Hidden Hazards in the Air: Why New Delhi’s Pollution May Be Worse Than We Think

Hù Nivicon

27-03-2025

“One day, he opens the cage and releases the bird. Rejoiced, Nightingale whooshes out and up, breathing in every bit of fresh air and clear sky”

In “Dream”; Wild Wise Weird [1]

New Delhi is known for having some of the worst air quality in the world, with fine particulate matter (PM₁) linked to over 10,000 premature deaths each year [2,3]. Yet a new study suggests that the true extent of this pollution has been significantly underestimated—due to a scientific blind spot involving humidity.

In this research, Chen [4] reveals that PM₁ particles in Delhi are highly hygroscopic, meaning they absorb water from the air and grow in size. This swelling causes many particles to become too large for standard air monitors to capture, leading to underreporting of pollution levels. The problem is most acute during the city’s humid winter mornings, when PM₁ readings may be underestimated by up to 20%, or as much as 50 micrograms per cubic meter.

Drawing on a year of detailed observations at a New Delhi monitoring site, the study is the first in India to quantify this sampling bias. It shows that the underestimation increases both with humidity and pollution levels, particularly when atmospheric chloride—largely from human activities like biomass burning—is abundant [5]. In contrast, during the monsoon season, rainfall removes these pollutants from the air, reducing both actual pollution and the extent of measurement bias [4].

These findings have serious implications. If pollution is routinely underestimated, public warnings may fall short, and policies may fail to protect vulnerable populations. The study also highlights a broader lesson: human impacts on the environment are often more complex—and more hidden—than they appear.

As we seek to cultivate our Nature Quotient (NQ) and address the global air quality crisis, improving the accuracy of our monitoring tools is essential [6]. This work underscores the need to better understand the nature-human nexus in a changing, urbanizing world [7].

references

[1] Vuong QH. (2024). Wild Wise Weird. https://www.amazon.com/dp/B0BG2NNHY6/

[2] Chowdhury S, Dey S. (2016). Cause-specific premature death from ambient PM2.5 exposure in India: Estimate adjusted for baseline mortality. Environment International, 91, 283-290. https://doi.org/10.1016/j.envint.2016.03.004 

[3] Chen Y, et al. (2020). Local characteristics of and exposure to fine particulate matter (PM2.5) in four indian megacities. Atmospheric Environment: X, 5, 100052. https://doi.org/10.1016/j.aeaoa.2019.100052 

[4] Chen Y. (2025). Air pollution in New Delhi is more severe than observed due to hygroscopicity-induced bias in aerosol sampling. npj Clean Air, 1, 1. https://www.nature.com/articles/s44407-024-00001-6 

[5] Zhang B, et al. (2022). Global emissions of hydrogen chloride and particulate chloride from continental sources. Environmental Science & Technology, 56(7), 3894-3904. https://doi.org/10.1021/acs.est.1c05634 

[6] Nguyen MH. (2024). How can satirical fables offer us a vision for sustainability? Visions for Sustainability. https://ojs.unito.it/index.php/visions/article/view/11267