Are you living in Delhi's high-rise buildings? IIT has this 'deadly' air pollution warning for you as region continues to choke every year

For years, Delhi’s air quality narrative has been shaped by surface-level monitoring, but new data suggests this only tells half the story. Researchers at IIT Delhi conducted a five-day study in March 2021 using drones to measure the vertical profile of fine particulate matter up to 100 metres. Published in the journal npj Clean Air, the findings suggest that residents living in 28–30-storey buildings are exposed to far more toxic air than official data indicates, particularly during humid, early-morning haze conditions.

While surface readings were lower, PM2.5 levels rose sharply with altitude, reaching 160 micrograms per cubic metre at 100 metres—a 60% increase compared to ground readings on the same day. This indicates that the air at the top of a 30-storey building can be vastly more hazardous than the air on the sidewalk.

Living at top floors in Delhi

The study attributes these elevated high-altitude concentrations to a phenomenon called a shallow atmospheric boundary layer. In the early morning, stable and humid conditions trap pollutants in a narrow band near the surface. When relative humidity exceeds 70%, hygroscopic inorganic aerosols absorb moisture and grow, intensifying the haze. This process, combined with low wind speeds of less than 2 metres per second, prevents pollutants from dispersing, causing them to accumulate and form secondary particles at specific heights before the sun can warm the atmosphere and expand the boundary layer.

Data Divergence

The drone measurements highlighted how volatile Delhi’s air can be at different elevations. On March 20, researchers observed a sharp inversion layer at 70 metres, which caused a 30% jump in PM2.5 levels within a mere 20-metre vertical span. However, the study also noted the dramatic cleaning effect of natural interventions. Following a major rain event prior to March 23, surface PM2.5 levels plummeted to 40 micrograms per cubic metre—a significant 60% drop from the peak levels recorded earlier in the week.

Addressing the Gap

The core takeaway from the "Drone measurements reveal high near surface urban haze" report is that conventional surface stations capture only a partial picture of the city's toxic air. Because high chloride and nitrate content drives secondary particle formation at higher altitudes, current monitoring infrastructure may be underestimating the health risks for millions of urban dwellers. The researchers suggest that future air quality management must account for this vertical distribution to better protect residents in the city’s increasingly vertical landscape.

(With TOI inputs)