Air pollution is a kerbside phenomenon measured city-wide

Treat the sections below as a sequence. With urban air quality, getting the early decisions right makes the later ones much easier.

Distance from the kerb changes exposure quickly

Concentrations of traffic-related pollutants are highest at the kerb and fall substantially within the first few metres. Walking on the building side of a wide footway rather than at the kerb edge measurably reduces exposure. Back streets parallel to a main road can show markedly lower concentrations, which makes route choice a meaningful personal lever.

None of this is captured by a city-wide average, which is the figure most public discussion uses.

Street canyons trap what is emitted

A narrow street lined with continuous tall buildings restricts the vertical mixing that would otherwise dilute emissions. A vortex can form within the canyon that recirculates air, concentrating pollutants on the leeward side.

This means two streets with identical traffic can have very different concentrations because of their proportions. Building form is therefore an air quality variable, which is rarely how it is discussed in planning.

Not all of it comes from exhausts

Brake wear, tyre wear and road surface abrasion generate particulate matter regardless of how the vehicle is powered. These non-exhaust sources have become a larger share of traffic particulates as exhaust standards tightened. Heavier vehicles generally produce more of them, which is relevant to the shift toward larger cars and battery weight.

In practice, electrification therefore addresses part of the pollution problem and not the whole of it.

Where you are at the wrong time matters most

Exposure is concentration multiplied by time, so commuting routes, school runs and outdoor work dominate personal exposure. Children in pushchairs sit closer to exhaust height than adults, which has been raised repeatedly as a design concern. Inside a vehicle in traffic, concentrations can be higher than on the adjacent footway because the car sits in the exhaust stream ahead of it.

This is one of the more counterintuitive findings and it has been reported in a number of measurement studies.

Barriers and planting have modest, real effects

Solid barriers between road and footway can reduce concentrations behind them and can also worsen conditions on the road side. Dense vegetation barriers can trap particulates and can also restrict airflow in a canyon, which makes results context-dependent. Evidence for street-level green barriers is genuinely mixed and depends heavily on the geometry and species involved.

Reducing emissions at source remains the intervention with predictable effects.

Local statute governs most of this, and it varies street by street.

Policy operates at a scale that hides the variation

Limit values are usually expressed as annual averages at monitoring stations, which is appropriate for regulation and poor for describing exposure. Diffusion tube networks and low-cost sensors give spatial detail at lower accuracy, and interpreting them requires care. Individual measures — school street closures, low emission zones, traffic reduction — act on the local concentration where people are.

Health advice for anyone with a respiratory condition should come from a qualified clinician rather than from monitoring data.

The takeaway

Move a few metres from the kerb. It is the cheapest air quality measure available to you.

Measure the walk, not the map.