The air we breathe,
& the effects it has

Brain

• PM2.5
• PM10
• NO₂
• SO₂
• O₃
• CO
• NH₃
• Pb

The smallest PM2.5 particles travel up the olfactory nerve from the nose, bypassing the blood-brain barrier. Larger ones cross from the lung into the blood and inflame the vessels feeding the brain, which is how air pollution can lead to strokes.

Lead imitates calcium, which lets it into a child’s brain while nerve connections are still forming. Carbon monoxide starves the brain of oxygen.

Pooling 21 studies of 24 million people, researchers found dementia risk 17% higher for every extra 10 micrograms of PM2.5 per cubic metre in the annual average. India’s Million Death Study, covering 200,000 deaths, found stroke deaths 9% higher at that level.

Eyes

• PM2.5
• PM10
• NO₂
• SO₂
• O₃
• CO
• NH₃
• Pb

PM2.5 and PM10 break up the oily layer that stops tears evaporating, inflaming the glands that make it. Red, itchy eyes track most closely with ozone and nitrogen dioxide.

In 2002, AIIMS doctors examined 400 people in and outside Delhi and found an unstable tear film in a quarter of city residents who had no complaints. A Chinese study of 387 dry-eye patients found symptoms peaking about a week after bad air, with inflammatory proteins found in the tears. Indoor smoke, according to a 2016 India Eye Disease Study, was linked to cataracts among women who cooked on solid fuels.

Lungs

• PM2.5
• PM10
• NO₂
• SO₂
• O₃
• CO
• NH₃
• Pb

In the lungs, PM2.5 reaches alveoli, the tiny sacs where oxygen passes into the blood through half a micrometre of tissue. Immune cells swallow the particles, cannot break them down, and stay inflamed.

Ozone and nitrogen dioxide barely dissolve, so they reach the small airways and burn the lining. Sulphur dioxide is absorbed higher up and tightens asthmatics’ airways. Traffic fumes are linked to an estimated 350,000 new cases of childhood asthma in India a year.

A 2024 study of 354 people found that those aged 15 to 29 who had never smoked could exhale 17% less air in one second in Delhi-NCR than peers in the hills in Uttarakhand.

Heart

• PM2.5
• PM10
• NO₂
• SO₂
• O₃
• CO
• NH₃
• Pb

PM2.5 inflames the lung, and within hours the inflammation can reach the blood. The heartbeat loses its normal variation and blood can clot more easily. Years of exposure thicken the artery walls, raising the risk of strokes.

Across a city, hospital admissions for heart attacks and heart failure rise about 2% in the two days after PM2.5 climbs by 10 micrograms. Carbon monoxide, at high levels found in traffic, can bind to red blood cells, triggering chest pain and irregular heartbeats in people with narrowed arteries. In an enclosed space, high concentrations can kill within hours.

Worldwide, heart disease and stroke account for 68% of deaths from outdoor air pollution, the WHO says.

Gut

• PM2.5
• PM10
• NO₂
• SO₂
• O₃
• CO
• NH₃
• Pb

Most of the PM10 that is breathed in ends up swallowed. The coarser particles, too big for the deep lung, are caught in the nose and throat, swept back up and sent down the gullet within a day. In the gut, they can shift the balance of bacteria and loosen the joins between the cells lining the intestine.

A 2024 review found 13 studies of inflammatory bowel disease in Europe, North America and China. Eleven saw a link to air pollution but disagreed on its strength. A British study tied early Crohn’s disease to nitrogen dioxide and sulphur dioxide, though it found no link when all age groups were counted together. The evidence here remains thin.

Kidneys

• PM2.5
• PM10
• NO₂
• SO₂
• O₃
• CO
• NH₃
• Pb

Kidneys filter about a fifth of everything the heart pumps, so PM2.5 arriving through the blood wears on the tiny filtering vessels the way it scars arteries.

Researchers followed 2.5 million US veterans for eight years, in air an eighth as polluted as Delhi’s, and found chronic kidney disease 27% more likely per 10 micrograms of PM2.5. A study of 871,295 people put carbon monoxide alongside PM2.5 as the worst for kidney function. In 12,271 adults in Delhi and Chennai, filtration through the kidneys fell as pollution rose, and even faster in Delhi. Too few of them fell ill for the study to prove a link to kidney disease.

Womb

• PM2.5
• PM10
• NO₂
• SO₂
• O₃
• CO
• NH₃
• Pb

PM2.5 inflames the mother’s body and can reduce the oxygen crossing the placenta. Lead crosses freely, including lead leaking from the woman’s bones. Carbon monoxide and nitrogen dioxide are both linked to babies growing more slowly in the womb.

In a 2025 analysis of India’s national family health survey, every 10 micrograms of PM2.5 averaged across a pregnancy was linked to 12% more premature births and 5% more underweight babies. Studies elsewhere suggest the first three months of pregnancy are the riskiest. Researchers still argue over whether PM2.5 can reach the foetus.

Bones

• PM2.5
• PM10
• NO₂
• SO₂
• O₃
• CO
• NH₃
• Pb

Inflammation can break down bones faster than the body can rebuild it. PM2.5 keeps the body mildly inflamed. Lead works differently: most of it ends up in the skeleton, where it stays for decades before leaking back into the blood.

Among 9.2 million older Americans, fractures from thinning bone were more common where PM2.5 levels were higher, in a 2017 study. A Taiwanese database linked carbon monoxide and nitrogen dioxide to osteoporosis. Near Hyderabad, 3,700 adults had less mineral in the spine and hip where the air quality was worse, measured at their own homes, in a 2020 study. As with the gut, the findings are preliminary.

All eight

• PM2.5
• PM10
• NO₂
• SO₂
• O₃
• CO
• NH₃
• Pb

PM2.5 is the pollutant linked to harm in all eight organs. It affects the most number of organs because it is small enough to travel to the deepest part of the lungs. The inflammation it sets off there spreads through the bloodstream, which is why the particles show up in studies of the heart, kidneys, bones and pregnancy.

Lead is linked to four: the brain, heart, kidneys and womb. The gases act mainly on the eyes and lungs. In Delhi’s air, ammonia and sulphur dioxide also add to particulate matter pollution.