New Delhi: Delhi, the national capital of India, is among the world's most polluted cities, with the air pollution levels spiking far beyond safety limits.

In November last year post Diwali, the national witnessed the worst smog, also known as the 'Great smog of Delhi'. Data showed the levels of PM2.5 and PM 10 particulate matter hit 999 micrograms per cubic meter, while the safe limits for those pollutants are 60 and 100 respectively.

Tiny suspended particles (PM2.5) that penetrate deep into the airways and lungs can cause asthma, bronchitis, heart disease, stroke and a host of other diseases.

According to a WHO survey of 1600 world cities, Delhi's air quality is the worst in the world. However, two other Indian cities - Gwalior in Madhya Pradesh, and Raipur in Chhattisgarh - were have worse air quality than Delhi.

It is said that 13 of the 25 cities worldwide with the highest levels of PM are in India.

Air pollution is the fifth largest killer in India, claiming 1.5 million lives every year. In the national capital alone, poor quality air damages irreversibly the lungs of 2.2 million or 50 percent of all children.

As Delhi's air pollution levels increase, cases of severe breathlessness, asthma and allergy have sharply risen in the national capital.

Market data shows there has been a 43 percent rise in the sales of medicines to treat respiratory disorders, particularly asthma, in India over the past four years. The year 2016 also saw a 15 percent growth in anti-asthma prescriptions across children and adults in the country.

Health experts say the number of people being diagnosed with asthma for the first time with no family history is rising sharply due to high air pollution.

Asthma is a chronic lung disease that inflames and narrows the airways, making it difficult to breathe.

Unfortunately, asthma can't be cured, but the symptoms can be controlled with medications, enabling patients to live full lives. When the symptoms are not under control, the airways can become inflamed and can cause frequent asthma attacks.

India has the world's highest death rate from chronic respiratory diseases and asthma, according to the WHO.

WHO estimates between 15 and 20 million people have asthma in India, with children (5-11 years) being the worst affected. However, some studies put the numbers higher at 30 million.

World Asthma Day is held on the first Tuesday of May each year to raise awareness, care and support for those affected by the lung condition. May is observed as Asthma Awareness Month.

Hence, on World Asthma Day (Tuesday, May 2) and throughout the month, people with asthma and organisations dedicated to asthma control and education come together to increase awareness about asthma and improve the lives of all people with the condition.