I've been to rural areas in North Africa, India, Southeast Asia, the Caribbean, and Central America. One uniquitous and unexpected smell is that of smoke. The majority of cooking in these rural areas is over fire, and burning garbage is a commonplace activity; people will burn their household garbage in their back yards. That and the smell of food cooking (usually outdoors) are the most prevalent scents. In low- or moderate-population areas, it's actually a pleasant smell, and the lack of other pollution leaves the air fresh, with smoke wafting by occasionally. Notably, there's no sewage smell in most of these places, even when there's no plumbing.<p>Cities in these places, on the other hand, can have overwhelming scents. In addition to all the smoke from the same activities above, some of the large cities (particularly in Asia) have open sewers in the street, and standing bodies of water with algae overgrowth. I imagine this is roughly what large cities over the millenia have smelled like.<p>Now there are two major pollutants in the mix as well: cars (with no pollution standards in place) and industrial manufacturing. Of the places I've been, this is most prevalent in large cities in India and Thailand.