A brown lid on the horizon is easy to photograph and easy to misread. That haze is an average of a basin, a wind, a morning commute. The air at your mouth is local: the junction, the courtyard, the twelfth floor, and the kitchen are four different rooms. The city does not share one air.
Smog is the visible version of that fact — particles and gases piled up until the light goes yellow. It is not a weather event. Weather only decides whether the mess can leave. What you are breathing is traffic, heating, industry, dust, and chemistry that started a few hours upwind.
Two smogs, and a list that is not smog
The word covers two different atmospheres. Classic, “London” smog is sulfurous: coal or heavy fuel, sulfur dioxide, fog, and a winter inversion that holds the mix at street height. The Great Smog of 1952 is the textbook. It is rarer in cities that left coal, and still ordinary where winter heating is solid fuel in a cold basin.
Photochemical, “Los Angeles” smog is cooked in sunlight. Nitrogen oxides from engines and VOCs from fuel, solvents and plants turn into ozone and secondary particles. It peaks on still, bright afternoons — often worse downwind of the core than in it. A mask that stops dust does nothing to ozone. You cannot smell the difference until your throat does.
Most days the lid is neither museum piece. It is PM2.5: soot, metals, and droplets small enough to ride deep into the lung, from exhaust, brakes, burning, cooking, and from gases that became particles overnight. Add NO2 at the kerb, ozone in the afternoon, and occasional SO2 or dust storms, and you have the urban mix. WHO’s 2021 guideline for annual PM2.5 is 5 µg/m³. A great many city stations never see that number.
The city does not share one air.
Where it actually sits
Inside one city the gradient is brutal. A street canyon — tall walls, slow wind, a bus lane — loads NO2 and ultrafines at the kerb and drops them a few metres in, or one floor up. The courtyard behind the same block can look like a different town. Highways and industrial edges write a second map on top of the first.
Basins and inversions do the rest. Cold air slides under a lid; the morning commute has nowhere to go. Winter cities with coal or wood heat sit in that trap for days. Summer cities cook ozone instead. Rankings of “the most polluted capital” go stale in a season. The pattern does not: dense traffic, poor dispersion, and something burning.
Indoors is not automatically safer. A gas hob, a pan of oil, a cigarette, incense — with the windows shut on a bad outdoor day — can beat the street. The reverse is also true: when the outdoor reading is high, opening up to “air the room” imports the episode. You have to know which side of the glass is worse right now.
What the numbers are counting
AQI is a scale, not a substance. The same micrograms colour differently on US EPA, China AQI, and European CAQI. “Unhealthy” on one ladder can be “moderate” on another. Pick the scale you already know and stay on it — then read the pollutant underneath the colour. A purple day driven by ozone is a different day from a purple day driven by PM2.5.
PM2.5 is the number most clearly tied to long-term harm. NO2 marks traffic. Ozone marks the photochemical afternoon. A station that only publishes a city-wide index is telling you about a basin, not a pavement. The useful question is: which station is near the air I will actually be in?
What to do today
When PM is high: skip hard outdoor exercise, especially along arterial roads; if you have to be out, a well-fitted FFP2 or N95 stops particles, not gases. Close the windows during the peak, then ventilate when the reading drops. Don’t fry with no hood and no crack of air — cooking is a source. A simple filter helps indoors; it does not replace knowing when to open the window.
When ozone is high: it is usually the afternoon. Exercise earlier. Masks will not help. Shade and less traffic help a little; the chemistry is regional. Wildfire smoke is PM, often for days — treat it like a long PM episode, not like a cloudy sky.
Don’t add to the pile if you have a choice: no trash or leaf burning, no idling, less solid fuel. The real levers are still policy — what the city burns, which engines sit in the canyon, how the basin is heated. Personal moves are damage control. They work better when they follow a local reading, not a national headline.
That is the whole habit: pin the stations you actually pass, open the map when you travel, and keep the scale you already understand. AQI Sense reads WAQI, Sensor.Community and OpenSenseMap — live, on iPhone — so the air you check is the air you are in, not the air the city averaged this morning.