Picture a grey box on a pole beside the A4 near Heathrow. Inside, a pump draws air through a sensor all day. It counts one gas in particular: nitrogen dioxide, a brown gas that engines make, which hurts lungs. The law says the average over a year must stay under 40 micrograms in each cubic metre of air.
Engine makes the gas→It spreads and fades→A station reads it→Compared with 40
Carbon is counted; dirty air is measured. The difference matters. Carbon warms the planet wherever it is made, so a tonne is a tonne. Nitrogen dioxide hurts the people breathing it, here, today, so what matters is how much is in the air at a given street. That is a reading, not a sum.
Station reading32 micrograms per cubic metre
Legal line40 micrograms per cubic metre
A station near Heathrow reads a yearly average of 32, a made-up round number. The legal line is 40. What does that reading tell you?
The air is clean.
Not yet. Inside the legal line is not the same as clean; health bodies set lower lines than the law does. The reading says the law's line was met, which is what it measures.
The air at that spot was inside the legal line for the year.
Right. A reading is a comparison with a line at one place. Thirty-two is under forty, at that station, over that year. It says nothing about a street two miles away.
The airport made 32 tonnes of the gas.
Not yet. A reading is not a count. It says how much gas was in each cubic metre of air at the station, not how many tonnes anyone made.
Airport-related traffic11 in 100, Heathrow's upper figure
Everything else, mostly roads89 in 100, Heathrow's upper figure
Heathrow says road traffic as a whole is the main source of the gas in the streets around it, and that airport-related traffic makes 3 to 11 in 100 of local air pollution. Aircraft matter most right at the airfield, and their share falls quickly with distance.
Move the control away from the airport. The frames are from a 2024 study of six European airports, Heathrow among them, so this is an average shape, not a Heathrow reading: the share of nitrogen dioxide that came from aircraft.
From aircraft38 in 100
From everything else62 in 100
Distance from the airport: At the airport
At the airport fence: 38 in 100 of the gas is from aircraft, on the study's average.
From aircraft19 in 100
From everything else81 in 100
Distance from the airport: 2.6 km
2.6 km away: 19. The share halves about every 2.6 km.
From aircraft10 in 100
From everything else90 in 100
Distance from the airport: 5.2 km
5.2 km: about 10.
From aircraft5 in 100
From everything else95 in 100
Distance from the airport: 7.8 km
7.8 km: about 5.
From aircraft2 in 100
From everything else98 in 100
Distance from the airport: 10.4 km
10.4 km: about 2. Nearly all of what is left is roads and heating.
A campaigner three miles from the airport blames the planes for the street's reading. What does the study's shape say?
None of it. Aircraft gas never leaves the airfield.
Not yet. It does leave, and it fades. At the fence it is more than a third of the total. The point is the fading, not an absence.
Mostly the planes. The airport is only three miles off.
Not yet. Three miles is two halvings. The aircraft share at the fence is 38; two halvings down it is near 10. Most of the gas on that street came from closer engines.
At that distance aircraft are roughly a tenth of it. The street's own traffic is most of the rest.
Right. The share halves every 2.6 km, so at nearly five km it is about 10 in 100. The planes are real and small there; the cars outside the door are the bigger part.
Move the control.
20+ stationsinside legal lines
++
Cleaner carsthe big reason
Heathrow runs five stations and reads from over twenty nearby. Its report says every key pollutant is now inside the legal lines and has fallen for a decade, mostly because cars across the country got cleaner. Its own levers are smaller: cleaner engines, planes plugged in at the gate, and from January 2026 a ban on dirty vehicles airside.
What share of Heathrow's aircraft met the latest engine standard for this gas in 2024, up from 35 in 100 in 2019?
Pick one. Nothing is scored.
201935 in 100 of aircraft
202462 in 100 of aircraft
About 60 in 100.Sixty-two in 100 in 2024, from 35 in 2019. Heathrow charges less to land a cleaner engine, which is the lever an airport has over planes it does not own. Guesses tend to land on nearly all, because the fleet looks new.
Each is a source of air pollution near Heathrow. Does it mostly affect the air right at the airport, or the air on the streets around it?
One card at a time. Tap the pile it belongs to.
Card 1 of 6
Jet engines idling at the gate
Taxis queuing on the terminal road
Aircraft at full power on the runway
Lorries on the M4 and M25
Diesel baggage tugs on the apron
Gas boilers in nearby homes
Jet engines idling at the gate → Mostly at the airport
Not yet. At the airport. The exhaust is at the stand, and the gas fades fast with distance from it.
Taxis queuing on the terminal road → Mostly on the streets
Not yet. Streets. Those taxis also drive the roads around the airport, which is where most people breathe them.
Aircraft at full power on the runway → Mostly at the airport
Not yet. At the airport. The engines are at full power on the runway, and the gas is made there.
Lorries on the M4 and M25 → Mostly on the streets
Not yet. Streets. The motorways carry far more traffic than the airport does, and they run through the neighbourhoods.
Diesel baggage tugs on the apron → Mostly at the airport
Not yet. At the airport. Tugs never leave the airfield, which is why the airside ban from 2026 targets them.
Gas boilers in nearby homes → Mostly on the streets
Not yet. Streets. Heating makes nitrogen dioxide too, and it is made inside the neighbourhoods themselves.
3At the airport
vsvs
3On the streets
Right. Three sources sit on the airfield and fade before the houses. Three sit in the streets, where people live. The airport's levers touch the first pile; the second is the bigger one, and it is mostly cars.
Bus depot
↓→
School, half a mile awayreading?
A town wants to know if its new bus depot is dirtying the air at a school half a mile away. Which number answers it?
The depot's carbon footprint in tonnes.
Not yet. Tonnes of carbon warm the planet and say nothing about the air in one playground. Carbon is counted; this question needs air measured.
A yearly average reading at the school, compared with the legal line, before and after the depot opened.
Right. Dirty air is a reading at a place over time. Before and after at the school, against the line, is the measurement the town's question actually asks for.
The number of buses.
Not yet. More buses is a cause, not a reading. Whether the school's air changed depends on distance, wind and the buses' engines, which only a station can say.
Lesson complete
Dirty air is measured at a station and compared with a legal line, and it fades fast with distance.