PUNE: That Delhi is breathing poison has been confirmed by yet another study, which has revealed that the national capital’s air has the highest concentration of mercury – a toxic metal linked to neurological, kidney and heart problems – among three Indian cities measured.A six-year study by Pune-based Indian Institute of Tropical Meteorology (IITM) has found that mercury levels in the air in Delhi, Ahmedabad and Pune are well above the natural background, with coal, traffic and industries identified as major contributors. There was a silver lining though – there were indications of a significant decline in mercury levels in Delhi during the period studied, compared with previous years.The global or natural background value of mercury in clean air is about 1.7 ng/m³ in the Northern Hemisphere and 1.3 ng/m³ in the Southern Hemisphere.The first-of-its-kind study – published recently in the journal Air Quality, Atmosphere & Health, by Springer – analysed data from 2018 to 2024 on gaseous elemental mercury, a key atmospheric pollutant. Researchers reported that Delhi’s air contained, on an average, 6.9±4.2 nanograms of mercury per cubic metre (ng/m³), compared to 2.1±0.7 ng/m³ in Ahmedabad and 1.5 ±0.4 ng/m³ in Pune.This means each cubic metre of air in Delhi – roughly the amount a person breathes in an hour at rest – carries around 7 billionths of a gram of mercury on an average, compared to about 2 billionths in Ahmedabad and 1.5 billionths in Pune. Delhi’s peak values were nearly 13 times higher than the global background level. The research team attributed 72%-92% of mercury in these cities to human activities. “Seasonal and nighttime spikes were especially prominent in Delhi and Ahmedabad during winter, linked to coal use, stubble burning, and stable atmospheric conditions,” a scientist from IITM said.“Mercury is among the top 10 chemicals of major public health concern, according to WHO. Even in small amounts, if exposure continues for 5-10 years, it can be dangerous. Long-term inhalation mainly affects the nervous system, digestive system, immune system, kidneys and lungs,” said Gufran Beig, chair professor at National Institute of Advanced Studies, IISc. The study also said Delhi’s mercury levels remained among the highest in south Asia.