About 75% of outdoor air pollution is due to human activities. Most sources of air pollution are well beyond the control of individuals and demand/require concerted action by local and national level policy-makers working in sectors like transport, energy, waste management, urbal planning, and agriculture.
Historically, big cities in China and India have had the worst outdoor air pollution. China has taken steps to control pollution, reducing it by 17% between 2010 and 2015 but the situation in India has not improved at all. It is same story year after year. The slide shows the PM2.5 levels on 7th November 2017 in Delhi:
The outdoor air pollution is a problem not only in Delhi but in most of India. According to WHO, india has 14 out of the 15 top most polluted cities in the world in terms of PM2.5 concentrations. These are:
Delhi (153), Patna (149), Gwaliar (144), Raipur (134), Ahemdabad (100), Lucknow (96), Firozabad (96), Kanpur (93), Amritsar (92), Ludhiana (91), Allahabad (88), Agra (88) and Khanna (88)
In the end, the problem really boils down to the number of people living on the Earth. As more and more people move to western lifestyles with unbridled consumption; the human footprint, that is already too big, will increase further and our planet will just not be able to cope with the demands put on it.
Air pollution is just one of the ways that the Earth is responding to our irresponsibility.
PS: 4th November 2019: The Delhi pollution continues with authorities unable to provide a solution.