Why Delhi's winter smog can leave you low on vitamin D

One in five Indians is low on vitamin D, and winter haze makes it worse. What a Delhi study found, what your blood test means, and what to do this week.

Every November, the sun over Delhi turns into a pale orange disc you can look at without squinting. That should tell you something. Light weak enough to stare at is probably too weak for one of sunlight's quieter jobs: making vitamin D in your skin.

India is a sunny country, so vitamin D deficiency sounds like a problem for people in Norway. It isn't. A 2025 report from the Indian Council for Research on International Economic Relations (ICRIER) estimated that one in five Indians is vitamin D deficient, with prevalence ranging from 9.4 per cent in North India to 38.81 per cent in East India ¹. Add winter, smog and long indoor hours, and the odds tilt further against you.

How your skin makes vitamin D from sunlight

Vitamin D is an odd vitamin. You can eat it, but your body can also make it when ultraviolet B (UVB) rays reach your skin ². That second route is the one most of us quietly depend on, and it is fussier than it looks.

The US National Institutes of Health lists the things that change how much UVB you get and how much vitamin D you make: season, time of day, length of day, cloud cover, smog, skin melanin and sunscreen ². A few details matter for everyday life:

• UVB does not pass through glass, so the sunny spot by your office window does nothing for your levels ².
• Sunscreen with an SPF of 8 or more appears to block the rays that make vitamin D ².
• Older adults and people with darker skin make less vitamin D from the same amount of sun ².

None of this is a reason to skip sunscreen on a long day outdoors. It simply explains why "but I live in India" is not a guarantee.

Can smog really block vitamin D?

The best-known study on this comes from Delhi itself. Researchers compared 34 children aged 9 to 24 months from two areas: Mori Gate in Old Delhi, which was heavily polluted, and Gurgaon (now Gurugram), which was less polluted at the time ³. They used a haze score as a stand-in for how much UVB was reaching the ground, and the sky over Mori Gate was measurably hazier ³.

The children's blood tests lined up with the sky. Average vitamin D, measured as 25-hydroxyvitamin D, was 12.4 ng/mL in the Mori Gate group against 27.1 ng/mL in Gurgaon ³. Parathyroid hormone and alkaline phosphatase, two markers that tend to climb when vitamin D runs short, were higher in Mori Gate as well ³. The authors concluded that children in areas of high air pollution are at risk of vitamin D deficiency rickets and should be offered supplements ³.

This was a small study in toddlers, published in 2002, and it shows an association rather than proof. Adults with office jobs are a different group. Still, the pattern is hard to ignore, and it fits with the way the haze readings suggested less UVB was getting through.

The haze that makes your chest feel tight in November is also filtering the light your skin was counting on.

What counts as low vitamin D on a blood test?

Your report will usually show a 25-hydroxyvitamin D value, in ng/mL or nmol/L. Here is how the NIH fact sheet describes the ranges ²:

• Below 12 ng/mL (30 nmol/L): associated with deficiency.
• 12 to under 20 ng/mL (30 to under 50 nmol/L): generally considered inadequate for bone and overall health.
• 20 ng/mL (50 nmol/L) or more: generally considered adequate for most people.
• Above 50 ng/mL (125 nmol/L): linked to potential adverse effects.

Put the Delhi study next to that list and the Mori Gate average of 12.4 ng/mL sits right on the edge of deficiency ²³.

Expert bodies do not fully agree on these thresholds, though. The Endocrine Society, for instance, does not set its own cut-offs for sufficiency or deficiency ². That is one reason a number flagged in red on one lab sheet may look perfectly ordinary to your doctor.

Should every adult get tested or take a supplement?

This is where the research gets less dramatic than social media. In 2024 the Endocrine Society published a guideline on using vitamin D to prevent disease. It suggests against routine vitamin D testing when there is no established reason for it, and against taking vitamin D above the recommended intake to lower disease risk in healthy adults younger than 75 ⁴.

The same guideline does suggest supplementation for some groups: children and adolescents aged 1 to 18, adults aged 75 and older, people who are pregnant, and people with high-risk prediabetes ⁴.

For everyone else, the target is modest. The recommended intake is 600 IU (15 mcg) a day for adults up to 70 and 800 IU (20 mcg) a day above 70 ². More is not better. The NIH sets the upper limit for adults at 4,000 IU (100 mcg) a day, and too much vitamin D can raise blood calcium, which can cause nausea, vomiting, muscle weakness and loss of appetite ². High-dose capsules and sachets are easy to buy over the counter, so take them only when a doctor has advised it.

Which foods have vitamin D?

Not many foods carry much of it, which is exactly why sunlight matters so much. Per serving, the NIH lists ²:

• Farmed trout, cooked, about 85 g: 645 IU.
• Sockeye salmon, cooked, about 85 g: 570 IU.
• White mushrooms exposed to UV light, half a cup sliced: 366 IU.
• Vitamin D fortified milk, one cup: 120 IU.
• One large egg: 44 IU, all of it in the yolk.

If you are vegetarian, your realistic options are fortified milk and other fortified foods, plus UV-treated mushrooms. Check labels, because fortification varies by brand. If you eat eggs, keep the yolk. It is where the vitamin D lives.

Your vitamin D plan for smog season

These steps are general and meant for healthy adults.

1. Use the clean-air windows. On days when the AQI eases, step outside in daylight with your forearms uncovered for a while. If you train outdoors, our piece on exercising in smog explains how to read the AQI before you head out.
2. Go outside, not to the window. Glass blocks UVB, so a sunny desk does not count ².
3. Put two vitamin D foods on rotation this week. Eggs with the yolk, fortified milk, fish if you eat it, or UV-treated mushrooms ².
4. Skip the self-prescribed megadose. Stay at or below the upper limit unless a doctor tells you otherwise ².
5. Test for a reason, not out of worry. If you are over 75, pregnant or have prediabetes, or your doctor has a specific concern, ask whether testing or a supplement makes sense for you ⁴.

If you have a health condition, are pregnant or take regular medication, speak to your doctor before starting any vitamin D supplement.

The grey months will pass. The small habits you build during them are the part that stays.

General information for healthy adults, not medical advice. Speak to a doctor before changing your training or diet if you have a health condition, are pregnant or take regular medication.