good): no health impact.
• AQI 51 to 100 (Moderate): people with asthma may notice mild irritation. Healthy adults unaffected.
• AQI 101 to 150 (Unhealthy for Sensitive Groups): children, elderly, asthma, COPD, cardiovascular disease. Sensitive groups should reduce prolonged outdoor exertion.
• AQI 151 to 200 (Unhealthy): healthy adults may experience irritation; sensitive groups should avoid outdoor exertion.
• AQI 201 to 300 (Very Unhealthy): serious health effects on healthy adults; sensitive groups should avoid all outdoor activity. Healthy adults’ lung function measurably drops within hours per the EPA cardiopulmonary exposure literature.
• AQI 301 to 500 (Hazardous): emergency conditions; everyone should remain indoors with filtered air.

The practical decision threshold for travellers without existing respiratory conditions: at AQI 150, mask outdoors; at AQI 200, stay indoors when possible; at AQI 300, consider leaving the city or postponing the trip if you have flexibility.

What masks actually do

• N95 / KN95 / FFP2 / KF94: filter at least 94 to 95 percent of particles 0.3 µm and larger. Effective against PM2.5 wildfire smoke when properly fitted (good seal around the nose and chin). The standard recommended choice for outdoor exposure above AQI 150.
• P100 elastomeric respirators: filter 99.97 percent. Overkill for most wildfire smoke but appropriate for prolonged exposure (multi-day high-AQI events for sensitive groups).
• Surgical masks: not designed for particle filtration. Provide minimal protection against PM2.5. Better than nothing but not adequate for AQI 200+ exposure.
• Cloth masks and bandanas: essentially no protection against PM2.5. Filter much-larger particles only.

Mask fit matters more than mask brand. A properly-fitted N95 outperforms a poorly-fitted P100. Air leak around the nose bridge defeats the filter. Common signs of poor fit: glasses fogging (air leaking up), feeling cool air on the cheeks during inhalation.

Indoor-air protection

Indoors during wildfire smoke is much safer than outdoors but not safe. Buildings exchange air with outside through mechanical HVAC and through infiltration leaks. The relevant interventions:

• Close windows and doors: the obvious first step. Reduces infiltration to whatever the building envelope leakage rate is.
• Set HVAC to recirculate, not fresh air intake: most central air systems can be set to closed-loop mode. Air conditioners with outside-air dampers should be set to recirculate.
• HEPA portable air purifier: the single most-effective intervention for a single room. Sized for the room (CADR rating must match floor area). For a standard hotel room, a small 200-CFM HEPA purifier reduces indoor PM2.5 by 80 percent within an hour.
• DIY: box-fan plus MERV-13 filter: ~USD 40, ~80 percent of HEPA-purifier performance for a single room. EPA has published instructions. Useful when caught without a real purifier.
• Wet towel under doors: reduces infiltration from external corridors and gaps. Marginal but cumulative.
• One clean room: if the whole accommodation cannot be filtered, set up one room (smallest, easiest to seal, with a HEPA purifier) as the safe sleep room.

Hotels in wildfire-prone regions (US West Coast, Australia, Greece, Türkiye) increasingly stock HEPA purifiers; ask at reception. The Marriott, Hyatt, and Hilton chains in California have policies addressing smoke events; smaller properties vary.

The 24-hour decision tree

When the AQI rises above 150 in a city you are visiting:

• Hour 0 to 6 (AQI 150 to 200): indoor preference. N95 outdoors for essential trips. Sensitive groups limit outdoor activity. Sightseeing degraded; outdoor dining unpleasant.
• Hour 6 to 24 (AQI 200 to 300): indoor stay. N95 mandatory for any outdoor minutes. Open windows closed; HVAC on recirculate; portable HEPA if available. Consider whether the trip’s value justifies the health cost. If the forecast shows clearing within 24 to 48 hours, hunker down indoors. If the forecast shows multi-day persistence, evaluate departure.
• Hour 24 to 72 (AQI 300+): depart if possible. Healthy adult lung function measurably drops; sensitive groups (asthma, COPD, cardiovascular, children, elderly, pregnancy) face elevated emergency-room risk. Cancellation insurance often activates at this threshold in major markets.

The forecast quality matters: EPA AirNow Fire and Smoke Map, BC AirQuality, IQAir, and PurpleAir all publish 12 to 48-hour forecasts. Smoke plumes are driven by wind patterns; conditions can shift dramatically in 6 hours.

Vulnerable groups

• Children: smaller airways, higher ventilation rate per kg body weight, less ability to mask properly. Threshold for action is roughly one AQI category lower than adults.
• Elderly: existing cardiopulmonary comorbidities; reduced reserve. Same one-category-earlier threshold.
• Asthma and COPD: any AQI elevation can trigger attacks; carry rescue medication; mask early.
• Cardiovascular disease: PM2.5 exposure increases acute MI and stroke risk within hours. AQI 100+ warrants caution.
• Pregnancy: emerging evidence of preterm-birth association with wildfire-smoke exposure; CDC and WHO advise treating as a sensitive group.
• Outdoor athletes: marathon training in AQI 200+ delivers the cardiopulmonary load of long-term heavy smoking. Postpone or move indoors.

Recurring smoke-affected cities

Wildfire-smoke exposure recurs predictably in certain regions and seasons:

• U.S. West Coast (San Francisco, Portland, Seattle, Los Angeles): July through October. Annual exposure since at least 2017; the August 2020 and August 2023 events produced sustained AQI 300+.
• U.S. Northeast and Canadian Maritimes: the June 2023 Canadian wildfire smoke event was the worst multi-day exposure in modern record for New York, Boston, Philadelphia. Future recurrence likely.
• Athens and Greek mainland: July through September. The 2018 Mati, 2021 Evia, 2023 Rhodes fires produced sustained smoke across the metro region.
• Istanbul and Turkish Aegean coast: July through September. The 2021 fires affected Antalya and Bodrum tourist zones.
• Sydney, Melbourne, Canberra: November through February (Southern Hemisphere fire season). The 2019-2020 Black Summer produced months of Cat-Hazardous air across Sydney.
• Singapore, Kuala Lumpur, southern Thailand: September and October. The recurring Indonesian peat-fire haze. 2015 was the worst recent year (PSI 471 in Singapore).
• Northern California Central Valley and Sacramento: August through October.
• Vancouver and Calgary: July through August. BC interior fires affect both cities recurrently.
• Santiago de Chile, Buenos Aires: January through March (Southern Hemisphere). Chilean Central Valley fires affect Santiago air quality.

Country brief

• United States: EPA AirNow Fire and Smoke Map is the practical authority; CARB for California-specific.
• Canada: BCCDC, Health Canada, and Air Quality Health Index.
• Greece: 2018 Mati, 2021 Evia, 2023 Rhodes reference events.
• Türkiye: 2021 Aegean coast fires; MGM weather service publishes AQI.
• Spain: Iberian peat and forest fires summer-autumn.
• Australia: BOM and state EPA monitoring; 2019-2020 Black Summer reference.
• Singapore: NEA PSI and PM2.5 readings; Indonesian peat-fire haze September to October.
• Malaysia: APIMS readings; same haze season.
• Indonesia: BMKG; Sumatra and Kalimantan peat fires the source.
• Chile: Central Valley fires affect Santiago; SHOA publishes AQ via SINCA.
• Portugal: IPMA and APA air-quality monitoring; recurring summer wildfire smoke.

One more time

AQI 150 mask outdoors. AQI 200 indoor preference. AQI 300+ consider leaving the city. N95 or KN95 with good fit; cloth masks do not protect against PM2.5. HVAC on recirculate; portable HEPA in a sealed safe room if available. Sensitive groups act one category earlier. Forecasts from EPA AirNow, BCCDC, IQAir, or PurpleAir help time the decision. The Field Manual’s wildfire survival guide covers fire-front response; this guide covers the smoke that remains for days or weeks afterwards.