SPM a major cause of visible pollution in India as well, our semi-arid climate and low green cover kicks up enormous amounts of dust (containing organic & inorganic matter) in and outside cities. This website reports data on PM 2.5 and SPM10 - presumably 2.5 microns and 10 microns thick particles, and also on O3 - Ozone, NO2 - Nitrogen di oxide, SO2 - Sulphur Dioxide and CO - Carbon monoxide. PM may be unsightly, coat everything with dust, cause irritations and asthmas etc, but the other gases are more potent killers over time. These gases mostly come from automobiles and industries.
I couldn't make out if this ranking focuses only on PM2.5 or combines all the above for a composite score, with what weightages assigned to each. The website doesnt specify the data collection and analysis methods. Also its a pity one cant rank cities on other parameters and see.
Not so say India is any less polluted than it really is, but a mere SPM based index is misleading. This website is a bit dubious in its purpose since it doesn't clarify its data sources or its scientific methodology.
Oh wait, its a front for a business that sells air purifiers - thats why it focuses only on PM. With the Swiss cross in the logo triggering subconscious images of beautiful Swiss mountain's clean air and scare you to death with orange and read colours for your cities to sell cheap made in China air purifiers (basically a low power fan + filter).
Clever thinly disguised marketing



Caveat emptor !!