While rain is understood to wash away pollution, as evident from Delhi’s ‘satisfactory’ air quality in July, a new study has found a troubling twist.
According to new research published in the journal Environmental Advances, clouds are ferrying heavy metals from the polluted lowlands to ecologically sensitive regions, such as the Eastern Himalayas and the Western Ghats, posing a serious risk to human health.
The heavy metals transported include Cadmium (Cd), Nickel (Ni), Copper (Cu), Chromium (Cr), and Zinc (Zn), presenting risks of both carcinogenic and non-carcinogenic diseases and even developmental disorders, according to scientists from the Bose Institute in Kolkata.
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For the study, the researchers investigated metals in non-precipitating low-level clouds collected on the rooftop of the Indian Institute of Tropical Meteorology (IITM) campus at Mahabaleshwar in the Western Ghats, and the Bose Institute campus located at Darjeeling in the Eastern Himalayas.
Samples were collected from late summer to the onset of the monsoon in 2022 over the two sites. Notably, monsoonal clouds enter India through the Western Ghats and then reach the Eastern Himalayas via the Bay of Bengal.
Entry route and health implications
“Airborne metals like Cd, Ni, and Cr are recognised as major threats to human health due to their toxicity, poor biodegradability, and tendency to bioaccumulate,” the scientists noted.
“The toxicity of most airborne metals arises from their ability to generate reactive oxygen species (ROS), inhibit enzyme activities, and disrupt the antioxidant defence system. These processes result in structural and functional damage to DNA, leading to mutations, cancer development, apoptosis, and alterations in the cell cycle.”
“Clouds are reported to enhance ROS generation under sunlight through the photolysis of metals,” the experts noted.
Notably, the health implications of the heavy metals happen through inhalation, ingestion, and dermal absorption. The health implications are also dependent on the entry route.
"For instance, Ni and Cr are known to cause allergic contact dermatitis through dermal absorption, while Cd, through ingestion routes, leads to osteoporosis, osteomalacia, and kidney dysfunction. Inhalation of Cr is mainly associated with non-carcinogenic diseases like asthma, pneumonia, and acute bronchitis,” the group of researchers, led by Dr Sanat Kumar Das, Associate Professor, Bose Institute, noted.
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Not only that, the prolonged inhalation of metals such as Cr, Cd, and Ni increases the risk of lung cancer. On top of that, excessive exposure to such metals has been linked to chronic diseases of the lungs, liver, and kidneys, along with asthma, encephalopathy, and cardiovascular diseases.
"Accumulation of Ni in the body leads to several diseases such as lung fibrosis, kidney and cardiovascular infections, and malignant growth of the respiratory tract,” they noted.
Children were found to be at a greater risk of the metals with an increased risk of lung cancer, cardiovascular diseases, and “mental retardation.”
What’s the source?
The study noted that the metals were carried to the mountains via clouds from lowland areas.
Here, anthropogenic activities such as traffic, fossil fuel consumption, and urban waste incineration are to be blamed as they contribute significantly to metal concentrations in clouds.
“Road dust and soil erosion are also significant contributors to the accumulation of metals in clouds,” the researchers noted. Naturally, “clouds over polluted urban areas generally contain higher metal concentrations than those over unpolluted rural areas.”
Also, non-precipitating clouds typically have higher metal concentrations than precipitating ones.
Having said that, tourism needs to be factored in as many travel to the mountains not only for their scenic beauty, but also assuming that the mountains are less polluted and the clouds are cleaner.
“Monsoonal clouds attract tourists due to the scenic beauty of the mountainous rainforests. As a result, Mahabaleshwar experiences large traffic movements and other local anthropogenic activities during the monsoon season,” the scientists explained.
“On the other hand, Darjeeling is also a popular tourist spot experiencing air pollution primarily from vehicular emissions, biomass burning, and emissions from the iconic Toy Train (Darjeeling Himalayan Railway), run by coal and diesel,” they added.
Clearly, the study questions the idea that the mountain air is clean. It, in fact, furthers the fact that pollution isn’t contained within a geographical area, can travel far and wide, and has implications on both ecology and human health.
