How useful is the air quality index?

Imagine you're walking through polluted city streets. Despite wearing a N95 mask, you can still smell the smoky, sulphuric, gassy, solvent-like air. The pollution is so bad you can even taste the air's oily, dirty residue. The skyline is obscured by a thick layer of smog, transforming the horizon into a hazy, fog-laden panorama.

From the annual transboundary haze shrouding Southeast Asia for decades to Delhi's recent acrid smog, a toxic tide of air pollution is choking cities worldwide. Beijing's winter smog, a noxious cocktail of traffic fumes and factory spew, paints a horrifying picture: this invisible enemy isn't a distant threat, it's in the air we breathe.