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Delhi’s toxic air pushed Indian scientists to attempt something extraordinary — making it rain. On October 28, the city carried out its first phase of artificial rain trial, firing silver iodide flares into the clouds in the hope that the chemical would trigger precipitation and wash away some of the pollution choking the capital. While the exercise is claimed to bring notable reduction in air pollution level, it did raise a familiar concern: what happens when a chemical is sprayed into the sky above a crowded city? Experts, however, say the science — and evidence — should allay fears. “Cloud seeding is a technique used to induce artificial rain by adding certain chemicals to clouds,” explained Dr Rajeev Jayadevan, convener of the research cell, Kerala State Indian Medical Association, and Chairman of the Scientific Committee, IMA Cochin. “When we hear about planes depositing a chemical in the clouds above to create artificial rain, it is natural to feel concerned about its effects on our own health and environment. However, it is reassuring to know that silver iodide being an insoluble compound, will not accumulate in natural environments. It is also not toxic to man for the same reason.” The compound, a yellowish insoluble salt, is commonly used for glaciogenic cloud seeding — the “cold” method — across the world. “Multiple scientific studies — from the World Meteorological Organisation and the World Health Organization (WHO) — have looked for potential harmful effects and found none so far,” said Dr Jayadevan. “Besides, the amount deposited into the cloud is also too small to accumulate significantly in the ground or water far below.” In Delhi’s case, silver iodide was used sparingly — just a few kilograms dispersed across the NCR skies. How Silver Iodide Behaves in the Environment “Almost all recent experiments indicate that the silver in cloud seeding finds its way into rainwater in very low concentrations, far below those harmful to human beings,” Dr Tushar Tayal, associate director, internal medicine, CK Birla Hospital, Gurugram told News18. “Silver iodide is insoluble, so a majority of it precipitates as particles in water or soil instead of being absorbed by the body.” However, he added a note of caution. “When cloud seeding is repeated over the same area, silver can gradually build up in the environment. This is expected to impact aquatic life forms, soil microorganisms, and plant growth, as silver can disrupt photosynthesis and root growth at higher doses.” For humans, the risks are minimal — “any risk would be primarily through long-term accumulation in crops or water, but as of now, measurements indicate these quantities are too negligible to be dangerous,” Dr Tayal said. Who Is Most at Risk “The maximum effect of silver iodide is observed within the environment and not directly on individuals,” Tayal noted. “Fish, small aquatic animals, and microorganisms in the soil are more sensitive since silver will influence their growth and reproduction even at extremely low concentrations if it accumulates in the long term.” Short-term exposure — through inhalation or rainwater — is considered harmless. However, Tayal pointed out, “Exposure to high or chronic levels of silver (not from cloud seeding but from medical or industrial exposure) has been associated with a very rare disorder known as argyria, where the skin is bluish-grey because of silver deposition in tissues.” Also, those with chronic kidney disease or who live in areas where seeding happens repeatedly could be observed more closely, since their bodies may process silver differently. “At extremely high occupational exposures, experts said, the chemical also affects the liver, kidneys, and lungs. Individuals with existing kidney issues or who reside in locations where cloud seeding occurs regularly should be kept under closer observation, as their bodies might process silver differently.” What the Data Shows Global safety reviews back this up. The US Environmental Protection Agency and the World Health Organization both note that post-seeding silver levels in rain or soil typically measure below 0.01 micrograms per litre — far below the WHO’s drinking water safety threshold of 100 micrograms per litre. “Standard cloud seeding with silver iodide, when done per current protocols, is not shown to pose a significant long-term risk to humans, animals, or aquatic life, though environmental caution and monitoring continue to be recommended,” said Dr Sudhir Kumar, a neurologist at Apollo Hospitals, Hyderabad. “Most toxic effects occur with long-term or repeated exposures to silver, and one time or infrequent exposures generally have not been shown to cause adverse effects.” He added that silver ions, not silver iodide, are the main concern in aquatic ecosystems — “most aquatic toxicity data uses highly soluble silver salts like silver nitrate, not silver iodide, which is much less soluble and less bioavailable.” The Bottom Line Delhi has already used silver iodide once this season — but the quantities were small, and the risk to health or the environment remains negligible. As experts point out, the compound is not toxic, does not dissolve easily, and does not accumulate significantly in the short term.
