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Delhi’s toxic air is as bad indoors as it is outdoors
HT tracked pollution levels not only outdoors but inside spaces where vulnerable populations spend much of their day.
Millions of Delhi residents cling to a comforting belief: that shutting the door is enough to keep the city’s poisonous air at bay. But a week-long field experiment by Hindustan Times has punctured that illusion. It showed that the boundary between outdoor and indoor air is porous, fragile, and often meaningless. It found that polluted outdoor air is seeping indoors with limited attenuation, entering through ventilation systems, open doors and windows, or simply through the everyday leakage of buildings not designed to withstand such extreme environmental stress.
More troublingly, it shows no evidence that institutions like hospitals and schools – where those most vulnerable spend their days – act as protective buffers.
To understand just how severe this exposure is, Hindustan Times tracked pollution levels not only outdoors but inside spaces where vulnerable populations spend much of their day.
Over a week from January 14 to 20, three reporters logged PM2.5 levels at the same time each day — between 10am and 11.30am — at a private school, a major hospital and a residential home in different parts of the city. At all three locations, pollution levels exceeded national safety limits by as much as seven to eight times nearly throughout the week.
The data showed that unless one is inside a sealed, sanitised room with a high-efficiency air purifier running continuously, there is no real escape during severe pollution episodes. Schools, hospitals and waiting areas — spaces meant to protect children, newborns and the sick — are, in many cases, breathing almost the toxic haze as those beyond their walls.
To establish a control environment, an air purifier was kept running in a room inside a Vikaspuri house. There, PM2.5 levels dropped as low as 18 micrograms per cubic metre (µg/m³) – comfortably under India’s national safe limit of 60µg/m³ and the World Health Organization’s guideline of 15 µg/m³.
In stark contrast, readings at the school and hospital — both without purifiers — routinely hovered between 200 and 300 µg/m³, even indoors. At one point, an outdoor reading of 502µg/m³ was recorded outside the school, eight to nine times India’s relatively lax safe standards.
The measurements were taken using pocket monitors from PranaAir, pre-calibrated for the exercise and capable of recording PM2.5, PM10 and PM1 in real time. The monitors were allowed to stabilise before readings were logged, ensuring consistency. The school chosen was in Rohini’s Sector 5, while hospital readings were taken at the All India Institute of Medical Sciences (AIIMS), Delhi – specifically in the cancer ward and the mother and child ward. The objective was simple: to capture the real-life impact of Delhi’s air on those least equipped to cope with it.
At the school, outdoor PM2.5 levels at the playground and parking area remained extremely high throughout the week. The lowest recorded was 246 µg/m³ – over four times the safety threshold. The worst day saw levels soar to a whopping 502 µg/m³. Inside classrooms, the story barely improved. On January 15, when outdoor levels were at their lowest for the week, indoor PM2.5 still stood at 196 µg/m³, over three times the safe limit. On January 19, when pollution outside peaked, classroom air registered a PM 2.5 level of 432 µg/m³ -- an astonishing 7.2 times above what is considered safe.
The air at AIIMS, where some of the city’s sickest patients are treated, fared similarly poorly. Inside the cancer ward, PM2.5 levels hovered largely between the mid-200s and high-200sµg/m³, occasionally crossing 300µg/m³. The lowest reading recorded during the week was 193µg/m³, while the highest within a ward touched 328µg/m³. Particularly sharp spikes were seen in crowded OPD areas for cancer patients, where levels climbed as high as 431µg/m³.
The mother and child ward fared just as bad. PM2.5 levels again remained in the 200-300µg/m³ range across the week. Even deeper interiors – waiting areas and main halls – failed to show a meaningful drop, suggesting that polluted air was circulating freely through the building. Day-to-day fluctuations were also modest at best and consistently matched the readings at the cancer ward – suggesting the entire complex breathed the same air.
By contrast, the home environment with an air purifier told a different story. PM2.5 readings there largely stayed between 20 and 40 µg/m³, dropping as low as 18µg/m³. On January 16, the daily average stood at just 27.8µg/m³ – dramatically lower than what we saw elsewhere.
In terms of days, the worst readings coincided with January 17 to 19, when Delhi’s air quality index (AQI) was 400+ for three consecutive days during the city’s worst January spell since 2021. The average AQI stood at 400 on January 17, rose to 440 on January 18, and was 410 the following day.
Experts said the findings reinforce a hard truth: outdoor air is penetrating indoor spaces with limited attenuation – either through ventilation, open windows, or general building leakage – which means that being indoors does not meaningfully insulate anyone from poor air.
“We often see that due to higher concentrations outside, there is diffusion indoors,” said Dr Rakesh Kumar, president of the Society for Indoor Environment. While air purifiers can help, he cautioned they are only a temporary fix. “Even when used, gases like CO2 can spike due to poor ventilation,” he said, calling the school and hospital readings “alarming”.
In schools, he noted, open doors and windows often allow outdoor air to mix freely with indoor spaces. In hospitals, the implications are even more serious. “A proper HVAC system can help, but we often see filters that haven’t been cleaned or replaced for years, which leads to extremely high pollution,” Kumar said.
Environmental activists described the findings as nothing short of a health emergency. “As parents we used to believe closing doors would keep our families safe. But that’s not the case… none can escape polluted air. This is a reminder that even hybrid schooling does not make children safe… Pollution is so high, indoors and outdoors, that the only way to escape it is to leave the city,” said Bhavreen Kandhari of Warrior Moms.
Others emphasised that indoor fixes can’t save people from the worst of the air. “Hospitals for instance should see the highest protection as patients are at their most vulnerable… The only solution remains is to cleaning our outdoor air and Delhi’s air is seemingly not improving,” said Sunil Dahiya, an air pollution expert, part of the think-tank Envirocatalysts.
With inputs from Aheli Das and Aadtiya Khatwani
