India has topped a global indoor air pollution chart with the highest average annual PM2.5 levels followed by China, Turkey, UAE and South Korea, according to a study.

Moreover, on the list of cities with highest average annual PM2.5 levels, national capital Delhi was on the top amongst the studied cities, followed by Beijing, Shanghai, Shenzhen and Busan, a Dyson statement said.

The Dyson Global Connected Air Quality Data project analyses more than half a trillion data points, to understand user behaviours and compare indoor and outdoor air quality.

The national sample size for India is 37,013 connected Dyson air purifiers.

"India recorded the highest average annual PM2.5 levels, followed by China, Turkey, UAE, and South Korea, while in terms of indoor average VOCs, India ranked 7th among the top 10," said the report for which the data were collected from Dyson Purifiers connected to its app.

It further said: "Moreover, Delhi topped the list of PM2.5 levels, amongst the studied cities, with the highest indoor annual average PM2.5 levels of 69.29, followed by Beijing, Shanghai, Shenzhen and Busan." PM (Particulate Matter) 2.5, which is a mixture of solid and liquid particles, is used for describing pollutant levels.

PM 2.5 levels are fine particles that have a diameter less than 2.5 micrometers, more than 100 times thinner than a human hair and remain suspended in the air for longer durations and pose health risks as they can enter into the respiratory tract.

"The study reveals that air inside our homes can be significantly worse, particularly during the winter season, with PM2.5 levels exceeding health standards set by WHO. Winter was, in general, the most polluted season across the markets – even in the southern hemisphere – where colder weather traps polluted air nearer to ground level," it said.

Pollution peaks in the winter season, especially in Delhi, with November and December being the most polluted months.

"Shockingly, the air inside homes during winter was found to be a staggering 15 per cent worse than outside, effectively trapping smoke within our walls," it said adding "In fact, across India, the indoor air gets 41 per cent dirtier during these months, and in Delhi, it's even worse at 48 per cent." Unlike most places where the air is worst at night, in India, the most polluted time inside was between 7 am and midday.

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