What pollution does to sunlight and why vitamin D levels are crashing across Indian cities – Firstpost

As northern India battles yet another season of toxic winter smog, health experts are warning of a lesser-known but increasingly urgent consequence: the region’s severe air pollution is now undermining the population’s vitamin D levels and with it, long-term bone health.

The respiratory dangers of smog have long been documented but emerging research indicates that the crisis extends beyond lungs and airways. Scientists are now drawing a direct connection between extreme pollution and reduced exposure to ultraviolet B (UVB) radiation, the essential wavelength that enables the skin to synthesise vitamin D.

A study published in The Lancet Planetary Health reports that fine particulate matter such as PM2.5 can cut ground-level UVB radiation by up to 60% in highly polluted urban centres. This finding is particularly alarming in India, where ICMR data shows that 75–90% of the population is already deficient in vitamin D, with levels dropping even further during the winter.

Taken together, doctors say the convergence of thick smog, dimmed sunlight, prolonged indoor confinement and heavy winter clothing has created a “perfect storm” for worsening vitamin D deficiency across North India, with Delhi-NCR standing out as the most at-risk region.

Smog blocks UVB, raising vitamin D deficiency risks

According to Dr Ashwani Maichand, Director of Orthopaedics at CK Birla Hospital (Delhi) the connection between air pollution and vitamin D deficiency is now undeniable. He said that rising smog levels particularly during winter can significantly increase the risk of vitamin D deficiency and bone health problems.

“UVB radiation from sunlight is the most natural and effective way for the skin to produce vitamin D, but heavy smog acts as a barrier, limiting the amount of UVB that reaches the ground,” Dr Maichand said. “This reduction disrupts the body’s ability to produce enough vitamin D, which is crucial for calcium absorption, bone strength, immunity and overall metabolic health.”

Research cited by Dr Maichand shows that people living in heavily polluted regions tend to have lower vitamin D levels than those in cleaner environments. Pollutants such as PM2.5, PM10, nitrogen oxides and ozone scatter and absorb UVB radiation, leaving the body unable to synthesise adequate vitamin D even during peak sunlight hours.

“This issue is especially worrying in places like Delhi-NCR, where chronic air pollution is common in winter. People often stay indoors to avoid the smog, which further limits their sun exposure,” he said.

Bone health at risk: From fatigue to fractures

Dr Maichand warned that reduced UVB exposure due to smog can trigger a cascade of health issues. Prolonged vitamin D deficiency can weaken bones, reduce muscle strength, increase the risk of fractures and lead to rickets in children. Older adults who are already vulnerable to osteoporosis may experience accelerated bone loss because of pollution-induced vitamin D deficiency, he added.

He also said that the immune system, mood levels, inflammatory responses and overall energy levels can be affected. “Low vitamin D levels are associated with a weakened immune system, increased inflammation, fatigue and mood disturbances,” he said.

A ‘triple barrier’ to vitamin D absorption in winter

According to Dr Maichand, the problem intensifies because smog in northern India coincides with colder temperatures. People naturally spend more time indoors, and when outside, they wear multiple layers of clothing that block sunlight from reaching the skin. The combination of pollution, reduced outdoor activity and winter clothing forms what he called a “triple barrier” to UVB absorption.

“In such conditions, vitamin D deficiency in winter becomes almost unavoidable unless people consciously take preventive measures,” he said.

Who is most vulnerable?

While the entire population is affected to some degree, some groups face much higher risks. Dr Maichand said children, older adults, pregnant women, office workers who spend most of their time indoors and people with existing fatigue or chronic pain are more susceptible to pollution-linked vitamin D deficiency. He stressed the need for regular vitamin D testing for these groups during high-pollution months.

Dietary support and lifestyle changes are essential

Dr Maichand recommended increasing the intake of vitamin D–rich foods such as fortified milk, egg yolks, mushrooms, fatty fish like salmon and mackerel and fortified cereals during winter. Supplements may also be necessary if recommended by a doctor, he said.

He added that maintaining adequate calcium intake and staying physically active particularly through weight-bearing exercises is essential for supporting bone health during the winter months.

“Short, safe sun exposure during clearer times of the day can help, but it may not be enough in extremely polluted conditions. That’s why dietary measures, supplements when needed and lifestyle adjustments become essential,” he said.

Global evidence is strengthening the pollution–vitamin D link

Several studies from China, South Korea, the West Asia and Europe have documented links between high levels of air pollution and reduced vitamin D levels. A Korean study published in Environmental Health Perspectives found that every 10 μg/m³ increase in PM2.5 concentration was associated with a marked drop in serum vitamin D levels in adults.

In Beijing, researchers observed that children had significantly lower vitamin D levels during the city’s worst smog episodes. Similarly, a study from Saudi Arabia, a region with abundant sunshine, demonstrated that air pollution was one of the key predictors of vitamin D deficiency despite high solar exposure.

These findings align with observations from Indian doctors, who say the phenomenon is increasingly visible in Delhi, Gurugram, Noida and Lucknow.

A growing public health concern

Public health experts argue that the issue deserves far more attention. India’s massive vitamin D deficiency burden intersects with rising pollution and sedentary lifestyles, posing long-term risks for population-level bone health and chronic disease.

Urban planners and health policymakers are urging cities to integrate UVB access into environmental and health strategies, especially during the winter months when pollution levels peak.

Some global cities have already begun issuing winter vitamin D advisories along with air-quality alerts, an approach experts say India may also need to adopt.

According to Dr Maichand, awareness remains the first line of defence. “People need to understand that pollution doesn’t just harm the lungs. It can silently weaken bones, lower immunity and affect long-term health,” he said.

He emphasised routine vitamin D evaluations, especially for high-risk groups and timely nutritional and lifestyle interventions.

“Smog significantly raises the risk of vitamin D deficiency and related bone problems by blocking UVB radiation. Awareness, preventive actions and timely medical guidance are crucial to tackle these growing winter health challenges in polluted areas,” he added.