In 2014, WHO classed Delhi as the world's most polluted capital city, with air quality levels worse than Beijing. And so it remains today: India's capital is one of most polluted cities on Earth. India together with China had the highest absolute numbers of deaths attributable to fine particulate matter (<2·5 μm in diameter [PM2·5]) in 2015—according to a report by the Global Burden of Disease on air pollution. Such matter is the most damaging to health, and is minuscule enough to penetrate the pulmonary alveolus. Sustained exposure to high concentrations of PM2·5 is linked to respiratory diseases such as bronchitis, asthma, and inflammation of the lungs, and even to heart attack and stroke.
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