Why Air Quality Changes from Street to Street in Kathmandu

1. One city does not breathe one kind of air

When people check an air-quality application, they usually see one number assigned to an entire city. That number is useful for understanding the general condition, but it can create the impression that everyone in the city is breathing the same air.

In reality, air pollution is uneven. A person waiting beside a congested intersection may experience different conditions from someone inside a residential courtyard only a short distance away. A school next to a major road may face a different exposure pattern from a school surrounded by open land.

This street-to-street variation is known as hyperlocal air pollution. It matters because people experience pollution where they actually live, walk, study and work—not at the location of a distant monitoring station.

2. Traffic creates local pollution hotspots

Busy roads are among the clearest sources of hyperlocal variation. Vehicles release pollutants through exhaust, while brake wear, tyre wear and resuspended road dust add more particles to the air.

Congestion can make conditions worse. Vehicles repeatedly accelerating, braking and idling concentrate emissions around intersections and narrow road corridors. Heavy diesel vehicles can make a disproportionate contribution to particulate matter and nitrogen oxides.

Research reviewed by the United States Environmental Protection Agency shows that roadway pollution is generally highest on or immediately downwind of busy roads. Concentrations usually decline with distance, although the pattern changes according to the pollutant, traffic volume, wind, terrain and surrounding buildings.

This means that a city-level AQI may correctly describe Kathmandu’s general air quality while still missing a temporary hotspot beside a crowded road.

3. Buildings can trap or redirect pollution

Kathmandu’s urban form also influences how pollution moves. In areas where tall or closely spaced buildings line both sides of a road, the street can behave like a canyon. Air circulation becomes restricted, allowing emissions to accumulate near pedestrians, shops and homes.

Buildings can also redirect wind. One side of a street may receive cleaner moving air while another side experiences slower airflow and higher pollutant concentrations. Courtyards, walls, flyovers and narrow lanes can further change the local pattern.

Open spaces generally allow pollutants to disperse more easily, although they are not automatically pollution-free. Their conditions still depend on nearby sources, wind direction and regional pollution.

Vegetation may help intercept some particles and separate people from traffic, but poorly positioned or extremely dense vegetation can sometimes restrict airflow. Urban design therefore needs to consider both greenery and ventilation.

4. Weather and terrain change the picture

Air pollution does not remain fixed after it is emitted. Wind speed and direction determine where pollutants travel. Stronger winds often disperse pollution, while calm conditions allow it to accumulate near the source.

Temperature inversions can trap polluted air close to the ground, particularly during cold and calm mornings. Kathmandu Valley’s surrounding hills can limit regional ventilation, making these conditions more persistent.

Humidity and rainfall also affect particulate pollution. Rain can remove particles from the atmosphere, while high humidity can change particle size and influence the readings of some optical sensors.

These factors explain why the same road may show different pollution conditions in the morning, afternoon and evening. They also explain why a hotspot can move as the weather changes.

5. Why a single monitoring station is not enough

Reference-grade monitoring stations are essential because they provide highly controlled and reliable measurements. However, they are expensive to purchase, operate and maintain. A small number of stations cannot describe every neighbourhood, roadside or industrial zone.

Low-cost sensors can help fill this spatial gap by increasing the number of measurement locations. They are particularly useful for identifying patterns, comparing locations and detecting possible hotspots.

However, low-cost sensors must be calibrated, maintained and interpreted carefully. Humidity, temperature, sensor ageing and particle composition can influence their readings. They should complement reference stations rather than replace them.

Satellite observations provide broader regional coverage but do not directly measure what an individual is breathing at street level. Weather data, land use, road networks, satellite observations and ground measurements therefore need to be combined.

6. From city averages to local intelligence

The next generation of air-quality services must answer more than “What is Kathmandu’s AQI?” It should help people understand where pollution is rising, when exposure is likely to be highest and what action they should take.

A locally useful system can combine ground sensors, reference monitors, weather conditions, satellite observations, traffic indicators and statistical or machine-learning models. Each source contributes a different part of the picture.

Ground sensors provide local measurements. Reference stations provide calibration and quality control. Satellites provide regional atmospheric context. Weather explains how pollutants are likely to move and disperse. Models connect these inputs to estimate conditions in places without a physical monitor.

This is the direction VayuDrishti is working toward: turning scattered environmental observations into practical intelligence for citizens, schools, hospitals, industries and cities.

Now act

Practical recommendations

• Check the citywide AQI before leaving home, but remember that roadside exposure may be higher than the city average.
• Avoid prolonged exercise beside congested roads, especially during morning and evening traffic peaks.
• Choose quieter side streets for walking or cycling when a safe alternative is available.
• Keep children’s play and school assembly areas as far from major traffic corridors as practical.
• Use local monitoring at schools, hospitals, construction sites and workplaces where decisions depend on conditions at the exact location.
• Compare readings over time instead of making decisions from one unexplained sensor value.
• Compare readings over time instead of making decisions from one unexplained sensor value.
• People with respiratory or cardiovascular conditions should follow guidance from qualified health professionals.